5 :Author: Takashi Iwai <tiwai@suse.de>
10 This document describes how to write an `ALSA (Advanced Linux Sound
11 Architecture) <http://www.alsa-project.org/>`__ driver. The document
12 focuses mainly on PCI soundcards. In the case of other device types, the
13 API might be different, too. However, at least the ALSA kernel API is
14 consistent, and therefore it would be still a bit help for writing them.
16 This document targets people who already have enough C language skills
17 and have basic linux kernel programming knowledge. This document doesn't
18 explain the general topic of linux kernel coding and doesn't cover
19 low-level driver implementation details. It only describes the standard
20 way to write a PCI sound driver on ALSA.
22 This document is still a draft version. Any feedback and corrections,
31 The file tree structure of ALSA driver is depicted below.
63 This directory contains the middle layer which is the heart of ALSA
64 drivers. In this directory, the native ALSA modules are stored. The
65 sub-directories contain different modules and are dependent upon the
71 The codes for PCM and mixer OSS emulation modules are stored in this
72 directory. The rawmidi OSS emulation is included in the ALSA rawmidi
73 code since it's quite small. The sequencer code is stored in
74 ``core/seq/oss`` directory (see `below <#core-seq-oss>`__).
79 This directory and its sub-directories are for the ALSA sequencer. This
80 directory contains the sequencer core and primary sequencer modules such
81 like snd-seq-midi, snd-seq-virmidi, etc. They are compiled only when
82 ``CONFIG_SND_SEQUENCER`` is set in the kernel config.
87 This contains the OSS sequencer emulation codes.
92 This is the place for the public header files of ALSA drivers, which are
93 to be exported to user-space, or included by several files at different
94 directories. Basically, the private header files should not be placed in
95 this directory, but you may still find files there, due to historical
101 This directory contains code shared among different drivers on different
102 architectures. They are hence supposed not to be architecture-specific.
103 For example, the dummy pcm driver and the serial MIDI driver are found
104 in this directory. In the sub-directories, there is code for components
105 which are independent from bus and cpu architectures.
110 The MPU401 and MPU401-UART modules are stored here.
112 drivers/opl3 and opl4
113 ~~~~~~~~~~~~~~~~~~~~~
115 The OPL3 and OPL4 FM-synth stuff is found here.
120 This contains the ALSA i2c components.
122 Although there is a standard i2c layer on Linux, ALSA has its own i2c
123 code for some cards, because the soundcard needs only a simple operation
124 and the standard i2c API is too complicated for such a purpose.
129 This contains the synth middle-level modules.
131 So far, there is only Emu8000/Emu10k1 synth driver under the
132 ``synth/emux`` sub-directory.
137 This directory and its sub-directories hold the top-level card modules
138 for PCI soundcards and the code specific to the PCI BUS.
140 The drivers compiled from a single file are stored directly in the pci
141 directory, while the drivers with several source files are stored on
142 their own sub-directory (e.g. emu10k1, ice1712).
147 This directory and its sub-directories hold the top-level card modules
150 arm, ppc, and sparc directories
151 -------------------------------
153 They are used for top-level card modules which are specific to one of
159 This directory contains the USB-audio driver. In the latest version, the
160 USB MIDI driver is integrated in the usb-audio driver.
165 The PCMCIA, especially PCCard drivers will go here. CardBus drivers will
166 be in the pci directory, because their API is identical to that of
172 This directory contains the codes for ASoC (ALSA System on Chip)
173 layer including ASoC core, codec and machine drivers.
178 Here contains OSS/Lite codes.
179 All codes have been deprecated except for dmasound on m68k as of
183 Basic Flow for PCI Drivers
184 ==========================
189 The minimum flow for PCI soundcards is as follows:
191 - define the PCI ID table (see the section `PCI Entries`_).
193 - create ``probe`` callback.
195 - create ``remove`` callback.
197 - create a :c:type:`struct pci_driver <pci_driver>` structure
198 containing the three pointers above.
200 - create an ``init`` function just calling the
201 :c:func:`pci_register_driver()` to register the pci_driver
204 - create an ``exit`` function to call the
205 :c:func:`pci_unregister_driver()` function.
210 The code example is shown below. Some parts are kept unimplemented at
211 this moment but will be filled in the next sections. The numbers in the
212 comment lines of the :c:func:`snd_mychip_probe()` function refer
213 to details explained in the following section.
217 #include <linux/init.h>
218 #include <linux/pci.h>
219 #include <linux/slab.h>
220 #include <sound/core.h>
221 #include <sound/initval.h>
223 /* module parameters (see "Module Parameters") */
224 /* SNDRV_CARDS: maximum number of cards supported by this module */
225 static int index[SNDRV_CARDS] = SNDRV_DEFAULT_IDX;
226 static char *id[SNDRV_CARDS] = SNDRV_DEFAULT_STR;
227 static bool enable[SNDRV_CARDS] = SNDRV_DEFAULT_ENABLE_PNP;
229 /* definition of the chip-specific record */
231 struct snd_card *card;
232 /* the rest of the implementation will be in section
233 * "PCI Resource Management"
237 /* chip-specific destructor
238 * (see "PCI Resource Management")
240 static int snd_mychip_free(struct mychip *chip)
242 .... /* will be implemented later... */
245 /* component-destructor
246 * (see "Management of Cards and Components")
248 static int snd_mychip_dev_free(struct snd_device *device)
250 return snd_mychip_free(device->device_data);
253 /* chip-specific constructor
254 * (see "Management of Cards and Components")
256 static int snd_mychip_create(struct snd_card *card,
258 struct mychip **rchip)
262 static struct snd_device_ops ops = {
263 .dev_free = snd_mychip_dev_free,
268 /* check PCI availability here
269 * (see "PCI Resource Management")
273 /* allocate a chip-specific data with zero filled */
274 chip = kzalloc(sizeof(*chip), GFP_KERNEL);
280 /* rest of initialization here; will be implemented
281 * later, see "PCI Resource Management"
285 err = snd_device_new(card, SNDRV_DEV_LOWLEVEL, chip, &ops);
287 snd_mychip_free(chip);
295 /* constructor -- see "Driver Constructor" sub-section */
296 static int snd_mychip_probe(struct pci_dev *pci,
297 const struct pci_device_id *pci_id)
300 struct snd_card *card;
305 if (dev >= SNDRV_CARDS)
313 err = snd_card_new(&pci->dev, index[dev], id[dev], THIS_MODULE,
319 err = snd_mychip_create(card, pci, &chip);
324 strcpy(card->driver, "My Chip");
325 strcpy(card->shortname, "My Own Chip 123");
326 sprintf(card->longname, "%s at 0x%lx irq %i",
327 card->shortname, chip->ioport, chip->irq);
330 .... /* implemented later */
333 err = snd_card_register(card);
338 pci_set_drvdata(pci, card);
347 /* destructor -- see the "Destructor" sub-section */
348 static void snd_mychip_remove(struct pci_dev *pci)
350 snd_card_free(pci_get_drvdata(pci));
358 The real constructor of PCI drivers is the ``probe`` callback. The
359 ``probe`` callback and other component-constructors which are called
360 from the ``probe`` callback cannot be used with the ``__init`` prefix
361 because any PCI device could be a hotplug device.
363 In the ``probe`` callback, the following scheme is often used.
365 1) Check and increment the device index.
366 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
372 if (dev >= SNDRV_CARDS)
380 where ``enable[dev]`` is the module option.
382 Each time the ``probe`` callback is called, check the availability of
383 the device. If not available, simply increment the device index and
384 returns. dev will be incremented also later (`step 7
385 <#set-the-pci-driver-data-and-return-zero>`__).
387 2) Create a card instance
388 ~~~~~~~~~~~~~~~~~~~~~~~~~
392 struct snd_card *card;
395 err = snd_card_new(&pci->dev, index[dev], id[dev], THIS_MODULE,
399 The details will be explained in the section `Management of Cards and
402 3) Create a main component
403 ~~~~~~~~~~~~~~~~~~~~~~~~~~
405 In this part, the PCI resources are allocated.
411 err = snd_mychip_create(card, pci, &chip);
415 The details will be explained in the section `PCI Resource
418 When something goes wrong, the probe function needs to deal with the
419 error. In this example, we have a single error handling path placed
420 at the end of the function.
428 Since each component can be properly freed, the single
429 :c:func:`snd_card_free()` call should suffice in most cases.
432 4) Set the driver ID and name strings.
433 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
437 strcpy(card->driver, "My Chip");
438 strcpy(card->shortname, "My Own Chip 123");
439 sprintf(card->longname, "%s at 0x%lx irq %i",
440 card->shortname, chip->ioport, chip->irq);
442 The driver field holds the minimal ID string of the chip. This is used
443 by alsa-lib's configurator, so keep it simple but unique. Even the
444 same driver can have different driver IDs to distinguish the
445 functionality of each chip type.
447 The shortname field is a string shown as more verbose name. The longname
448 field contains the information shown in ``/proc/asound/cards``.
450 5) Create other components, such as mixer, MIDI, etc.
451 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
453 Here you define the basic components such as `PCM <#PCM-Interface>`__,
454 mixer (e.g. `AC97 <#API-for-AC97-Codec>`__), MIDI (e.g.
455 `MPU-401 <#MIDI-MPU401-UART-Interface>`__), and other interfaces.
456 Also, if you want a `proc file <#Proc-Interface>`__, define it here,
459 6) Register the card instance.
460 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
464 err = snd_card_register(card);
468 Will be explained in the section `Management of Cards and
471 7) Set the PCI driver data and return zero.
472 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
476 pci_set_drvdata(pci, card);
480 In the above, the card record is stored. This pointer is used in the
481 remove callback and power-management callbacks, too.
486 The destructor, remove callback, simply releases the card instance. Then
487 the ALSA middle layer will release all the attached components
490 It would be typically just :c:func:`calling snd_card_free()`:
494 static void snd_mychip_remove(struct pci_dev *pci)
496 snd_card_free(pci_get_drvdata(pci));
500 The above code assumes that the card pointer is set to the PCI driver
506 For the above example, at least the following include files are
511 #include <linux/init.h>
512 #include <linux/pci.h>
513 #include <linux/slab.h>
514 #include <sound/core.h>
515 #include <sound/initval.h>
517 where the last one is necessary only when module options are defined
518 in the source file. If the code is split into several files, the files
519 without module options don't need them.
521 In addition to these headers, you'll need ``<linux/interrupt.h>`` for
522 interrupt handling, and ``<linux/io.h>`` for I/O access. If you use the
523 :c:func:`mdelay()` or :c:func:`udelay()` functions, you'll need
524 to include ``<linux/delay.h>`` too.
526 The ALSA interfaces like the PCM and control APIs are defined in other
527 ``<sound/xxx.h>`` header files. They have to be included after
530 Management of Cards and Components
531 ==================================
536 For each soundcard, a “card” record must be allocated.
538 A card record is the headquarters of the soundcard. It manages the whole
539 list of devices (components) on the soundcard, such as PCM, mixers,
540 MIDI, synthesizer, and so on. Also, the card record holds the ID and the
541 name strings of the card, manages the root of proc files, and controls
542 the power-management states and hotplug disconnections. The component
543 list on the card record is used to manage the correct release of
544 resources at destruction.
546 As mentioned above, to create a card instance, call
547 :c:func:`snd_card_new()`.
551 struct snd_card *card;
553 err = snd_card_new(&pci->dev, index, id, module, extra_size, &card);
556 The function takes six arguments: the parent device pointer, the
557 card-index number, the id string, the module pointer (usually
558 ``THIS_MODULE``), the size of extra-data space, and the pointer to
559 return the card instance. The extra_size argument is used to allocate
560 card->private_data for the chip-specific data. Note that these data are
561 allocated by :c:func:`snd_card_new()`.
563 The first argument, the pointer of struct :c:type:`struct device
564 <device>`, specifies the parent device. For PCI devices, typically
565 ``&pci->`` is passed there.
570 After the card is created, you can attach the components (devices) to
571 the card instance. In an ALSA driver, a component is represented as a
572 :c:type:`struct snd_device <snd_device>` object. A component
573 can be a PCM instance, a control interface, a raw MIDI interface, etc.
574 Each such instance has one component entry.
576 A component can be created via :c:func:`snd_device_new()`
581 snd_device_new(card, SNDRV_DEV_XXX, chip, &ops);
583 This takes the card pointer, the device-level (``SNDRV_DEV_XXX``), the
584 data pointer, and the callback pointers (``&ops``). The device-level
585 defines the type of components and the order of registration and
586 de-registration. For most components, the device-level is already
587 defined. For a user-defined component, you can use
588 ``SNDRV_DEV_LOWLEVEL``.
590 This function itself doesn't allocate the data space. The data must be
591 allocated manually beforehand, and its pointer is passed as the
592 argument. This pointer (``chip`` in the above example) is used as the
593 identifier for the instance.
595 Each pre-defined ALSA component such as ac97 and pcm calls
596 :c:func:`snd_device_new()` inside its constructor. The destructor
597 for each component is defined in the callback pointers. Hence, you don't
598 need to take care of calling a destructor for such a component.
600 If you wish to create your own component, you need to set the destructor
601 function to the dev_free callback in the ``ops``, so that it can be
602 released automatically via :c:func:`snd_card_free()`. The next
603 example will show an implementation of chip-specific data.
608 Chip-specific information, e.g. the I/O port address, its resource
609 pointer, or the irq number, is stored in the chip-specific record.
618 In general, there are two ways of allocating the chip record.
620 1. Allocating via :c:func:`snd_card_new()`.
621 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
623 As mentioned above, you can pass the extra-data-length to the 5th
624 argument of :c:func:`snd_card_new()`, i.e.
628 err = snd_card_new(&pci->dev, index[dev], id[dev], THIS_MODULE,
629 sizeof(struct mychip), &card);
631 :c:type:`struct mychip <mychip>` is the type of the chip record.
633 In return, the allocated record can be accessed as
637 struct mychip *chip = card->private_data;
639 With this method, you don't have to allocate twice. The record is
640 released together with the card instance.
642 2. Allocating an extra device.
643 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
645 After allocating a card instance via :c:func:`snd_card_new()`
646 (with ``0`` on the 4th arg), call :c:func:`kzalloc()`.
650 struct snd_card *card;
652 err = snd_card_new(&pci->dev, index[dev], id[dev], THIS_MODULE,
655 chip = kzalloc(sizeof(*chip), GFP_KERNEL);
657 The chip record should have the field to hold the card pointer at least,
662 struct snd_card *card;
667 Then, set the card pointer in the returned chip instance.
673 Next, initialize the fields, and register this chip record as a
674 low-level device with a specified ``ops``,
678 static struct snd_device_ops ops = {
679 .dev_free = snd_mychip_dev_free,
682 snd_device_new(card, SNDRV_DEV_LOWLEVEL, chip, &ops);
684 :c:func:`snd_mychip_dev_free()` is the device-destructor
685 function, which will call the real destructor.
689 static int snd_mychip_dev_free(struct snd_device *device)
691 return snd_mychip_free(device->device_data);
694 where :c:func:`snd_mychip_free()` is the real destructor.
696 The demerit of this method is the obviously more amount of codes.
697 The merit is, however, you can trigger the own callback at registering
698 and disconnecting the card via setting in snd_device_ops.
699 About the registering and disconnecting the card, see the subsections
703 Registration and Release
704 ------------------------
706 After all components are assigned, register the card instance by calling
707 :c:func:`snd_card_register()`. Access to the device files is
708 enabled at this point. That is, before
709 :c:func:`snd_card_register()` is called, the components are safely
710 inaccessible from external side. If this call fails, exit the probe
711 function after releasing the card via :c:func:`snd_card_free()`.
713 For releasing the card instance, you can call simply
714 :c:func:`snd_card_free()`. As mentioned earlier, all components
715 are released automatically by this call.
717 For a device which allows hotplugging, you can use
718 :c:func:`snd_card_free_when_closed()`. This one will postpone
719 the destruction until all devices are closed.
721 PCI Resource Management
722 =======================
727 In this section, we'll complete the chip-specific constructor,
728 destructor and PCI entries. Example code is shown first, below.
733 struct snd_card *card;
740 static int snd_mychip_free(struct mychip *chip)
742 /* disable hardware here if any */
743 .... /* (not implemented in this document) */
745 /* release the irq */
747 free_irq(chip->irq, chip);
748 /* release the I/O ports & memory */
749 pci_release_regions(chip->pci);
750 /* disable the PCI entry */
751 pci_disable_device(chip->pci);
752 /* release the data */
757 /* chip-specific constructor */
758 static int snd_mychip_create(struct snd_card *card,
760 struct mychip **rchip)
764 static struct snd_device_ops ops = {
765 .dev_free = snd_mychip_dev_free,
770 /* initialize the PCI entry */
771 err = pci_enable_device(pci);
774 /* check PCI availability (28bit DMA) */
775 if (pci_set_dma_mask(pci, DMA_BIT_MASK(28)) < 0 ||
776 pci_set_consistent_dma_mask(pci, DMA_BIT_MASK(28)) < 0) {
777 printk(KERN_ERR "error to set 28bit mask DMA\n");
778 pci_disable_device(pci);
782 chip = kzalloc(sizeof(*chip), GFP_KERNEL);
784 pci_disable_device(pci);
788 /* initialize the stuff */
793 /* (1) PCI resource allocation */
794 err = pci_request_regions(pci, "My Chip");
797 pci_disable_device(pci);
800 chip->port = pci_resource_start(pci, 0);
801 if (request_irq(pci->irq, snd_mychip_interrupt,
802 IRQF_SHARED, KBUILD_MODNAME, chip)) {
803 printk(KERN_ERR "cannot grab irq %d\n", pci->irq);
804 snd_mychip_free(chip);
807 chip->irq = pci->irq;
809 /* (2) initialization of the chip hardware */
810 .... /* (not implemented in this document) */
812 err = snd_device_new(card, SNDRV_DEV_LOWLEVEL, chip, &ops);
814 snd_mychip_free(chip);
823 static struct pci_device_id snd_mychip_ids[] = {
824 { PCI_VENDOR_ID_FOO, PCI_DEVICE_ID_BAR,
825 PCI_ANY_ID, PCI_ANY_ID, 0, 0, 0, },
829 MODULE_DEVICE_TABLE(pci, snd_mychip_ids);
831 /* pci_driver definition */
832 static struct pci_driver driver = {
833 .name = KBUILD_MODNAME,
834 .id_table = snd_mychip_ids,
835 .probe = snd_mychip_probe,
836 .remove = snd_mychip_remove,
839 /* module initialization */
840 static int __init alsa_card_mychip_init(void)
842 return pci_register_driver(&driver);
845 /* module clean up */
846 static void __exit alsa_card_mychip_exit(void)
848 pci_unregister_driver(&driver);
851 module_init(alsa_card_mychip_init)
852 module_exit(alsa_card_mychip_exit)
854 EXPORT_NO_SYMBOLS; /* for old kernels only */
859 The allocation of PCI resources is done in the ``probe`` function, and
860 usually an extra :c:func:`xxx_create()` function is written for this
863 In the case of PCI devices, you first have to call the
864 :c:func:`pci_enable_device()` function before allocating
865 resources. Also, you need to set the proper PCI DMA mask to limit the
866 accessed I/O range. In some cases, you might need to call
867 :c:func:`pci_set_master()` function, too.
869 Suppose the 28bit mask, and the code to be added would be like:
873 err = pci_enable_device(pci);
876 if (pci_set_dma_mask(pci, DMA_BIT_MASK(28)) < 0 ||
877 pci_set_consistent_dma_mask(pci, DMA_BIT_MASK(28)) < 0) {
878 printk(KERN_ERR "error to set 28bit mask DMA\n");
879 pci_disable_device(pci);
887 The allocation of I/O ports and irqs is done via standard kernel
888 functions. These resources must be released in the destructor
889 function (see below).
891 Now assume that the PCI device has an I/O port with 8 bytes and an
892 interrupt. Then :c:type:`struct mychip <mychip>` will have the
898 struct snd_card *card;
905 For an I/O port (and also a memory region), you need to have the
906 resource pointer for the standard resource management. For an irq, you
907 have to keep only the irq number (integer). But you need to initialize
908 this number as -1 before actual allocation, since irq 0 is valid. The
909 port address and its resource pointer can be initialized as null by
910 :c:func:`kzalloc()` automatically, so you don't have to take care of
913 The allocation of an I/O port is done like this:
917 err = pci_request_regions(pci, "My Chip");
920 pci_disable_device(pci);
923 chip->port = pci_resource_start(pci, 0);
925 It will reserve the I/O port region of 8 bytes of the given PCI device.
926 The returned value, ``chip->res_port``, is allocated via
927 :c:func:`kmalloc()` by :c:func:`request_region()`. The pointer
928 must be released via :c:func:`kfree()`, but there is a problem with
929 this. This issue will be explained later.
931 The allocation of an interrupt source is done like this:
935 if (request_irq(pci->irq, snd_mychip_interrupt,
936 IRQF_SHARED, KBUILD_MODNAME, chip)) {
937 printk(KERN_ERR "cannot grab irq %d\n", pci->irq);
938 snd_mychip_free(chip);
941 chip->irq = pci->irq;
943 where :c:func:`snd_mychip_interrupt()` is the interrupt handler
944 defined `later <#pcm-interface-interrupt-handler>`__. Note that
945 ``chip->irq`` should be defined only when :c:func:`request_irq()`
948 On the PCI bus, interrupts can be shared. Thus, ``IRQF_SHARED`` is used
949 as the interrupt flag of :c:func:`request_irq()`.
951 The last argument of :c:func:`request_irq()` is the data pointer
952 passed to the interrupt handler. Usually, the chip-specific record is
953 used for that, but you can use what you like, too.
955 I won't give details about the interrupt handler at this point, but at
956 least its appearance can be explained now. The interrupt handler looks
957 usually like the following:
961 static irqreturn_t snd_mychip_interrupt(int irq, void *dev_id)
963 struct mychip *chip = dev_id;
969 Now let's write the corresponding destructor for the resources above.
970 The role of destructor is simple: disable the hardware (if already
971 activated) and release the resources. So far, we have no hardware part,
972 so the disabling code is not written here.
974 To release the resources, the “check-and-release” method is a safer way.
975 For the interrupt, do like this:
980 free_irq(chip->irq, chip);
982 Since the irq number can start from 0, you should initialize
983 ``chip->irq`` with a negative value (e.g. -1), so that you can check
984 the validity of the irq number as above.
986 When you requested I/O ports or memory regions via
987 :c:func:`pci_request_region()` or
988 :c:func:`pci_request_regions()` like in this example, release the
989 resource(s) using the corresponding function,
990 :c:func:`pci_release_region()` or
991 :c:func:`pci_release_regions()`.
995 pci_release_regions(chip->pci);
997 When you requested manually via :c:func:`request_region()` or
998 :c:func:`request_mem_region()`, you can release it via
999 :c:func:`release_resource()`. Suppose that you keep the resource
1000 pointer returned from :c:func:`request_region()` in
1001 chip->res_port, the release procedure looks like:
1005 release_and_free_resource(chip->res_port);
1007 Don't forget to call :c:func:`pci_disable_device()` before the
1010 And finally, release the chip-specific record.
1016 We didn't implement the hardware disabling part in the above. If you
1017 need to do this, please note that the destructor may be called even
1018 before the initialization of the chip is completed. It would be better
1019 to have a flag to skip hardware disabling if the hardware was not
1022 When the chip-data is assigned to the card using
1023 :c:func:`snd_device_new()` with ``SNDRV_DEV_LOWLELVEL`` , its
1024 destructor is called at the last. That is, it is assured that all other
1025 components like PCMs and controls have already been released. You don't
1026 have to stop PCMs, etc. explicitly, but just call low-level hardware
1029 The management of a memory-mapped region is almost as same as the
1030 management of an I/O port. You'll need three fields like the
1037 unsigned long iobase_phys;
1038 void __iomem *iobase_virt;
1041 and the allocation would be like below:
1045 err = pci_request_regions(pci, "My Chip");
1050 chip->iobase_phys = pci_resource_start(pci, 0);
1051 chip->iobase_virt = ioremap_nocache(chip->iobase_phys,
1052 pci_resource_len(pci, 0));
1054 and the corresponding destructor would be:
1058 static int snd_mychip_free(struct mychip *chip)
1061 if (chip->iobase_virt)
1062 iounmap(chip->iobase_virt);
1064 pci_release_regions(chip->pci);
1068 Of course, a modern way with :c:func:`pci_iomap()` will make things a
1073 err = pci_request_regions(pci, "My Chip");
1078 chip->iobase_virt = pci_iomap(pci, 0, 0);
1080 which is paired with :c:func:`pci_iounmap()` at destructor.
1086 So far, so good. Let's finish the missing PCI stuff. At first, we need a
1087 :c:type:`struct pci_device_id <pci_device_id>` table for
1088 this chipset. It's a table of PCI vendor/device ID number, and some
1095 static struct pci_device_id snd_mychip_ids[] = {
1096 { PCI_VENDOR_ID_FOO, PCI_DEVICE_ID_BAR,
1097 PCI_ANY_ID, PCI_ANY_ID, 0, 0, 0, },
1101 MODULE_DEVICE_TABLE(pci, snd_mychip_ids);
1103 The first and second fields of the :c:type:`struct pci_device_id
1104 <pci_device_id>` structure are the vendor and device IDs. If you
1105 have no reason to filter the matching devices, you can leave the
1106 remaining fields as above. The last field of the :c:type:`struct
1107 pci_device_id <pci_device_id>` struct contains private data
1108 for this entry. You can specify any value here, for example, to define
1109 specific operations for supported device IDs. Such an example is found
1110 in the intel8x0 driver.
1112 The last entry of this list is the terminator. You must specify this
1115 Then, prepare the :c:type:`struct pci_driver <pci_driver>`
1120 static struct pci_driver driver = {
1121 .name = KBUILD_MODNAME,
1122 .id_table = snd_mychip_ids,
1123 .probe = snd_mychip_probe,
1124 .remove = snd_mychip_remove,
1127 The ``probe`` and ``remove`` functions have already been defined in
1128 the previous sections. The ``name`` field is the name string of this
1129 device. Note that you must not use a slash “/” in this string.
1131 And at last, the module entries:
1135 static int __init alsa_card_mychip_init(void)
1137 return pci_register_driver(&driver);
1140 static void __exit alsa_card_mychip_exit(void)
1142 pci_unregister_driver(&driver);
1145 module_init(alsa_card_mychip_init)
1146 module_exit(alsa_card_mychip_exit)
1148 Note that these module entries are tagged with ``__init`` and ``__exit``
1159 The PCM middle layer of ALSA is quite powerful and it is only necessary
1160 for each driver to implement the low-level functions to access its
1163 For accessing to the PCM layer, you need to include ``<sound/pcm.h>``
1164 first. In addition, ``<sound/pcm_params.h>`` might be needed if you
1165 access to some functions related with hw_param.
1167 Each card device can have up to four pcm instances. A pcm instance
1168 corresponds to a pcm device file. The limitation of number of instances
1169 comes only from the available bit size of the Linux's device numbers.
1170 Once when 64bit device number is used, we'll have more pcm instances
1173 A pcm instance consists of pcm playback and capture streams, and each
1174 pcm stream consists of one or more pcm substreams. Some soundcards
1175 support multiple playback functions. For example, emu10k1 has a PCM
1176 playback of 32 stereo substreams. In this case, at each open, a free
1177 substream is (usually) automatically chosen and opened. Meanwhile, when
1178 only one substream exists and it was already opened, the successful open
1179 will either block or error with ``EAGAIN`` according to the file open
1180 mode. But you don't have to care about such details in your driver. The
1181 PCM middle layer will take care of such work.
1186 The example code below does not include any hardware access routines but
1187 shows only the skeleton, how to build up the PCM interfaces.
1191 #include <sound/pcm.h>
1194 /* hardware definition */
1195 static struct snd_pcm_hardware snd_mychip_playback_hw = {
1196 .info = (SNDRV_PCM_INFO_MMAP |
1197 SNDRV_PCM_INFO_INTERLEAVED |
1198 SNDRV_PCM_INFO_BLOCK_TRANSFER |
1199 SNDRV_PCM_INFO_MMAP_VALID),
1200 .formats = SNDRV_PCM_FMTBIT_S16_LE,
1201 .rates = SNDRV_PCM_RATE_8000_48000,
1206 .buffer_bytes_max = 32768,
1207 .period_bytes_min = 4096,
1208 .period_bytes_max = 32768,
1210 .periods_max = 1024,
1213 /* hardware definition */
1214 static struct snd_pcm_hardware snd_mychip_capture_hw = {
1215 .info = (SNDRV_PCM_INFO_MMAP |
1216 SNDRV_PCM_INFO_INTERLEAVED |
1217 SNDRV_PCM_INFO_BLOCK_TRANSFER |
1218 SNDRV_PCM_INFO_MMAP_VALID),
1219 .formats = SNDRV_PCM_FMTBIT_S16_LE,
1220 .rates = SNDRV_PCM_RATE_8000_48000,
1225 .buffer_bytes_max = 32768,
1226 .period_bytes_min = 4096,
1227 .period_bytes_max = 32768,
1229 .periods_max = 1024,
1233 static int snd_mychip_playback_open(struct snd_pcm_substream *substream)
1235 struct mychip *chip = snd_pcm_substream_chip(substream);
1236 struct snd_pcm_runtime *runtime = substream->runtime;
1238 runtime->hw = snd_mychip_playback_hw;
1239 /* more hardware-initialization will be done here */
1244 /* close callback */
1245 static int snd_mychip_playback_close(struct snd_pcm_substream *substream)
1247 struct mychip *chip = snd_pcm_substream_chip(substream);
1248 /* the hardware-specific codes will be here */
1255 static int snd_mychip_capture_open(struct snd_pcm_substream *substream)
1257 struct mychip *chip = snd_pcm_substream_chip(substream);
1258 struct snd_pcm_runtime *runtime = substream->runtime;
1260 runtime->hw = snd_mychip_capture_hw;
1261 /* more hardware-initialization will be done here */
1266 /* close callback */
1267 static int snd_mychip_capture_close(struct snd_pcm_substream *substream)
1269 struct mychip *chip = snd_pcm_substream_chip(substream);
1270 /* the hardware-specific codes will be here */
1276 /* hw_params callback */
1277 static int snd_mychip_pcm_hw_params(struct snd_pcm_substream *substream,
1278 struct snd_pcm_hw_params *hw_params)
1280 return snd_pcm_lib_malloc_pages(substream,
1281 params_buffer_bytes(hw_params));
1284 /* hw_free callback */
1285 static int snd_mychip_pcm_hw_free(struct snd_pcm_substream *substream)
1287 return snd_pcm_lib_free_pages(substream);
1290 /* prepare callback */
1291 static int snd_mychip_pcm_prepare(struct snd_pcm_substream *substream)
1293 struct mychip *chip = snd_pcm_substream_chip(substream);
1294 struct snd_pcm_runtime *runtime = substream->runtime;
1296 /* set up the hardware with the current configuration
1299 mychip_set_sample_format(chip, runtime->format);
1300 mychip_set_sample_rate(chip, runtime->rate);
1301 mychip_set_channels(chip, runtime->channels);
1302 mychip_set_dma_setup(chip, runtime->dma_addr,
1308 /* trigger callback */
1309 static int snd_mychip_pcm_trigger(struct snd_pcm_substream *substream,
1313 case SNDRV_PCM_TRIGGER_START:
1314 /* do something to start the PCM engine */
1317 case SNDRV_PCM_TRIGGER_STOP:
1318 /* do something to stop the PCM engine */
1326 /* pointer callback */
1327 static snd_pcm_uframes_t
1328 snd_mychip_pcm_pointer(struct snd_pcm_substream *substream)
1330 struct mychip *chip = snd_pcm_substream_chip(substream);
1331 unsigned int current_ptr;
1333 /* get the current hardware pointer */
1334 current_ptr = mychip_get_hw_pointer(chip);
1339 static struct snd_pcm_ops snd_mychip_playback_ops = {
1340 .open = snd_mychip_playback_open,
1341 .close = snd_mychip_playback_close,
1342 .ioctl = snd_pcm_lib_ioctl,
1343 .hw_params = snd_mychip_pcm_hw_params,
1344 .hw_free = snd_mychip_pcm_hw_free,
1345 .prepare = snd_mychip_pcm_prepare,
1346 .trigger = snd_mychip_pcm_trigger,
1347 .pointer = snd_mychip_pcm_pointer,
1351 static struct snd_pcm_ops snd_mychip_capture_ops = {
1352 .open = snd_mychip_capture_open,
1353 .close = snd_mychip_capture_close,
1354 .ioctl = snd_pcm_lib_ioctl,
1355 .hw_params = snd_mychip_pcm_hw_params,
1356 .hw_free = snd_mychip_pcm_hw_free,
1357 .prepare = snd_mychip_pcm_prepare,
1358 .trigger = snd_mychip_pcm_trigger,
1359 .pointer = snd_mychip_pcm_pointer,
1363 * definitions of capture are omitted here...
1366 /* create a pcm device */
1367 static int snd_mychip_new_pcm(struct mychip *chip)
1369 struct snd_pcm *pcm;
1372 err = snd_pcm_new(chip->card, "My Chip", 0, 1, 1, &pcm);
1375 pcm->private_data = chip;
1376 strcpy(pcm->name, "My Chip");
1379 snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_PLAYBACK,
1380 &snd_mychip_playback_ops);
1381 snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_CAPTURE,
1382 &snd_mychip_capture_ops);
1383 /* pre-allocation of buffers */
1384 /* NOTE: this may fail */
1385 snd_pcm_lib_preallocate_pages_for_all(pcm, SNDRV_DMA_TYPE_DEV,
1386 snd_dma_pci_data(chip->pci),
1395 A pcm instance is allocated by the :c:func:`snd_pcm_new()`
1396 function. It would be better to create a constructor for pcm, namely,
1400 static int snd_mychip_new_pcm(struct mychip *chip)
1402 struct snd_pcm *pcm;
1405 err = snd_pcm_new(chip->card, "My Chip", 0, 1, 1, &pcm);
1408 pcm->private_data = chip;
1409 strcpy(pcm->name, "My Chip");
1415 The :c:func:`snd_pcm_new()` function takes four arguments. The
1416 first argument is the card pointer to which this pcm is assigned, and
1417 the second is the ID string.
1419 The third argument (``index``, 0 in the above) is the index of this new
1420 pcm. It begins from zero. If you create more than one pcm instances,
1421 specify the different numbers in this argument. For example, ``index =
1422 1`` for the second PCM device.
1424 The fourth and fifth arguments are the number of substreams for playback
1425 and capture, respectively. Here 1 is used for both arguments. When no
1426 playback or capture substreams are available, pass 0 to the
1427 corresponding argument.
1429 If a chip supports multiple playbacks or captures, you can specify more
1430 numbers, but they must be handled properly in open/close, etc.
1431 callbacks. When you need to know which substream you are referring to,
1432 then it can be obtained from :c:type:`struct snd_pcm_substream
1433 <snd_pcm_substream>` data passed to each callback as follows:
1437 struct snd_pcm_substream *substream;
1438 int index = substream->number;
1441 After the pcm is created, you need to set operators for each pcm stream.
1445 snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_PLAYBACK,
1446 &snd_mychip_playback_ops);
1447 snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_CAPTURE,
1448 &snd_mychip_capture_ops);
1450 The operators are defined typically like this:
1454 static struct snd_pcm_ops snd_mychip_playback_ops = {
1455 .open = snd_mychip_pcm_open,
1456 .close = snd_mychip_pcm_close,
1457 .ioctl = snd_pcm_lib_ioctl,
1458 .hw_params = snd_mychip_pcm_hw_params,
1459 .hw_free = snd_mychip_pcm_hw_free,
1460 .prepare = snd_mychip_pcm_prepare,
1461 .trigger = snd_mychip_pcm_trigger,
1462 .pointer = snd_mychip_pcm_pointer,
1465 All the callbacks are described in the Operators_ subsection.
1467 After setting the operators, you probably will want to pre-allocate the
1468 buffer. For the pre-allocation, simply call the following:
1472 snd_pcm_lib_preallocate_pages_for_all(pcm, SNDRV_DMA_TYPE_DEV,
1473 snd_dma_pci_data(chip->pci),
1476 It will allocate a buffer up to 64kB as default. Buffer management
1477 details will be described in the later section `Buffer and Memory
1480 Additionally, you can set some extra information for this pcm in
1481 ``pcm->info_flags``. The available values are defined as
1482 ``SNDRV_PCM_INFO_XXX`` in ``<sound/asound.h>``, which is used for the
1483 hardware definition (described later). When your soundchip supports only
1484 half-duplex, specify like this:
1488 pcm->info_flags = SNDRV_PCM_INFO_HALF_DUPLEX;
1491 ... And the Destructor?
1492 -----------------------
1494 The destructor for a pcm instance is not always necessary. Since the pcm
1495 device will be released by the middle layer code automatically, you
1496 don't have to call the destructor explicitly.
1498 The destructor would be necessary if you created special records
1499 internally and needed to release them. In such a case, set the
1500 destructor function to ``pcm->private_free``:
1504 static void mychip_pcm_free(struct snd_pcm *pcm)
1506 struct mychip *chip = snd_pcm_chip(pcm);
1507 /* free your own data */
1508 kfree(chip->my_private_pcm_data);
1509 /* do what you like else */
1513 static int snd_mychip_new_pcm(struct mychip *chip)
1515 struct snd_pcm *pcm;
1517 /* allocate your own data */
1518 chip->my_private_pcm_data = kmalloc(...);
1519 /* set the destructor */
1520 pcm->private_data = chip;
1521 pcm->private_free = mychip_pcm_free;
1527 Runtime Pointer - The Chest of PCM Information
1528 ----------------------------------------------
1530 When the PCM substream is opened, a PCM runtime instance is allocated
1531 and assigned to the substream. This pointer is accessible via
1532 ``substream->runtime``. This runtime pointer holds most information you
1533 need to control the PCM: the copy of hw_params and sw_params
1534 configurations, the buffer pointers, mmap records, spinlocks, etc.
1536 The definition of runtime instance is found in ``<sound/pcm.h>``. Here
1537 are the contents of this file:
1541 struct _snd_pcm_runtime {
1543 struct snd_pcm_substream *trigger_master;
1544 snd_timestamp_t trigger_tstamp; /* trigger timestamp */
1546 snd_pcm_uframes_t avail_max;
1547 snd_pcm_uframes_t hw_ptr_base; /* Position at buffer restart */
1548 snd_pcm_uframes_t hw_ptr_interrupt; /* Position at interrupt time*/
1550 /* -- HW params -- */
1551 snd_pcm_access_t access; /* access mode */
1552 snd_pcm_format_t format; /* SNDRV_PCM_FORMAT_* */
1553 snd_pcm_subformat_t subformat; /* subformat */
1554 unsigned int rate; /* rate in Hz */
1555 unsigned int channels; /* channels */
1556 snd_pcm_uframes_t period_size; /* period size */
1557 unsigned int periods; /* periods */
1558 snd_pcm_uframes_t buffer_size; /* buffer size */
1559 unsigned int tick_time; /* tick time */
1560 snd_pcm_uframes_t min_align; /* Min alignment for the format */
1562 unsigned int frame_bits;
1563 unsigned int sample_bits;
1565 unsigned int rate_num;
1566 unsigned int rate_den;
1568 /* -- SW params -- */
1569 struct timespec tstamp_mode; /* mmap timestamp is updated */
1570 unsigned int period_step;
1571 unsigned int sleep_min; /* min ticks to sleep */
1572 snd_pcm_uframes_t start_threshold;
1573 snd_pcm_uframes_t stop_threshold;
1574 snd_pcm_uframes_t silence_threshold; /* Silence filling happens when
1575 noise is nearest than this */
1576 snd_pcm_uframes_t silence_size; /* Silence filling size */
1577 snd_pcm_uframes_t boundary; /* pointers wrap point */
1579 snd_pcm_uframes_t silenced_start;
1580 snd_pcm_uframes_t silenced_size;
1582 snd_pcm_sync_id_t sync; /* hardware synchronization ID */
1585 volatile struct snd_pcm_mmap_status *status;
1586 volatile struct snd_pcm_mmap_control *control;
1587 atomic_t mmap_count;
1589 /* -- locking / scheduling -- */
1591 wait_queue_head_t sleep;
1592 struct timer_list tick_timer;
1593 struct fasync_struct *fasync;
1595 /* -- private section -- */
1597 void (*private_free)(struct snd_pcm_runtime *runtime);
1599 /* -- hardware description -- */
1600 struct snd_pcm_hardware hw;
1601 struct snd_pcm_hw_constraints hw_constraints;
1604 unsigned int timer_resolution; /* timer resolution */
1607 unsigned char *dma_area; /* DMA area */
1608 dma_addr_t dma_addr; /* physical bus address (not accessible from main CPU) */
1609 size_t dma_bytes; /* size of DMA area */
1611 struct snd_dma_buffer *dma_buffer_p; /* allocated buffer */
1613 #if defined(CONFIG_SND_PCM_OSS) || defined(CONFIG_SND_PCM_OSS_MODULE)
1614 /* -- OSS things -- */
1615 struct snd_pcm_oss_runtime oss;
1620 For the operators (callbacks) of each sound driver, most of these
1621 records are supposed to be read-only. Only the PCM middle-layer changes
1622 / updates them. The exceptions are the hardware description (hw) DMA
1623 buffer information and the private data. Besides, if you use the
1624 standard buffer allocation method via
1625 :c:func:`snd_pcm_lib_malloc_pages()`, you don't need to set the
1626 DMA buffer information by yourself.
1628 In the sections below, important records are explained.
1630 Hardware Description
1631 ~~~~~~~~~~~~~~~~~~~~
1633 The hardware descriptor (:c:type:`struct snd_pcm_hardware
1634 <snd_pcm_hardware>`) contains the definitions of the fundamental
1635 hardware configuration. Above all, you'll need to define this in the
1636 `PCM open callback`_. Note that the runtime instance holds the copy of
1637 the descriptor, not the pointer to the existing descriptor. That is,
1638 in the open callback, you can modify the copied descriptor
1639 (``runtime->hw``) as you need. For example, if the maximum number of
1640 channels is 1 only on some chip models, you can still use the same
1641 hardware descriptor and change the channels_max later:
1645 struct snd_pcm_runtime *runtime = substream->runtime;
1647 runtime->hw = snd_mychip_playback_hw; /* common definition */
1648 if (chip->model == VERY_OLD_ONE)
1649 runtime->hw.channels_max = 1;
1651 Typically, you'll have a hardware descriptor as below:
1655 static struct snd_pcm_hardware snd_mychip_playback_hw = {
1656 .info = (SNDRV_PCM_INFO_MMAP |
1657 SNDRV_PCM_INFO_INTERLEAVED |
1658 SNDRV_PCM_INFO_BLOCK_TRANSFER |
1659 SNDRV_PCM_INFO_MMAP_VALID),
1660 .formats = SNDRV_PCM_FMTBIT_S16_LE,
1661 .rates = SNDRV_PCM_RATE_8000_48000,
1666 .buffer_bytes_max = 32768,
1667 .period_bytes_min = 4096,
1668 .period_bytes_max = 32768,
1670 .periods_max = 1024,
1673 - The ``info`` field contains the type and capabilities of this
1674 pcm. The bit flags are defined in ``<sound/asound.h>`` as
1675 ``SNDRV_PCM_INFO_XXX``. Here, at least, you have to specify whether
1676 the mmap is supported and which interleaved format is
1677 supported. When the hardware supports mmap, add the
1678 ``SNDRV_PCM_INFO_MMAP`` flag here. When the hardware supports the
1679 interleaved or the non-interleaved formats,
1680 ``SNDRV_PCM_INFO_INTERLEAVED`` or ``SNDRV_PCM_INFO_NONINTERLEAVED``
1681 flag must be set, respectively. If both are supported, you can set
1684 In the above example, ``MMAP_VALID`` and ``BLOCK_TRANSFER`` are
1685 specified for the OSS mmap mode. Usually both are set. Of course,
1686 ``MMAP_VALID`` is set only if the mmap is really supported.
1688 The other possible flags are ``SNDRV_PCM_INFO_PAUSE`` and
1689 ``SNDRV_PCM_INFO_RESUME``. The ``PAUSE`` bit means that the pcm
1690 supports the “pause” operation, while the ``RESUME`` bit means that
1691 the pcm supports the full “suspend/resume” operation. If the
1692 ``PAUSE`` flag is set, the ``trigger`` callback below must handle
1693 the corresponding (pause push/release) commands. The suspend/resume
1694 trigger commands can be defined even without the ``RESUME``
1695 flag. See `Power Management`_ section for details.
1697 When the PCM substreams can be synchronized (typically,
1698 synchronized start/stop of a playback and a capture streams), you
1699 can give ``SNDRV_PCM_INFO_SYNC_START``, too. In this case, you'll
1700 need to check the linked-list of PCM substreams in the trigger
1701 callback. This will be described in the later section.
1703 - ``formats`` field contains the bit-flags of supported formats
1704 (``SNDRV_PCM_FMTBIT_XXX``). If the hardware supports more than one
1705 format, give all or'ed bits. In the example above, the signed 16bit
1706 little-endian format is specified.
1708 - ``rates`` field contains the bit-flags of supported rates
1709 (``SNDRV_PCM_RATE_XXX``). When the chip supports continuous rates,
1710 pass ``CONTINUOUS`` bit additionally. The pre-defined rate bits are
1711 provided only for typical rates. If your chip supports
1712 unconventional rates, you need to add the ``KNOT`` bit and set up
1713 the hardware constraint manually (explained later).
1715 - ``rate_min`` and ``rate_max`` define the minimum and maximum sample
1716 rate. This should correspond somehow to ``rates`` bits.
1718 - ``channel_min`` and ``channel_max`` define, as you might already
1719 expected, the minimum and maximum number of channels.
1721 - ``buffer_bytes_max`` defines the maximum buffer size in
1722 bytes. There is no ``buffer_bytes_min`` field, since it can be
1723 calculated from the minimum period size and the minimum number of
1724 periods. Meanwhile, ``period_bytes_min`` and define the minimum and
1725 maximum size of the period in bytes. ``periods_max`` and
1726 ``periods_min`` define the maximum and minimum number of periods in
1729 The “period” is a term that corresponds to a fragment in the OSS
1730 world. The period defines the size at which a PCM interrupt is
1731 generated. This size strongly depends on the hardware. Generally,
1732 the smaller period size will give you more interrupts, that is,
1733 more controls. In the case of capture, this size defines the input
1734 latency. On the other hand, the whole buffer size defines the
1735 output latency for the playback direction.
1737 - There is also a field ``fifo_size``. This specifies the size of the
1738 hardware FIFO, but currently it is neither used in the driver nor
1739 in the alsa-lib. So, you can ignore this field.
1744 Ok, let's go back again to the PCM runtime records. The most
1745 frequently referred records in the runtime instance are the PCM
1746 configurations. The PCM configurations are stored in the runtime
1747 instance after the application sends ``hw_params`` data via
1748 alsa-lib. There are many fields copied from hw_params and sw_params
1749 structs. For example, ``format`` holds the format type chosen by the
1750 application. This field contains the enum value
1751 ``SNDRV_PCM_FORMAT_XXX``.
1753 One thing to be noted is that the configured buffer and period sizes
1754 are stored in “frames” in the runtime. In the ALSA world, ``1 frame =
1755 channels \* samples-size``. For conversion between frames and bytes,
1756 you can use the :c:func:`frames_to_bytes()` and
1757 :c:func:`bytes_to_frames()` helper functions.
1761 period_bytes = frames_to_bytes(runtime, runtime->period_size);
1763 Also, many software parameters (sw_params) are stored in frames, too.
1764 Please check the type of the field. ``snd_pcm_uframes_t`` is for the
1765 frames as unsigned integer while ``snd_pcm_sframes_t`` is for the
1766 frames as signed integer.
1768 DMA Buffer Information
1769 ~~~~~~~~~~~~~~~~~~~~~~
1771 The DMA buffer is defined by the following four fields, ``dma_area``,
1772 ``dma_addr``, ``dma_bytes`` and ``dma_private``. The ``dma_area``
1773 holds the buffer pointer (the logical address). You can call
1774 :c:func:`memcpy()` from/to this pointer. Meanwhile, ``dma_addr`` holds
1775 the physical address of the buffer. This field is specified only when
1776 the buffer is a linear buffer. ``dma_bytes`` holds the size of buffer
1777 in bytes. ``dma_private`` is used for the ALSA DMA allocator.
1779 If you use a standard ALSA function,
1780 :c:func:`snd_pcm_lib_malloc_pages()`, for allocating the buffer,
1781 these fields are set by the ALSA middle layer, and you should *not*
1782 change them by yourself. You can read them but not write them. On the
1783 other hand, if you want to allocate the buffer by yourself, you'll
1784 need to manage it in hw_params callback. At least, ``dma_bytes`` is
1785 mandatory. ``dma_area`` is necessary when the buffer is mmapped. If
1786 your driver doesn't support mmap, this field is not
1787 necessary. ``dma_addr`` is also optional. You can use dma_private as
1793 The running status can be referred via ``runtime->status``. This is
1794 the pointer to the :c:type:`struct snd_pcm_mmap_status
1795 <snd_pcm_mmap_status>` record. For example, you can get the current
1796 DMA hardware pointer via ``runtime->status->hw_ptr``.
1798 The DMA application pointer can be referred via ``runtime->control``,
1799 which points to the :c:type:`struct snd_pcm_mmap_control
1800 <snd_pcm_mmap_control>` record. However, accessing directly to
1801 this value is not recommended.
1806 You can allocate a record for the substream and store it in
1807 ``runtime->private_data``. Usually, this is done in the `PCM open
1808 callback`_. Don't mix this with ``pcm->private_data``. The
1809 ``pcm->private_data`` usually points to the chip instance assigned
1810 statically at the creation of PCM, while the ``runtime->private_data``
1811 points to a dynamic data structure created at the PCM open
1816 static int snd_xxx_open(struct snd_pcm_substream *substream)
1818 struct my_pcm_data *data;
1820 data = kmalloc(sizeof(*data), GFP_KERNEL);
1821 substream->runtime->private_data = data;
1826 The allocated object must be released in the `close callback`_.
1831 OK, now let me give details about each pcm callback (``ops``). In
1832 general, every callback must return 0 if successful, or a negative
1833 error number such as ``-EINVAL``. To choose an appropriate error
1834 number, it is advised to check what value other parts of the kernel
1835 return when the same kind of request fails.
1837 The callback function takes at least the argument with :c:type:`struct
1838 snd_pcm_substream <snd_pcm_substream>` pointer. To retrieve the chip
1839 record from the given substream instance, you can use the following
1845 struct mychip *chip = snd_pcm_substream_chip(substream);
1849 The macro reads ``substream->private_data``, which is a copy of
1850 ``pcm->private_data``. You can override the former if you need to
1851 assign different data records per PCM substream. For example, the
1852 cmi8330 driver assigns different ``private_data`` for playback and
1853 capture directions, because it uses two different codecs (SB- and
1854 AD-compatible) for different directions.
1861 static int snd_xxx_open(struct snd_pcm_substream *substream);
1863 This is called when a pcm substream is opened.
1865 At least, here you have to initialize the ``runtime->hw``
1866 record. Typically, this is done by like this:
1870 static int snd_xxx_open(struct snd_pcm_substream *substream)
1872 struct mychip *chip = snd_pcm_substream_chip(substream);
1873 struct snd_pcm_runtime *runtime = substream->runtime;
1875 runtime->hw = snd_mychip_playback_hw;
1879 where ``snd_mychip_playback_hw`` is the pre-defined hardware
1882 You can allocate a private data in this callback, as described in
1883 `Private Data`_ section.
1885 If the hardware configuration needs more constraints, set the hardware
1886 constraints here, too. See Constraints_ for more details.
1893 static int snd_xxx_close(struct snd_pcm_substream *substream);
1896 Obviously, this is called when a pcm substream is closed.
1898 Any private instance for a pcm substream allocated in the ``open``
1899 callback will be released here.
1903 static int snd_xxx_close(struct snd_pcm_substream *substream)
1906 kfree(substream->runtime->private_data);
1913 This is used for any special call to pcm ioctls. But usually you can
1914 pass a generic ioctl callback, :c:func:`snd_pcm_lib_ioctl()`.
1921 static int snd_xxx_hw_params(struct snd_pcm_substream *substream,
1922 struct snd_pcm_hw_params *hw_params);
1924 This is called when the hardware parameter (``hw_params``) is set up
1925 by the application, that is, once when the buffer size, the period
1926 size, the format, etc. are defined for the pcm substream.
1928 Many hardware setups should be done in this callback, including the
1929 allocation of buffers.
1931 Parameters to be initialized are retrieved by
1932 :c:func:`params_xxx()` macros. To allocate buffer, you can call a
1937 snd_pcm_lib_malloc_pages(substream, params_buffer_bytes(hw_params));
1939 :c:func:`snd_pcm_lib_malloc_pages()` is available only when the
1940 DMA buffers have been pre-allocated. See the section `Buffer Types`_
1943 Note that this and ``prepare`` callbacks may be called multiple times
1944 per initialization. For example, the OSS emulation may call these
1945 callbacks at each change via its ioctl.
1947 Thus, you need to be careful not to allocate the same buffers many
1948 times, which will lead to memory leaks! Calling the helper function
1949 above many times is OK. It will release the previous buffer
1950 automatically when it was already allocated.
1952 Another note is that this callback is non-atomic (schedulable) as
1953 default, i.e. when no ``nonatomic`` flag set. This is important,
1954 because the ``trigger`` callback is atomic (non-schedulable). That is,
1955 mutexes or any schedule-related functions are not available in
1956 ``trigger`` callback. Please see the subsection Atomicity_ for
1964 static int snd_xxx_hw_free(struct snd_pcm_substream *substream);
1966 This is called to release the resources allocated via
1967 ``hw_params``. For example, releasing the buffer via
1968 :c:func:`snd_pcm_lib_malloc_pages()` is done by calling the
1973 snd_pcm_lib_free_pages(substream);
1975 This function is always called before the close callback is called.
1976 Also, the callback may be called multiple times, too. Keep track
1977 whether the resource was already released.
1984 static int snd_xxx_prepare(struct snd_pcm_substream *substream);
1986 This callback is called when the pcm is “prepared”. You can set the
1987 format type, sample rate, etc. here. The difference from ``hw_params``
1988 is that the ``prepare`` callback will be called each time
1989 :c:func:`snd_pcm_prepare()` is called, i.e. when recovering after
1992 Note that this callback is now non-atomic. You can use
1993 schedule-related functions safely in this callback.
1995 In this and the following callbacks, you can refer to the values via
1996 the runtime record, ``substream->runtime``. For example, to get the
1997 current rate, format or channels, access to ``runtime->rate``,
1998 ``runtime->format`` or ``runtime->channels``, respectively. The
1999 physical address of the allocated buffer is set to
2000 ``runtime->dma_area``. The buffer and period sizes are in
2001 ``runtime->buffer_size`` and ``runtime->period_size``, respectively.
2003 Be careful that this callback will be called many times at each setup,
2011 static int snd_xxx_trigger(struct snd_pcm_substream *substream, int cmd);
2013 This is called when the pcm is started, stopped or paused.
2015 Which action is specified in the second argument,
2016 ``SNDRV_PCM_TRIGGER_XXX`` in ``<sound/pcm.h>``. At least, the ``START``
2017 and ``STOP`` commands must be defined in this callback.
2022 case SNDRV_PCM_TRIGGER_START:
2023 /* do something to start the PCM engine */
2025 case SNDRV_PCM_TRIGGER_STOP:
2026 /* do something to stop the PCM engine */
2032 When the pcm supports the pause operation (given in the info field of
2033 the hardware table), the ``PAUSE_PUSH`` and ``PAUSE_RELEASE`` commands
2034 must be handled here, too. The former is the command to pause the pcm,
2035 and the latter to restart the pcm again.
2037 When the pcm supports the suspend/resume operation, regardless of full
2038 or partial suspend/resume support, the ``SUSPEND`` and ``RESUME``
2039 commands must be handled, too. These commands are issued when the
2040 power-management status is changed. Obviously, the ``SUSPEND`` and
2041 ``RESUME`` commands suspend and resume the pcm substream, and usually,
2042 they are identical to the ``STOP`` and ``START`` commands, respectively.
2043 See the `Power Management`_ section for details.
2045 As mentioned, this callback is atomic as default unless ``nonatomic``
2046 flag set, and you cannot call functions which may sleep. The
2047 ``trigger`` callback should be as minimal as possible, just really
2048 triggering the DMA. The other stuff should be initialized
2049 ``hw_params`` and ``prepare`` callbacks properly beforehand.
2056 static snd_pcm_uframes_t snd_xxx_pointer(struct snd_pcm_substream *substream)
2058 This callback is called when the PCM middle layer inquires the current
2059 hardware position on the buffer. The position must be returned in
2060 frames, ranging from 0 to ``buffer_size - 1``.
2062 This is called usually from the buffer-update routine in the pcm
2063 middle layer, which is invoked when :c:func:`snd_pcm_period_elapsed()`
2064 is called in the interrupt routine. Then the pcm middle layer updates
2065 the position and calculates the available space, and wakes up the
2066 sleeping poll threads, etc.
2068 This callback is also atomic as default.
2070 copy_user, copy_kernel and fill_silence ops
2071 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2073 These callbacks are not mandatory, and can be omitted in most cases.
2074 These callbacks are used when the hardware buffer cannot be in the
2075 normal memory space. Some chips have their own buffer on the hardware
2076 which is not mappable. In such a case, you have to transfer the data
2077 manually from the memory buffer to the hardware buffer. Or, if the
2078 buffer is non-contiguous on both physical and virtual memory spaces,
2079 these callbacks must be defined, too.
2081 If these two callbacks are defined, copy and set-silence operations
2082 are done by them. The detailed will be described in the later section
2083 `Buffer and Memory Management`_.
2088 This callback is also not mandatory. This callback is called when the
2089 ``appl_ptr`` is updated in read or write operations. Some drivers like
2090 emu10k1-fx and cs46xx need to track the current ``appl_ptr`` for the
2091 internal buffer, and this callback is useful only for such a purpose.
2093 This callback is atomic as default.
2098 This callback is optional too. This callback is used mainly for
2099 non-contiguous buffers. The mmap calls this callback to get the page
2100 address. Some examples will be explained in the later section `Buffer
2101 and Memory Management`_, too.
2106 This is another optional callback for controlling mmap behavior.
2107 Once when defined, PCM core calls this callback when a page is
2108 memory-mapped instead of dealing via the standard helper.
2109 If you need special handling (due to some architecture or
2110 device-specific issues), implement everything here as you like.
2113 PCM Interrupt Handler
2114 ---------------------
2116 The rest of pcm stuff is the PCM interrupt handler. The role of PCM
2117 interrupt handler in the sound driver is to update the buffer position
2118 and to tell the PCM middle layer when the buffer position goes across
2119 the prescribed period size. To inform this, call the
2120 :c:func:`snd_pcm_period_elapsed()` function.
2122 There are several types of sound chips to generate the interrupts.
2124 Interrupts at the period (fragment) boundary
2125 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2127 This is the most frequently found type: the hardware generates an
2128 interrupt at each period boundary. In this case, you can call
2129 :c:func:`snd_pcm_period_elapsed()` at each interrupt.
2131 :c:func:`snd_pcm_period_elapsed()` takes the substream pointer as
2132 its argument. Thus, you need to keep the substream pointer accessible
2133 from the chip instance. For example, define ``substream`` field in the
2134 chip record to hold the current running substream pointer, and set the
2135 pointer value at ``open`` callback (and reset at ``close`` callback).
2137 If you acquire a spinlock in the interrupt handler, and the lock is used
2138 in other pcm callbacks, too, then you have to release the lock before
2139 calling :c:func:`snd_pcm_period_elapsed()`, because
2140 :c:func:`snd_pcm_period_elapsed()` calls other pcm callbacks
2143 Typical code would be like:
2148 static irqreturn_t snd_mychip_interrupt(int irq, void *dev_id)
2150 struct mychip *chip = dev_id;
2151 spin_lock(&chip->lock);
2153 if (pcm_irq_invoked(chip)) {
2154 /* call updater, unlock before it */
2155 spin_unlock(&chip->lock);
2156 snd_pcm_period_elapsed(chip->substream);
2157 spin_lock(&chip->lock);
2158 /* acknowledge the interrupt if necessary */
2161 spin_unlock(&chip->lock);
2167 High frequency timer interrupts
2168 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2170 This happens when the hardware doesn't generate interrupts at the period
2171 boundary but issues timer interrupts at a fixed timer rate (e.g. es1968
2172 or ymfpci drivers). In this case, you need to check the current hardware
2173 position and accumulate the processed sample length at each interrupt.
2174 When the accumulated size exceeds the period size, call
2175 :c:func:`snd_pcm_period_elapsed()` and reset the accumulator.
2177 Typical code would be like the following.
2182 static irqreturn_t snd_mychip_interrupt(int irq, void *dev_id)
2184 struct mychip *chip = dev_id;
2185 spin_lock(&chip->lock);
2187 if (pcm_irq_invoked(chip)) {
2188 unsigned int last_ptr, size;
2189 /* get the current hardware pointer (in frames) */
2190 last_ptr = get_hw_ptr(chip);
2191 /* calculate the processed frames since the
2194 if (last_ptr < chip->last_ptr)
2195 size = runtime->buffer_size + last_ptr
2198 size = last_ptr - chip->last_ptr;
2199 /* remember the last updated point */
2200 chip->last_ptr = last_ptr;
2201 /* accumulate the size */
2203 /* over the period boundary? */
2204 if (chip->size >= runtime->period_size) {
2205 /* reset the accumulator */
2206 chip->size %= runtime->period_size;
2208 spin_unlock(&chip->lock);
2209 snd_pcm_period_elapsed(substream);
2210 spin_lock(&chip->lock);
2212 /* acknowledge the interrupt if necessary */
2215 spin_unlock(&chip->lock);
2221 On calling :c:func:`snd_pcm_period_elapsed()`
2222 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2224 In both cases, even if more than one period are elapsed, you don't have
2225 to call :c:func:`snd_pcm_period_elapsed()` many times. Call only
2226 once. And the pcm layer will check the current hardware pointer and
2227 update to the latest status.
2232 One of the most important (and thus difficult to debug) problems in
2233 kernel programming are race conditions. In the Linux kernel, they are
2234 usually avoided via spin-locks, mutexes or semaphores. In general, if a
2235 race condition can happen in an interrupt handler, it has to be managed
2236 atomically, and you have to use a spinlock to protect the critical
2237 session. If the critical section is not in interrupt handler code and if
2238 taking a relatively long time to execute is acceptable, you should use
2239 mutexes or semaphores instead.
2241 As already seen, some pcm callbacks are atomic and some are not. For
2242 example, the ``hw_params`` callback is non-atomic, while ``trigger``
2243 callback is atomic. This means, the latter is called already in a
2244 spinlock held by the PCM middle layer. Please take this atomicity into
2245 account when you choose a locking scheme in the callbacks.
2247 In the atomic callbacks, you cannot use functions which may call
2248 :c:func:`schedule()` or go to :c:func:`sleep()`. Semaphores and
2249 mutexes can sleep, and hence they cannot be used inside the atomic
2250 callbacks (e.g. ``trigger`` callback). To implement some delay in such a
2251 callback, please use :c:func:`udelay()` or :c:func:`mdelay()`.
2253 All three atomic callbacks (trigger, pointer, and ack) are called with
2254 local interrupts disabled.
2256 The recent changes in PCM core code, however, allow all PCM operations
2257 to be non-atomic. This assumes that the all caller sides are in
2258 non-atomic contexts. For example, the function
2259 :c:func:`snd_pcm_period_elapsed()` is called typically from the
2260 interrupt handler. But, if you set up the driver to use a threaded
2261 interrupt handler, this call can be in non-atomic context, too. In such
2262 a case, you can set ``nonatomic`` filed of :c:type:`struct snd_pcm
2263 <snd_pcm>` object after creating it. When this flag is set, mutex
2264 and rwsem are used internally in the PCM core instead of spin and
2265 rwlocks, so that you can call all PCM functions safely in a non-atomic
2271 If your chip supports unconventional sample rates, or only the limited
2272 samples, you need to set a constraint for the condition.
2274 For example, in order to restrict the sample rates in the some supported
2275 values, use :c:func:`snd_pcm_hw_constraint_list()`. You need to
2276 call this function in the open callback.
2280 static unsigned int rates[] =
2281 {4000, 10000, 22050, 44100};
2282 static struct snd_pcm_hw_constraint_list constraints_rates = {
2283 .count = ARRAY_SIZE(rates),
2288 static int snd_mychip_pcm_open(struct snd_pcm_substream *substream)
2292 err = snd_pcm_hw_constraint_list(substream->runtime, 0,
2293 SNDRV_PCM_HW_PARAM_RATE,
2294 &constraints_rates);
2302 There are many different constraints. Look at ``sound/pcm.h`` for a
2303 complete list. You can even define your own constraint rules. For
2304 example, let's suppose my_chip can manage a substream of 1 channel if
2305 and only if the format is ``S16_LE``, otherwise it supports any format
2306 specified in the :c:type:`struct snd_pcm_hardware
2307 <snd_pcm_hardware>` structure (or in any other
2308 constraint_list). You can build a rule like this:
2312 static int hw_rule_channels_by_format(struct snd_pcm_hw_params *params,
2313 struct snd_pcm_hw_rule *rule)
2315 struct snd_interval *c = hw_param_interval(params,
2316 SNDRV_PCM_HW_PARAM_CHANNELS);
2317 struct snd_mask *f = hw_param_mask(params, SNDRV_PCM_HW_PARAM_FORMAT);
2318 struct snd_interval ch;
2320 snd_interval_any(&ch);
2321 if (f->bits[0] == SNDRV_PCM_FMTBIT_S16_LE) {
2322 ch.min = ch.max = 1;
2324 return snd_interval_refine(c, &ch);
2330 Then you need to call this function to add your rule:
2334 snd_pcm_hw_rule_add(substream->runtime, 0, SNDRV_PCM_HW_PARAM_CHANNELS,
2335 hw_rule_channels_by_format, NULL,
2336 SNDRV_PCM_HW_PARAM_FORMAT, -1);
2338 The rule function is called when an application sets the PCM format, and
2339 it refines the number of channels accordingly. But an application may
2340 set the number of channels before setting the format. Thus you also need
2341 to define the inverse rule:
2345 static int hw_rule_format_by_channels(struct snd_pcm_hw_params *params,
2346 struct snd_pcm_hw_rule *rule)
2348 struct snd_interval *c = hw_param_interval(params,
2349 SNDRV_PCM_HW_PARAM_CHANNELS);
2350 struct snd_mask *f = hw_param_mask(params, SNDRV_PCM_HW_PARAM_FORMAT);
2351 struct snd_mask fmt;
2353 snd_mask_any(&fmt); /* Init the struct */
2355 fmt.bits[0] &= SNDRV_PCM_FMTBIT_S16_LE;
2356 return snd_mask_refine(f, &fmt);
2362 ... and in the open callback:
2366 snd_pcm_hw_rule_add(substream->runtime, 0, SNDRV_PCM_HW_PARAM_FORMAT,
2367 hw_rule_format_by_channels, NULL,
2368 SNDRV_PCM_HW_PARAM_CHANNELS, -1);
2370 One typical usage of the hw constraints is to align the buffer size
2371 with the period size. As default, ALSA PCM core doesn't enforce the
2372 buffer size to be aligned with the period size. For example, it'd be
2373 possible to have a combination like 256 period bytes with 999 buffer
2376 Many device chips, however, require the buffer to be a multiple of
2377 periods. In such a case, call
2378 :c:func:`snd_pcm_hw_constraint_integer()` for
2379 ``SNDRV_PCM_HW_PARAM_PERIODS``.
2383 snd_pcm_hw_constraint_integer(substream->runtime,
2384 SNDRV_PCM_HW_PARAM_PERIODS);
2386 This assures that the number of periods is integer, hence the buffer
2387 size is aligned with the period size.
2389 The hw constraint is a very much powerful mechanism to define the
2390 preferred PCM configuration, and there are relevant helpers.
2391 I won't give more details here, rather I would like to say, “Luke, use
2400 The control interface is used widely for many switches, sliders, etc.
2401 which are accessed from user-space. Its most important use is the mixer
2402 interface. In other words, since ALSA 0.9.x, all the mixer stuff is
2403 implemented on the control kernel API.
2405 ALSA has a well-defined AC97 control module. If your chip supports only
2406 the AC97 and nothing else, you can skip this section.
2408 The control API is defined in ``<sound/control.h>``. Include this file
2409 if you want to add your own controls.
2411 Definition of Controls
2412 ----------------------
2414 To create a new control, you need to define the following three
2415 callbacks: ``info``, ``get`` and ``put``. Then, define a
2416 :c:type:`struct snd_kcontrol_new <snd_kcontrol_new>` record, such as:
2421 static struct snd_kcontrol_new my_control = {
2422 .iface = SNDRV_CTL_ELEM_IFACE_MIXER,
2423 .name = "PCM Playback Switch",
2425 .access = SNDRV_CTL_ELEM_ACCESS_READWRITE,
2426 .private_value = 0xffff,
2427 .info = my_control_info,
2428 .get = my_control_get,
2429 .put = my_control_put
2433 The ``iface`` field specifies the control type,
2434 ``SNDRV_CTL_ELEM_IFACE_XXX``, which is usually ``MIXER``. Use ``CARD``
2435 for global controls that are not logically part of the mixer. If the
2436 control is closely associated with some specific device on the sound
2437 card, use ``HWDEP``, ``PCM``, ``RAWMIDI``, ``TIMER``, or ``SEQUENCER``,
2438 and specify the device number with the ``device`` and ``subdevice``
2441 The ``name`` is the name identifier string. Since ALSA 0.9.x, the
2442 control name is very important, because its role is classified from
2443 its name. There are pre-defined standard control names. The details
2444 are described in the `Control Names`_ subsection.
2446 The ``index`` field holds the index number of this control. If there
2447 are several different controls with the same name, they can be
2448 distinguished by the index number. This is the case when several
2449 codecs exist on the card. If the index is zero, you can omit the
2452 The ``access`` field contains the access type of this control. Give
2453 the combination of bit masks, ``SNDRV_CTL_ELEM_ACCESS_XXX``,
2454 there. The details will be explained in the `Access Flags`_
2457 The ``private_value`` field contains an arbitrary long integer value
2458 for this record. When using the generic ``info``, ``get`` and ``put``
2459 callbacks, you can pass a value through this field. If several small
2460 numbers are necessary, you can combine them in bitwise. Or, it's
2461 possible to give a pointer (casted to unsigned long) of some record to
2464 The ``tlv`` field can be used to provide metadata about the control;
2465 see the `Metadata`_ subsection.
2467 The other three are `Control Callbacks`_.
2472 There are some standards to define the control names. A control is
2473 usually defined from the three parts as “SOURCE DIRECTION FUNCTION”.
2475 The first, ``SOURCE``, specifies the source of the control, and is a
2476 string such as “Master”, “PCM”, “CD” and “Line”. There are many
2477 pre-defined sources.
2479 The second, ``DIRECTION``, is one of the following strings according to
2480 the direction of the control: “Playback”, “Capture”, “Bypass Playback”
2481 and “Bypass Capture”. Or, it can be omitted, meaning both playback and
2484 The third, ``FUNCTION``, is one of the following strings according to
2485 the function of the control: “Switch”, “Volume” and “Route”.
2487 The example of control names are, thus, “Master Capture Switch” or “PCM
2490 There are some exceptions:
2492 Global capture and playback
2493 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
2495 “Capture Source”, “Capture Switch” and “Capture Volume” are used for the
2496 global capture (input) source, switch and volume. Similarly, “Playback
2497 Switch” and “Playback Volume” are used for the global output gain switch
2503 tone-control switch and volumes are specified like “Tone Control - XXX”,
2504 e.g. “Tone Control - Switch”, “Tone Control - Bass”, “Tone Control -
2510 3D-control switches and volumes are specified like “3D Control - XXX”,
2511 e.g. “3D Control - Switch”, “3D Control - Center”, “3D Control - Space”.
2516 Mic-boost switch is set as “Mic Boost” or “Mic Boost (6dB)”.
2518 More precise information can be found in
2519 ``Documentation/sound/designs/control-names.rst``.
2524 The access flag is the bitmask which specifies the access type of the
2525 given control. The default access type is
2526 ``SNDRV_CTL_ELEM_ACCESS_READWRITE``, which means both read and write are
2527 allowed to this control. When the access flag is omitted (i.e. = 0), it
2528 is considered as ``READWRITE`` access as default.
2530 When the control is read-only, pass ``SNDRV_CTL_ELEM_ACCESS_READ``
2531 instead. In this case, you don't have to define the ``put`` callback.
2532 Similarly, when the control is write-only (although it's a rare case),
2533 you can use the ``WRITE`` flag instead, and you don't need the ``get``
2536 If the control value changes frequently (e.g. the VU meter),
2537 ``VOLATILE`` flag should be given. This means that the control may be
2538 changed without `Change notification`_. Applications should poll such
2539 a control constantly.
2541 When the control is inactive, set the ``INACTIVE`` flag, too. There are
2542 ``LOCK`` and ``OWNER`` flags to change the write permissions.
2550 The ``info`` callback is used to get detailed information on this
2551 control. This must store the values of the given :c:type:`struct
2552 snd_ctl_elem_info <snd_ctl_elem_info>` object. For example,
2553 for a boolean control with a single element:
2558 static int snd_myctl_mono_info(struct snd_kcontrol *kcontrol,
2559 struct snd_ctl_elem_info *uinfo)
2561 uinfo->type = SNDRV_CTL_ELEM_TYPE_BOOLEAN;
2563 uinfo->value.integer.min = 0;
2564 uinfo->value.integer.max = 1;
2570 The ``type`` field specifies the type of the control. There are
2571 ``BOOLEAN``, ``INTEGER``, ``ENUMERATED``, ``BYTES``, ``IEC958`` and
2572 ``INTEGER64``. The ``count`` field specifies the number of elements in
2573 this control. For example, a stereo volume would have count = 2. The
2574 ``value`` field is a union, and the values stored are depending on the
2575 type. The boolean and integer types are identical.
2577 The enumerated type is a bit different from others. You'll need to set
2578 the string for the currently given item index.
2582 static int snd_myctl_enum_info(struct snd_kcontrol *kcontrol,
2583 struct snd_ctl_elem_info *uinfo)
2585 static char *texts[4] = {
2586 "First", "Second", "Third", "Fourth"
2588 uinfo->type = SNDRV_CTL_ELEM_TYPE_ENUMERATED;
2590 uinfo->value.enumerated.items = 4;
2591 if (uinfo->value.enumerated.item > 3)
2592 uinfo->value.enumerated.item = 3;
2593 strcpy(uinfo->value.enumerated.name,
2594 texts[uinfo->value.enumerated.item]);
2598 The above callback can be simplified with a helper function,
2599 :c:func:`snd_ctl_enum_info()`. The final code looks like below.
2600 (You can pass ``ARRAY_SIZE(texts)`` instead of 4 in the third argument;
2601 it's a matter of taste.)
2605 static int snd_myctl_enum_info(struct snd_kcontrol *kcontrol,
2606 struct snd_ctl_elem_info *uinfo)
2608 static char *texts[4] = {
2609 "First", "Second", "Third", "Fourth"
2611 return snd_ctl_enum_info(uinfo, 1, 4, texts);
2615 Some common info callbacks are available for your convenience:
2616 :c:func:`snd_ctl_boolean_mono_info()` and
2617 :c:func:`snd_ctl_boolean_stereo_info()`. Obviously, the former
2618 is an info callback for a mono channel boolean item, just like
2619 :c:func:`snd_myctl_mono_info()` above, and the latter is for a
2620 stereo channel boolean item.
2625 This callback is used to read the current value of the control and to
2626 return to user-space.
2633 static int snd_myctl_get(struct snd_kcontrol *kcontrol,
2634 struct snd_ctl_elem_value *ucontrol)
2636 struct mychip *chip = snd_kcontrol_chip(kcontrol);
2637 ucontrol->value.integer.value[0] = get_some_value(chip);
2643 The ``value`` field depends on the type of control as well as on the
2644 info callback. For example, the sb driver uses this field to store the
2645 register offset, the bit-shift and the bit-mask. The ``private_value``
2646 field is set as follows:
2650 .private_value = reg | (shift << 16) | (mask << 24)
2652 and is retrieved in callbacks like
2656 static int snd_sbmixer_get_single(struct snd_kcontrol *kcontrol,
2657 struct snd_ctl_elem_value *ucontrol)
2659 int reg = kcontrol->private_value & 0xff;
2660 int shift = (kcontrol->private_value >> 16) & 0xff;
2661 int mask = (kcontrol->private_value >> 24) & 0xff;
2665 In the ``get`` callback, you have to fill all the elements if the
2666 control has more than one elements, i.e. ``count > 1``. In the example
2667 above, we filled only one element (``value.integer.value[0]``) since
2668 it's assumed as ``count = 1``.
2673 This callback is used to write a value from user-space.
2680 static int snd_myctl_put(struct snd_kcontrol *kcontrol,
2681 struct snd_ctl_elem_value *ucontrol)
2683 struct mychip *chip = snd_kcontrol_chip(kcontrol);
2685 if (chip->current_value !=
2686 ucontrol->value.integer.value[0]) {
2687 change_current_value(chip,
2688 ucontrol->value.integer.value[0]);
2696 As seen above, you have to return 1 if the value is changed. If the
2697 value is not changed, return 0 instead. If any fatal error happens,
2698 return a negative error code as usual.
2700 As in the ``get`` callback, when the control has more than one
2701 elements, all elements must be evaluated in this callback, too.
2703 Callbacks are not atomic
2704 ~~~~~~~~~~~~~~~~~~~~~~~~
2706 All these three callbacks are basically not atomic.
2711 When everything is ready, finally we can create a new control. To create
2712 a control, there are two functions to be called,
2713 :c:func:`snd_ctl_new1()` and :c:func:`snd_ctl_add()`.
2715 In the simplest way, you can do like this:
2719 err = snd_ctl_add(card, snd_ctl_new1(&my_control, chip));
2723 where ``my_control`` is the :c:type:`struct snd_kcontrol_new
2724 <snd_kcontrol_new>` object defined above, and chip is the object
2725 pointer to be passed to kcontrol->private_data which can be referred
2728 :c:func:`snd_ctl_new1()` allocates a new :c:type:`struct
2729 snd_kcontrol <snd_kcontrol>` instance, and
2730 :c:func:`snd_ctl_add()` assigns the given control component to the
2736 If you need to change and update a control in the interrupt routine, you
2737 can call :c:func:`snd_ctl_notify()`. For example,
2741 snd_ctl_notify(card, SNDRV_CTL_EVENT_MASK_VALUE, id_pointer);
2743 This function takes the card pointer, the event-mask, and the control id
2744 pointer for the notification. The event-mask specifies the types of
2745 notification, for example, in the above example, the change of control
2746 values is notified. The id pointer is the pointer of :c:type:`struct
2747 snd_ctl_elem_id <snd_ctl_elem_id>` to be notified. You can
2748 find some examples in ``es1938.c`` or ``es1968.c`` for hardware volume
2754 To provide information about the dB values of a mixer control, use on of
2755 the ``DECLARE_TLV_xxx`` macros from ``<sound/tlv.h>`` to define a
2756 variable containing this information, set the ``tlv.p`` field to point to
2757 this variable, and include the ``SNDRV_CTL_ELEM_ACCESS_TLV_READ`` flag
2758 in the ``access`` field; like this:
2762 static DECLARE_TLV_DB_SCALE(db_scale_my_control, -4050, 150, 0);
2764 static struct snd_kcontrol_new my_control = {
2766 .access = SNDRV_CTL_ELEM_ACCESS_READWRITE |
2767 SNDRV_CTL_ELEM_ACCESS_TLV_READ,
2769 .tlv.p = db_scale_my_control,
2773 The :c:func:`DECLARE_TLV_DB_SCALE()` macro defines information
2774 about a mixer control where each step in the control's value changes the
2775 dB value by a constant dB amount. The first parameter is the name of the
2776 variable to be defined. The second parameter is the minimum value, in
2777 units of 0.01 dB. The third parameter is the step size, in units of 0.01
2778 dB. Set the fourth parameter to 1 if the minimum value actually mutes
2781 The :c:func:`DECLARE_TLV_DB_LINEAR()` macro defines information
2782 about a mixer control where the control's value affects the output
2783 linearly. The first parameter is the name of the variable to be defined.
2784 The second parameter is the minimum value, in units of 0.01 dB. The
2785 third parameter is the maximum value, in units of 0.01 dB. If the
2786 minimum value mutes the control, set the second parameter to
2787 ``TLV_DB_GAIN_MUTE``.
2795 The ALSA AC97 codec layer is a well-defined one, and you don't have to
2796 write much code to control it. Only low-level control routines are
2797 necessary. The AC97 codec API is defined in ``<sound/ac97_codec.h>``.
2806 struct snd_ac97 *ac97;
2810 static unsigned short snd_mychip_ac97_read(struct snd_ac97 *ac97,
2813 struct mychip *chip = ac97->private_data;
2815 /* read a register value here from the codec */
2816 return the_register_value;
2819 static void snd_mychip_ac97_write(struct snd_ac97 *ac97,
2820 unsigned short reg, unsigned short val)
2822 struct mychip *chip = ac97->private_data;
2824 /* write the given register value to the codec */
2827 static int snd_mychip_ac97(struct mychip *chip)
2829 struct snd_ac97_bus *bus;
2830 struct snd_ac97_template ac97;
2832 static struct snd_ac97_bus_ops ops = {
2833 .write = snd_mychip_ac97_write,
2834 .read = snd_mychip_ac97_read,
2837 err = snd_ac97_bus(chip->card, 0, &ops, NULL, &bus);
2840 memset(&ac97, 0, sizeof(ac97));
2841 ac97.private_data = chip;
2842 return snd_ac97_mixer(bus, &ac97, &chip->ac97);
2849 To create an ac97 instance, first call :c:func:`snd_ac97_bus()`
2850 with an ``ac97_bus_ops_t`` record with callback functions.
2854 struct snd_ac97_bus *bus;
2855 static struct snd_ac97_bus_ops ops = {
2856 .write = snd_mychip_ac97_write,
2857 .read = snd_mychip_ac97_read,
2860 snd_ac97_bus(card, 0, &ops, NULL, &pbus);
2862 The bus record is shared among all belonging ac97 instances.
2864 And then call :c:func:`snd_ac97_mixer()` with an :c:type:`struct
2865 snd_ac97_template <snd_ac97_template>` record together with
2866 the bus pointer created above.
2870 struct snd_ac97_template ac97;
2873 memset(&ac97, 0, sizeof(ac97));
2874 ac97.private_data = chip;
2875 snd_ac97_mixer(bus, &ac97, &chip->ac97);
2877 where chip->ac97 is a pointer to a newly created ``ac97_t``
2878 instance. In this case, the chip pointer is set as the private data,
2879 so that the read/write callback functions can refer to this chip
2880 instance. This instance is not necessarily stored in the chip
2881 record. If you need to change the register values from the driver, or
2882 need the suspend/resume of ac97 codecs, keep this pointer to pass to
2883 the corresponding functions.
2888 The standard callbacks are ``read`` and ``write``. Obviously they
2889 correspond to the functions for read and write accesses to the
2890 hardware low-level codes.
2892 The ``read`` callback returns the register value specified in the
2897 static unsigned short snd_mychip_ac97_read(struct snd_ac97 *ac97,
2900 struct mychip *chip = ac97->private_data;
2902 return the_register_value;
2905 Here, the chip can be cast from ``ac97->private_data``.
2907 Meanwhile, the ``write`` callback is used to set the register
2912 static void snd_mychip_ac97_write(struct snd_ac97 *ac97,
2913 unsigned short reg, unsigned short val)
2916 These callbacks are non-atomic like the control API callbacks.
2918 There are also other callbacks: ``reset``, ``wait`` and ``init``.
2920 The ``reset`` callback is used to reset the codec. If the chip
2921 requires a special kind of reset, you can define this callback.
2923 The ``wait`` callback is used to add some waiting time in the standard
2924 initialization of the codec. If the chip requires the extra waiting
2925 time, define this callback.
2927 The ``init`` callback is used for additional initialization of the
2930 Updating Registers in The Driver
2931 --------------------------------
2933 If you need to access to the codec from the driver, you can call the
2934 following functions: :c:func:`snd_ac97_write()`,
2935 :c:func:`snd_ac97_read()`, :c:func:`snd_ac97_update()` and
2936 :c:func:`snd_ac97_update_bits()`.
2938 Both :c:func:`snd_ac97_write()` and
2939 :c:func:`snd_ac97_update()` functions are used to set a value to
2940 the given register (``AC97_XXX``). The difference between them is that
2941 :c:func:`snd_ac97_update()` doesn't write a value if the given
2942 value has been already set, while :c:func:`snd_ac97_write()`
2943 always rewrites the value.
2947 snd_ac97_write(ac97, AC97_MASTER, 0x8080);
2948 snd_ac97_update(ac97, AC97_MASTER, 0x8080);
2950 :c:func:`snd_ac97_read()` is used to read the value of the given
2951 register. For example,
2955 value = snd_ac97_read(ac97, AC97_MASTER);
2957 :c:func:`snd_ac97_update_bits()` is used to update some bits in
2962 snd_ac97_update_bits(ac97, reg, mask, value);
2964 Also, there is a function to change the sample rate (of a given register
2965 such as ``AC97_PCM_FRONT_DAC_RATE``) when VRA or DRA is supported by the
2966 codec: :c:func:`snd_ac97_set_rate()`.
2970 snd_ac97_set_rate(ac97, AC97_PCM_FRONT_DAC_RATE, 44100);
2973 The following registers are available to set the rate:
2974 ``AC97_PCM_MIC_ADC_RATE``, ``AC97_PCM_FRONT_DAC_RATE``,
2975 ``AC97_PCM_LR_ADC_RATE``, ``AC97_SPDIF``. When ``AC97_SPDIF`` is
2976 specified, the register is not really changed but the corresponding
2977 IEC958 status bits will be updated.
2982 In some chips, the clock of the codec isn't 48000 but using a PCI clock
2983 (to save a quartz!). In this case, change the field ``bus->clock`` to
2984 the corresponding value. For example, intel8x0 and es1968 drivers have
2985 their own function to read from the clock.
2990 The ALSA AC97 interface will create a proc file such as
2991 ``/proc/asound/card0/codec97#0/ac97#0-0`` and ``ac97#0-0+regs``. You
2992 can refer to these files to see the current status and registers of
2998 When there are several codecs on the same card, you need to call
2999 :c:func:`snd_ac97_mixer()` multiple times with ``ac97.num=1`` or
3000 greater. The ``num`` field specifies the codec number.
3002 If you set up multiple codecs, you either need to write different
3003 callbacks for each codec or check ``ac97->num`` in the callback
3006 MIDI (MPU401-UART) Interface
3007 ============================
3012 Many soundcards have built-in MIDI (MPU401-UART) interfaces. When the
3013 soundcard supports the standard MPU401-UART interface, most likely you
3014 can use the ALSA MPU401-UART API. The MPU401-UART API is defined in
3015 ``<sound/mpu401.h>``.
3017 Some soundchips have a similar but slightly different implementation of
3018 mpu401 stuff. For example, emu10k1 has its own mpu401 routines.
3023 To create a rawmidi object, call :c:func:`snd_mpu401_uart_new()`.
3027 struct snd_rawmidi *rmidi;
3028 snd_mpu401_uart_new(card, 0, MPU401_HW_MPU401, port, info_flags,
3032 The first argument is the card pointer, and the second is the index of
3033 this component. You can create up to 8 rawmidi devices.
3035 The third argument is the type of the hardware, ``MPU401_HW_XXX``. If
3036 it's not a special one, you can use ``MPU401_HW_MPU401``.
3038 The 4th argument is the I/O port address. Many backward-compatible
3039 MPU401 have an I/O port such as 0x330. Or, it might be a part of its own
3040 PCI I/O region. It depends on the chip design.
3042 The 5th argument is a bitflag for additional information. When the I/O
3043 port address above is part of the PCI I/O region, the MPU401 I/O port
3044 might have been already allocated (reserved) by the driver itself. In
3045 such a case, pass a bit flag ``MPU401_INFO_INTEGRATED``, and the
3046 mpu401-uart layer will allocate the I/O ports by itself.
3048 When the controller supports only the input or output MIDI stream, pass
3049 the ``MPU401_INFO_INPUT`` or ``MPU401_INFO_OUTPUT`` bitflag,
3050 respectively. Then the rawmidi instance is created as a single stream.
3052 ``MPU401_INFO_MMIO`` bitflag is used to change the access method to MMIO
3053 (via readb and writeb) instead of iob and outb. In this case, you have
3054 to pass the iomapped address to :c:func:`snd_mpu401_uart_new()`.
3056 When ``MPU401_INFO_TX_IRQ`` is set, the output stream isn't checked in
3057 the default interrupt handler. The driver needs to call
3058 :c:func:`snd_mpu401_uart_interrupt_tx()` by itself to start
3059 processing the output stream in the irq handler.
3061 If the MPU-401 interface shares its interrupt with the other logical
3062 devices on the card, set ``MPU401_INFO_IRQ_HOOK`` (see
3063 `below <#MIDI-Interrupt-Handler>`__).
3065 Usually, the port address corresponds to the command port and port + 1
3066 corresponds to the data port. If not, you may change the ``cport``
3067 field of :c:type:`struct snd_mpu401 <snd_mpu401>` manually afterward.
3068 However, :c:type:`struct snd_mpu401 <snd_mpu401>` pointer is
3069 not returned explicitly by :c:func:`snd_mpu401_uart_new()`. You
3070 need to cast ``rmidi->private_data`` to :c:type:`struct snd_mpu401
3071 <snd_mpu401>` explicitly,
3075 struct snd_mpu401 *mpu;
3076 mpu = rmidi->private_data;
3078 and reset the ``cport`` as you like:
3082 mpu->cport = my_own_control_port;
3084 The 6th argument specifies the ISA irq number that will be allocated. If
3085 no interrupt is to be allocated (because your code is already allocating
3086 a shared interrupt, or because the device does not use interrupts), pass
3087 -1 instead. For a MPU-401 device without an interrupt, a polling timer
3088 will be used instead.
3090 MIDI Interrupt Handler
3091 ----------------------
3093 When the interrupt is allocated in
3094 :c:func:`snd_mpu401_uart_new()`, an exclusive ISA interrupt
3095 handler is automatically used, hence you don't have anything else to do
3096 than creating the mpu401 stuff. Otherwise, you have to set
3097 ``MPU401_INFO_IRQ_HOOK``, and call
3098 :c:func:`snd_mpu401_uart_interrupt()` explicitly from your own
3099 interrupt handler when it has determined that a UART interrupt has
3102 In this case, you need to pass the private_data of the returned rawmidi
3103 object from :c:func:`snd_mpu401_uart_new()` as the second
3104 argument of :c:func:`snd_mpu401_uart_interrupt()`.
3108 snd_mpu401_uart_interrupt(irq, rmidi->private_data, regs);
3117 The raw MIDI interface is used for hardware MIDI ports that can be
3118 accessed as a byte stream. It is not used for synthesizer chips that do
3119 not directly understand MIDI.
3121 ALSA handles file and buffer management. All you have to do is to write
3122 some code to move data between the buffer and the hardware.
3124 The rawmidi API is defined in ``<sound/rawmidi.h>``.
3129 To create a rawmidi device, call the :c:func:`snd_rawmidi_new()`
3134 struct snd_rawmidi *rmidi;
3135 err = snd_rawmidi_new(chip->card, "MyMIDI", 0, outs, ins, &rmidi);
3138 rmidi->private_data = chip;
3139 strcpy(rmidi->name, "My MIDI");
3140 rmidi->info_flags = SNDRV_RAWMIDI_INFO_OUTPUT |
3141 SNDRV_RAWMIDI_INFO_INPUT |
3142 SNDRV_RAWMIDI_INFO_DUPLEX;
3144 The first argument is the card pointer, the second argument is the ID
3147 The third argument is the index of this component. You can create up to
3150 The fourth and fifth arguments are the number of output and input
3151 substreams, respectively, of this device (a substream is the equivalent
3154 Set the ``info_flags`` field to specify the capabilities of the
3155 device. Set ``SNDRV_RAWMIDI_INFO_OUTPUT`` if there is at least one
3156 output port, ``SNDRV_RAWMIDI_INFO_INPUT`` if there is at least one
3157 input port, and ``SNDRV_RAWMIDI_INFO_DUPLEX`` if the device can handle
3158 output and input at the same time.
3160 After the rawmidi device is created, you need to set the operators
3161 (callbacks) for each substream. There are helper functions to set the
3162 operators for all the substreams of a device:
3166 snd_rawmidi_set_ops(rmidi, SNDRV_RAWMIDI_STREAM_OUTPUT, &snd_mymidi_output_ops);
3167 snd_rawmidi_set_ops(rmidi, SNDRV_RAWMIDI_STREAM_INPUT, &snd_mymidi_input_ops);
3169 The operators are usually defined like this:
3173 static struct snd_rawmidi_ops snd_mymidi_output_ops = {
3174 .open = snd_mymidi_output_open,
3175 .close = snd_mymidi_output_close,
3176 .trigger = snd_mymidi_output_trigger,
3179 These callbacks are explained in the `RawMIDI Callbacks`_ section.
3181 If there are more than one substream, you should give a unique name to
3186 struct snd_rawmidi_substream *substream;
3187 list_for_each_entry(substream,
3188 &rmidi->streams[SNDRV_RAWMIDI_STREAM_OUTPUT].substreams,
3190 sprintf(substream->name, "My MIDI Port %d", substream->number + 1);
3192 /* same for SNDRV_RAWMIDI_STREAM_INPUT */
3197 In all the callbacks, the private data that you've set for the rawmidi
3198 device can be accessed as ``substream->rmidi->private_data``.
3200 If there is more than one port, your callbacks can determine the port
3201 index from the struct snd_rawmidi_substream data passed to each
3206 struct snd_rawmidi_substream *substream;
3207 int index = substream->number;
3209 RawMIDI open callback
3210 ~~~~~~~~~~~~~~~~~~~~~
3214 static int snd_xxx_open(struct snd_rawmidi_substream *substream);
3217 This is called when a substream is opened. You can initialize the
3218 hardware here, but you shouldn't start transmitting/receiving data yet.
3220 RawMIDI close callback
3221 ~~~~~~~~~~~~~~~~~~~~~~
3225 static int snd_xxx_close(struct snd_rawmidi_substream *substream);
3229 The ``open`` and ``close`` callbacks of a rawmidi device are
3230 serialized with a mutex, and can sleep.
3232 Rawmidi trigger callback for output substreams
3233 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
3237 static void snd_xxx_output_trigger(struct snd_rawmidi_substream *substream, int up);
3240 This is called with a nonzero ``up`` parameter when there is some data
3241 in the substream buffer that must be transmitted.
3243 To read data from the buffer, call
3244 :c:func:`snd_rawmidi_transmit_peek()`. It will return the number
3245 of bytes that have been read; this will be less than the number of bytes
3246 requested when there are no more data in the buffer. After the data have
3247 been transmitted successfully, call
3248 :c:func:`snd_rawmidi_transmit_ack()` to remove the data from the
3254 while (snd_rawmidi_transmit_peek(substream, &data, 1) == 1) {
3255 if (snd_mychip_try_to_transmit(data))
3256 snd_rawmidi_transmit_ack(substream, 1);
3258 break; /* hardware FIFO full */
3261 If you know beforehand that the hardware will accept data, you can use
3262 the :c:func:`snd_rawmidi_transmit()` function which reads some
3263 data and removes them from the buffer at once:
3267 while (snd_mychip_transmit_possible()) {
3269 if (snd_rawmidi_transmit(substream, &data, 1) != 1)
3270 break; /* no more data */
3271 snd_mychip_transmit(data);
3274 If you know beforehand how many bytes you can accept, you can use a
3275 buffer size greater than one with the
3276 :c:func:`snd_rawmidi_transmit\*()` functions.
3278 The ``trigger`` callback must not sleep. If the hardware FIFO is full
3279 before the substream buffer has been emptied, you have to continue
3280 transmitting data later, either in an interrupt handler, or with a
3281 timer if the hardware doesn't have a MIDI transmit interrupt.
3283 The ``trigger`` callback is called with a zero ``up`` parameter when
3284 the transmission of data should be aborted.
3286 RawMIDI trigger callback for input substreams
3287 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
3291 static void snd_xxx_input_trigger(struct snd_rawmidi_substream *substream, int up);
3294 This is called with a nonzero ``up`` parameter to enable receiving data,
3295 or with a zero ``up`` parameter do disable receiving data.
3297 The ``trigger`` callback must not sleep; the actual reading of data
3298 from the device is usually done in an interrupt handler.
3300 When data reception is enabled, your interrupt handler should call
3301 :c:func:`snd_rawmidi_receive()` for all received data:
3305 void snd_mychip_midi_interrupt(...)
3307 while (mychip_midi_available()) {
3309 data = mychip_midi_read();
3310 snd_rawmidi_receive(substream, &data, 1);
3320 static void snd_xxx_drain(struct snd_rawmidi_substream *substream);
3323 This is only used with output substreams. This function should wait
3324 until all data read from the substream buffer have been transmitted.
3325 This ensures that the device can be closed and the driver unloaded
3326 without losing data.
3328 This callback is optional. If you do not set ``drain`` in the struct
3329 snd_rawmidi_ops structure, ALSA will simply wait for 50 milliseconds
3332 Miscellaneous Devices
3333 =====================
3338 The FM OPL3 is still used in many chips (mainly for backward
3339 compatibility). ALSA has a nice OPL3 FM control layer, too. The OPL3 API
3340 is defined in ``<sound/opl3.h>``.
3342 FM registers can be directly accessed through the direct-FM API, defined
3343 in ``<sound/asound_fm.h>``. In ALSA native mode, FM registers are
3344 accessed through the Hardware-Dependent Device direct-FM extension API,
3345 whereas in OSS compatible mode, FM registers can be accessed with the
3346 OSS direct-FM compatible API in ``/dev/dmfmX`` device.
3348 To create the OPL3 component, you have two functions to call. The first
3349 one is a constructor for the ``opl3_t`` instance.
3353 struct snd_opl3 *opl3;
3354 snd_opl3_create(card, lport, rport, OPL3_HW_OPL3_XXX,
3357 The first argument is the card pointer, the second one is the left port
3358 address, and the third is the right port address. In most cases, the
3359 right port is placed at the left port + 2.
3361 The fourth argument is the hardware type.
3363 When the left and right ports have been already allocated by the card
3364 driver, pass non-zero to the fifth argument (``integrated``). Otherwise,
3365 the opl3 module will allocate the specified ports by itself.
3367 When the accessing the hardware requires special method instead of the
3368 standard I/O access, you can create opl3 instance separately with
3369 :c:func:`snd_opl3_new()`.
3373 struct snd_opl3 *opl3;
3374 snd_opl3_new(card, OPL3_HW_OPL3_XXX, &opl3);
3376 Then set ``command``, ``private_data`` and ``private_free`` for the
3377 private access function, the private data and the destructor. The
3378 ``l_port`` and ``r_port`` are not necessarily set. Only the command
3379 must be set properly. You can retrieve the data from the
3380 ``opl3->private_data`` field.
3382 After creating the opl3 instance via :c:func:`snd_opl3_new()`,
3383 call :c:func:`snd_opl3_init()` to initialize the chip to the
3384 proper state. Note that :c:func:`snd_opl3_create()` always calls
3387 If the opl3 instance is created successfully, then create a hwdep device
3392 struct snd_hwdep *opl3hwdep;
3393 snd_opl3_hwdep_new(opl3, 0, 1, &opl3hwdep);
3395 The first argument is the ``opl3_t`` instance you created, and the
3396 second is the index number, usually 0.
3398 The third argument is the index-offset for the sequencer client assigned
3399 to the OPL3 port. When there is an MPU401-UART, give 1 for here (UART
3402 Hardware-Dependent Devices
3403 --------------------------
3405 Some chips need user-space access for special controls or for loading
3406 the micro code. In such a case, you can create a hwdep
3407 (hardware-dependent) device. The hwdep API is defined in
3408 ``<sound/hwdep.h>``. You can find examples in opl3 driver or
3409 ``isa/sb/sb16_csp.c``.
3411 The creation of the ``hwdep`` instance is done via
3412 :c:func:`snd_hwdep_new()`.
3416 struct snd_hwdep *hw;
3417 snd_hwdep_new(card, "My HWDEP", 0, &hw);
3419 where the third argument is the index number.
3421 You can then pass any pointer value to the ``private_data``. If you
3422 assign a private data, you should define the destructor, too. The
3423 destructor function is set in the ``private_free`` field.
3427 struct mydata *p = kmalloc(sizeof(*p), GFP_KERNEL);
3428 hw->private_data = p;
3429 hw->private_free = mydata_free;
3431 and the implementation of the destructor would be:
3435 static void mydata_free(struct snd_hwdep *hw)
3437 struct mydata *p = hw->private_data;
3441 The arbitrary file operations can be defined for this instance. The file
3442 operators are defined in the ``ops`` table. For example, assume that
3443 this chip needs an ioctl.
3447 hw->ops.open = mydata_open;
3448 hw->ops.ioctl = mydata_ioctl;
3449 hw->ops.release = mydata_release;
3451 And implement the callback functions as you like.
3456 Usually the controls for IEC958 devices are implemented via the control
3457 interface. There is a macro to compose a name string for IEC958
3458 controls, :c:func:`SNDRV_CTL_NAME_IEC958()` defined in
3459 ``<include/asound.h>``.
3461 There are some standard controls for IEC958 status bits. These controls
3462 use the type ``SNDRV_CTL_ELEM_TYPE_IEC958``, and the size of element is
3463 fixed as 4 bytes array (value.iec958.status[x]). For the ``info``
3464 callback, you don't specify the value field for this type (the count
3465 field must be set, though).
3467 “IEC958 Playback Con Mask” is used to return the bit-mask for the IEC958
3468 status bits of consumer mode. Similarly, “IEC958 Playback Pro Mask”
3469 returns the bitmask for professional mode. They are read-only controls,
3470 and are defined as MIXER controls (iface =
3471 ``SNDRV_CTL_ELEM_IFACE_MIXER``).
3473 Meanwhile, “IEC958 Playback Default” control is defined for getting and
3474 setting the current default IEC958 bits. Note that this one is usually
3475 defined as a PCM control (iface = ``SNDRV_CTL_ELEM_IFACE_PCM``),
3476 although in some places it's defined as a MIXER control.
3478 In addition, you can define the control switches to enable/disable or to
3479 set the raw bit mode. The implementation will depend on the chip, but
3480 the control should be named as “IEC958 xxx”, preferably using the
3481 :c:func:`SNDRV_CTL_NAME_IEC958()` macro.
3483 You can find several cases, for example, ``pci/emu10k1``,
3484 ``pci/ice1712``, or ``pci/cmipci.c``.
3486 Buffer and Memory Management
3487 ============================
3492 ALSA provides several different buffer allocation functions depending on
3493 the bus and the architecture. All these have a consistent API. The
3494 allocation of physically-contiguous pages is done via
3495 :c:func:`snd_malloc_xxx_pages()` function, where xxx is the bus
3498 The allocation of pages with fallback is
3499 :c:func:`snd_malloc_xxx_pages_fallback()`. This function tries
3500 to allocate the specified pages but if the pages are not available, it
3501 tries to reduce the page sizes until enough space is found.
3503 The release the pages, call :c:func:`snd_free_xxx_pages()`
3506 Usually, ALSA drivers try to allocate and reserve a large contiguous
3507 physical space at the time the module is loaded for the later use. This
3508 is called “pre-allocation”. As already written, you can call the
3509 following function at pcm instance construction time (in the case of PCI
3514 snd_pcm_lib_preallocate_pages_for_all(pcm, SNDRV_DMA_TYPE_DEV,
3515 snd_dma_pci_data(pci), size, max);
3517 where ``size`` is the byte size to be pre-allocated and the ``max`` is
3518 the maximum size to be changed via the ``prealloc`` proc file. The
3519 allocator will try to get an area as large as possible within the
3522 The second argument (type) and the third argument (device pointer) are
3523 dependent on the bus. For normal devices, pass the device pointer
3524 (typically identical as ``card->dev``) to the third argument with
3525 ``SNDRV_DMA_TYPE_DEV`` type. For the continuous buffer unrelated to the
3526 bus can be pre-allocated with ``SNDRV_DMA_TYPE_CONTINUOUS`` type and the
3527 ``snd_dma_continuous_data(GFP_KERNEL)`` device pointer, where
3528 ``GFP_KERNEL`` is the kernel allocation flag to use. For the
3529 scatter-gather buffers, use ``SNDRV_DMA_TYPE_DEV_SG`` with the device
3530 pointer (see the `Non-Contiguous Buffers`_
3533 Once the buffer is pre-allocated, you can use the allocator in the
3534 ``hw_params`` callback:
3538 snd_pcm_lib_malloc_pages(substream, size);
3540 Note that you have to pre-allocate to use this function.
3542 External Hardware Buffers
3543 -------------------------
3545 Some chips have their own hardware buffers and the DMA transfer from the
3546 host memory is not available. In such a case, you need to either 1)
3547 copy/set the audio data directly to the external hardware buffer, or 2)
3548 make an intermediate buffer and copy/set the data from it to the
3549 external hardware buffer in interrupts (or in tasklets, preferably).
3551 The first case works fine if the external hardware buffer is large
3552 enough. This method doesn't need any extra buffers and thus is more
3553 effective. You need to define the ``copy_user`` and ``copy_kernel``
3554 callbacks for the data transfer, in addition to ``fill_silence``
3555 callback for playback. However, there is a drawback: it cannot be
3556 mmapped. The examples are GUS's GF1 PCM or emu8000's wavetable PCM.
3558 The second case allows for mmap on the buffer, although you have to
3559 handle an interrupt or a tasklet to transfer the data from the
3560 intermediate buffer to the hardware buffer. You can find an example in
3561 the vxpocket driver.
3563 Another case is when the chip uses a PCI memory-map region for the
3564 buffer instead of the host memory. In this case, mmap is available only
3565 on certain architectures like the Intel one. In non-mmap mode, the data
3566 cannot be transferred as in the normal way. Thus you need to define the
3567 ``copy_user``, ``copy_kernel`` and ``fill_silence`` callbacks as well,
3568 as in the cases above. The examples are found in ``rme32.c`` and
3571 The implementation of the ``copy_user``, ``copy_kernel`` and
3572 ``silence`` callbacks depends upon whether the hardware supports
3573 interleaved or non-interleaved samples. The ``copy_user`` callback is
3574 defined like below, a bit differently depending whether the direction
3575 is playback or capture:
3579 static int playback_copy_user(struct snd_pcm_substream *substream,
3580 int channel, unsigned long pos,
3581 void __user *src, unsigned long count);
3582 static int capture_copy_user(struct snd_pcm_substream *substream,
3583 int channel, unsigned long pos,
3584 void __user *dst, unsigned long count);
3586 In the case of interleaved samples, the second argument (``channel``) is
3587 not used. The third argument (``pos``) points the current position
3590 The meaning of the fourth argument is different between playback and
3591 capture. For playback, it holds the source data pointer, and for
3592 capture, it's the destination data pointer.
3594 The last argument is the number of bytes to be copied.
3596 What you have to do in this callback is again different between playback
3597 and capture directions. In the playback case, you copy the given amount
3598 of data (``count``) at the specified pointer (``src``) to the specified
3599 offset (``pos``) on the hardware buffer. When coded like memcpy-like
3600 way, the copy would be like:
3604 my_memcpy_from_user(my_buffer + pos, src, count);
3606 For the capture direction, you copy the given amount of data (``count``)
3607 at the specified offset (``pos``) on the hardware buffer to the
3608 specified pointer (``dst``).
3612 my_memcpy_to_user(dst, my_buffer + pos, count);
3614 Here the functions are named as ``from_user`` and ``to_user`` because
3615 it's the user-space buffer that is passed to these callbacks. That
3616 is, the callback is supposed to copy from/to the user-space data
3617 directly to/from the hardware buffer.
3619 Careful readers might notice that these callbacks receive the
3620 arguments in bytes, not in frames like other callbacks. It's because
3621 it would make coding easier like the examples above, and also it makes
3622 easier to unify both the interleaved and non-interleaved cases, as
3623 explained in the following.
3625 In the case of non-interleaved samples, the implementation will be a bit
3626 more complicated. The callback is called for each channel, passed by
3627 the second argument, so totally it's called for N-channels times per
3630 The meaning of other arguments are almost same as the interleaved
3631 case. The callback is supposed to copy the data from/to the given
3632 user-space buffer, but only for the given channel. For the detailed
3633 implementations, please check ``isa/gus/gus_pcm.c`` or
3634 "pci/rme9652/rme9652.c" as examples.
3636 The above callbacks are the copy from/to the user-space buffer. There
3637 are some cases where we want copy from/to the kernel-space buffer
3638 instead. In such a case, ``copy_kernel`` callback is called. It'd
3643 static int playback_copy_kernel(struct snd_pcm_substream *substream,
3644 int channel, unsigned long pos,
3645 void *src, unsigned long count);
3646 static int capture_copy_kernel(struct snd_pcm_substream *substream,
3647 int channel, unsigned long pos,
3648 void *dst, unsigned long count);
3650 As found easily, the only difference is that the buffer pointer is
3651 without ``__user`` prefix; that is, a kernel-buffer pointer is passed
3652 in the fourth argument. Correspondingly, the implementation would be
3653 a version without the user-copy, such as:
3657 my_memcpy(my_buffer + pos, src, count);
3659 Usually for the playback, another callback ``fill_silence`` is
3660 defined. It's implemented in a similar way as the copy callbacks
3665 static int silence(struct snd_pcm_substream *substream, int channel,
3666 unsigned long pos, unsigned long count);
3668 The meanings of arguments are the same as in the ``copy_user`` and
3669 ``copy_kernel`` callbacks, although there is no buffer pointer
3670 argument. In the case of interleaved samples, the channel argument has
3671 no meaning, as well as on ``copy_*`` callbacks.
3673 The role of ``fill_silence`` callback is to set the given amount
3674 (``count``) of silence data at the specified offset (``pos``) on the
3675 hardware buffer. Suppose that the data format is signed (that is, the
3676 silent-data is 0), and the implementation using a memset-like function
3681 my_memset(my_buffer + pos, 0, count);
3683 In the case of non-interleaved samples, again, the implementation
3684 becomes a bit more complicated, as it's called N-times per transfer
3685 for each channel. See, for example, ``isa/gus/gus_pcm.c``.
3687 Non-Contiguous Buffers
3688 ----------------------
3690 If your hardware supports the page table as in emu10k1 or the buffer
3691 descriptors as in via82xx, you can use the scatter-gather (SG) DMA. ALSA
3692 provides an interface for handling SG-buffers. The API is provided in
3695 For creating the SG-buffer handler, call
3696 :c:func:`snd_pcm_lib_preallocate_pages()` or
3697 :c:func:`snd_pcm_lib_preallocate_pages_for_all()` with
3698 ``SNDRV_DMA_TYPE_DEV_SG`` in the PCM constructor like other PCI
3699 pre-allocator. You need to pass ``snd_dma_pci_data(pci)``, where pci is
3700 the :c:type:`struct pci_dev <pci_dev>` pointer of the chip as
3701 well. The ``struct snd_sg_buf`` instance is created as
3702 ``substream->dma_private``. You can cast the pointer like:
3706 struct snd_sg_buf *sgbuf = (struct snd_sg_buf *)substream->dma_private;
3708 Then call :c:func:`snd_pcm_lib_malloc_pages()` in the ``hw_params``
3709 callback as well as in the case of normal PCI buffer. The SG-buffer
3710 handler will allocate the non-contiguous kernel pages of the given size
3711 and map them onto the virtually contiguous memory. The virtual pointer
3712 is addressed in runtime->dma_area. The physical address
3713 (``runtime->dma_addr``) is set to zero, because the buffer is
3714 physically non-contiguous. The physical address table is set up in
3715 ``sgbuf->table``. You can get the physical address at a certain offset
3716 via :c:func:`snd_pcm_sgbuf_get_addr()`.
3718 When a SG-handler is used, you need to set
3719 :c:func:`snd_pcm_sgbuf_ops_page()` as the ``page`` callback. (See
3720 `page callback`_ section.)
3722 To release the data, call :c:func:`snd_pcm_lib_free_pages()` in
3723 the ``hw_free`` callback as usual.
3728 It's possible to use a buffer allocated via :c:func:`vmalloc()`, for
3729 example, for an intermediate buffer. Since the allocated pages are not
3730 contiguous, you need to set the ``page`` callback to obtain the physical
3731 address at every offset.
3733 The easiest way to achieve it would be to use
3734 :c:func:`snd_pcm_lib_alloc_vmalloc_buffer()` for allocating the buffer
3735 via :c:func:`vmalloc()`, and set :c:func:`snd_pcm_sgbuf_ops_page()` to
3736 the ``page`` callback. At release, you need to call
3737 :c:func:`snd_pcm_lib_free_vmalloc_buffer()`.
3739 If you want to implementation the ``page`` manually, it would be like
3744 #include <linux/vmalloc.h>
3746 /* get the physical page pointer on the given offset */
3747 static struct page *mychip_page(struct snd_pcm_substream *substream,
3748 unsigned long offset)
3750 void *pageptr = substream->runtime->dma_area + offset;
3751 return vmalloc_to_page(pageptr);
3757 ALSA provides an easy interface for procfs. The proc files are very
3758 useful for debugging. I recommend you set up proc files if you write a
3759 driver and want to get a running status or register dumps. The API is
3760 found in ``<sound/info.h>``.
3762 To create a proc file, call :c:func:`snd_card_proc_new()`.
3766 struct snd_info_entry *entry;
3767 int err = snd_card_proc_new(card, "my-file", &entry);
3769 where the second argument specifies the name of the proc file to be
3770 created. The above example will create a file ``my-file`` under the
3771 card directory, e.g. ``/proc/asound/card0/my-file``.
3773 Like other components, the proc entry created via
3774 :c:func:`snd_card_proc_new()` will be registered and released
3775 automatically in the card registration and release functions.
3777 When the creation is successful, the function stores a new instance in
3778 the pointer given in the third argument. It is initialized as a text
3779 proc file for read only. To use this proc file as a read-only text file
3780 as it is, set the read callback with a private data via
3781 :c:func:`snd_info_set_text_ops()`.
3785 snd_info_set_text_ops(entry, chip, my_proc_read);
3787 where the second argument (``chip``) is the private data to be used in
3788 the callbacks. The third parameter specifies the read buffer size and
3789 the fourth (``my_proc_read``) is the callback function, which is
3794 static void my_proc_read(struct snd_info_entry *entry,
3795 struct snd_info_buffer *buffer);
3797 In the read callback, use :c:func:`snd_iprintf()` for output
3798 strings, which works just like normal :c:func:`printf()`. For
3803 static void my_proc_read(struct snd_info_entry *entry,
3804 struct snd_info_buffer *buffer)
3806 struct my_chip *chip = entry->private_data;
3808 snd_iprintf(buffer, "This is my chip!\n");
3809 snd_iprintf(buffer, "Port = %ld\n", chip->port);
3812 The file permissions can be changed afterwards. As default, it's set as
3813 read only for all users. If you want to add write permission for the
3814 user (root as default), do as follows:
3818 entry->mode = S_IFREG | S_IRUGO | S_IWUSR;
3820 and set the write buffer size and the callback
3824 entry->c.text.write = my_proc_write;
3826 For the write callback, you can use :c:func:`snd_info_get_line()`
3827 to get a text line, and :c:func:`snd_info_get_str()` to retrieve
3828 a string from the line. Some examples are found in
3829 ``core/oss/mixer_oss.c``, core/oss/and ``pcm_oss.c``.
3831 For a raw-data proc-file, set the attributes as follows:
3835 static struct snd_info_entry_ops my_file_io_ops = {
3836 .read = my_file_io_read,
3839 entry->content = SNDRV_INFO_CONTENT_DATA;
3840 entry->private_data = chip;
3841 entry->c.ops = &my_file_io_ops;
3843 entry->mode = S_IFREG | S_IRUGO;
3845 For the raw data, ``size`` field must be set properly. This specifies
3846 the maximum size of the proc file access.
3848 The read/write callbacks of raw mode are more direct than the text mode.
3849 You need to use a low-level I/O functions such as
3850 :c:func:`copy_from/to_user()` to transfer the data.
3854 static ssize_t my_file_io_read(struct snd_info_entry *entry,
3855 void *file_private_data,
3861 if (copy_to_user(buf, local_data + pos, count))
3866 If the size of the info entry has been set up properly, ``count`` and
3867 ``pos`` are guaranteed to fit within 0 and the given size. You don't
3868 have to check the range in the callbacks unless any other condition is
3874 If the chip is supposed to work with suspend/resume functions, you need
3875 to add power-management code to the driver. The additional code for
3876 power-management should be ifdef-ed with ``CONFIG_PM``, or annotated
3877 with __maybe_unused attribute; otherwise the compiler will complain
3880 If the driver *fully* supports suspend/resume that is, the device can be
3881 properly resumed to its state when suspend was called, you can set the
3882 ``SNDRV_PCM_INFO_RESUME`` flag in the pcm info field. Usually, this is
3883 possible when the registers of the chip can be safely saved and restored
3884 to RAM. If this is set, the trigger callback is called with
3885 ``SNDRV_PCM_TRIGGER_RESUME`` after the resume callback completes.
3887 Even if the driver doesn't support PM fully but partial suspend/resume
3888 is still possible, it's still worthy to implement suspend/resume
3889 callbacks. In such a case, applications would reset the status by
3890 calling :c:func:`snd_pcm_prepare()` and restart the stream
3891 appropriately. Hence, you can define suspend/resume callbacks below but
3892 don't set ``SNDRV_PCM_INFO_RESUME`` info flag to the PCM.
3894 Note that the trigger with SUSPEND can always be called when
3895 :c:func:`snd_pcm_suspend_all()` is called, regardless of the
3896 ``SNDRV_PCM_INFO_RESUME`` flag. The ``RESUME`` flag affects only the
3897 behavior of :c:func:`snd_pcm_resume()`. (Thus, in theory,
3898 ``SNDRV_PCM_TRIGGER_RESUME`` isn't needed to be handled in the trigger
3899 callback when no ``SNDRV_PCM_INFO_RESUME`` flag is set. But, it's better
3900 to keep it for compatibility reasons.)
3902 In the earlier version of ALSA drivers, a common power-management layer
3903 was provided, but it has been removed. The driver needs to define the
3904 suspend/resume hooks according to the bus the device is connected to. In
3905 the case of PCI drivers, the callbacks look like below:
3909 static int __maybe_unused snd_my_suspend(struct device *dev)
3911 .... /* do things for suspend */
3914 static int __maybe_unused snd_my_resume(struct device *dev)
3916 .... /* do things for suspend */
3920 The scheme of the real suspend job is as follows.
3922 1. Retrieve the card and the chip data.
3924 2. Call :c:func:`snd_power_change_state()` with
3925 ``SNDRV_CTL_POWER_D3hot`` to change the power status.
3927 3. If AC97 codecs are used, call :c:func:`snd_ac97_suspend()` for
3930 4. Save the register values if necessary.
3932 5. Stop the hardware if necessary.
3934 A typical code would be like:
3938 static int __maybe_unused mychip_suspend(struct device *dev)
3941 struct snd_card *card = dev_get_drvdata(dev);
3942 struct mychip *chip = card->private_data;
3944 snd_power_change_state(card, SNDRV_CTL_POWER_D3hot);
3946 snd_ac97_suspend(chip->ac97);
3948 snd_mychip_save_registers(chip);
3950 snd_mychip_stop_hardware(chip);
3955 The scheme of the real resume job is as follows.
3957 1. Retrieve the card and the chip data.
3959 2. Re-initialize the chip.
3961 3. Restore the saved registers if necessary.
3963 4. Resume the mixer, e.g. calling :c:func:`snd_ac97_resume()`.
3965 5. Restart the hardware (if any).
3967 6. Call :c:func:`snd_power_change_state()` with
3968 ``SNDRV_CTL_POWER_D0`` to notify the processes.
3970 A typical code would be like:
3974 static int __maybe_unused mychip_resume(struct pci_dev *pci)
3977 struct snd_card *card = dev_get_drvdata(dev);
3978 struct mychip *chip = card->private_data;
3980 snd_mychip_reinit_chip(chip);
3982 snd_mychip_restore_registers(chip);
3984 snd_ac97_resume(chip->ac97);
3986 snd_mychip_restart_chip(chip);
3988 snd_power_change_state(card, SNDRV_CTL_POWER_D0);
3992 Note that, at the time this callback gets called, the PCM stream has
3993 been already suspended via its own PM ops calling
3994 :c:func:`snd_pcm_suspend_all()` internally.
3996 OK, we have all callbacks now. Let's set them up. In the initialization
3997 of the card, make sure that you can get the chip data from the card
3998 instance, typically via ``private_data`` field, in case you created the
3999 chip data individually.
4003 static int snd_mychip_probe(struct pci_dev *pci,
4004 const struct pci_device_id *pci_id)
4007 struct snd_card *card;
4008 struct mychip *chip;
4011 err = snd_card_new(&pci->dev, index[dev], id[dev], THIS_MODULE,
4014 chip = kzalloc(sizeof(*chip), GFP_KERNEL);
4016 card->private_data = chip;
4020 When you created the chip data with :c:func:`snd_card_new()`, it's
4021 anyway accessible via ``private_data`` field.
4025 static int snd_mychip_probe(struct pci_dev *pci,
4026 const struct pci_device_id *pci_id)
4029 struct snd_card *card;
4030 struct mychip *chip;
4033 err = snd_card_new(&pci->dev, index[dev], id[dev], THIS_MODULE,
4034 sizeof(struct mychip), &card);
4036 chip = card->private_data;
4040 If you need a space to save the registers, allocate the buffer for it
4041 here, too, since it would be fatal if you cannot allocate a memory in
4042 the suspend phase. The allocated buffer should be released in the
4043 corresponding destructor.
4045 And next, set suspend/resume callbacks to the pci_driver.
4049 static SIMPLE_DEV_PM_OPS(snd_my_pm_ops, mychip_suspend, mychip_resume);
4051 static struct pci_driver driver = {
4052 .name = KBUILD_MODNAME,
4053 .id_table = snd_my_ids,
4054 .probe = snd_my_probe,
4055 .remove = snd_my_remove,
4056 .driver.pm = &snd_my_pm_ops,
4062 There are standard module options for ALSA. At least, each module should
4063 have the ``index``, ``id`` and ``enable`` options.
4065 If the module supports multiple cards (usually up to 8 = ``SNDRV_CARDS``
4066 cards), they should be arrays. The default initial values are defined
4067 already as constants for easier programming:
4071 static int index[SNDRV_CARDS] = SNDRV_DEFAULT_IDX;
4072 static char *id[SNDRV_CARDS] = SNDRV_DEFAULT_STR;
4073 static int enable[SNDRV_CARDS] = SNDRV_DEFAULT_ENABLE_PNP;
4075 If the module supports only a single card, they could be single
4076 variables, instead. ``enable`` option is not always necessary in this
4077 case, but it would be better to have a dummy option for compatibility.
4079 The module parameters must be declared with the standard
4080 ``module_param()``, ``module_param_array()`` and
4081 :c:func:`MODULE_PARM_DESC()` macros.
4083 The typical coding would be like below:
4087 #define CARD_NAME "My Chip"
4089 module_param_array(index, int, NULL, 0444);
4090 MODULE_PARM_DESC(index, "Index value for " CARD_NAME " soundcard.");
4091 module_param_array(id, charp, NULL, 0444);
4092 MODULE_PARM_DESC(id, "ID string for " CARD_NAME " soundcard.");
4093 module_param_array(enable, bool, NULL, 0444);
4094 MODULE_PARM_DESC(enable, "Enable " CARD_NAME " soundcard.");
4096 Also, don't forget to define the module description and the license.
4097 Especially, the recent modprobe requires to define the
4098 module license as GPL, etc., otherwise the system is shown as “tainted”.
4102 MODULE_DESCRIPTION("Sound driver for My Chip");
4103 MODULE_LICENSE("GPL");
4106 How To Put Your Driver Into ALSA Tree
4107 =====================================
4112 So far, you've learned how to write the driver codes. And you might have
4113 a question now: how to put my own driver into the ALSA driver tree? Here
4114 (finally :) the standard procedure is described briefly.
4116 Suppose that you create a new PCI driver for the card “xyz”. The card
4117 module name would be snd-xyz. The new driver is usually put into the
4118 alsa-driver tree, ``sound/pci`` directory in the case of PCI
4121 In the following sections, the driver code is supposed to be put into
4122 Linux kernel tree. The two cases are covered: a driver consisting of a
4123 single source file and one consisting of several source files.
4125 Driver with A Single Source File
4126 --------------------------------
4128 1. Modify sound/pci/Makefile
4130 Suppose you have a file xyz.c. Add the following two lines
4134 snd-xyz-objs := xyz.o
4135 obj-$(CONFIG_SND_XYZ) += snd-xyz.o
4137 2. Create the Kconfig entry
4139 Add the new entry of Kconfig for your xyz driver. config SND_XYZ
4140 tristate "Foobar XYZ" depends on SND select SND_PCM help Say Y here
4141 to include support for Foobar XYZ soundcard. To compile this driver
4142 as a module, choose M here: the module will be called snd-xyz. the
4143 line, select SND_PCM, specifies that the driver xyz supports PCM. In
4144 addition to SND_PCM, the following components are supported for
4145 select command: SND_RAWMIDI, SND_TIMER, SND_HWDEP,
4146 SND_MPU401_UART, SND_OPL3_LIB, SND_OPL4_LIB, SND_VX_LIB,
4147 SND_AC97_CODEC. Add the select command for each supported
4150 Note that some selections imply the lowlevel selections. For example,
4151 PCM includes TIMER, MPU401_UART includes RAWMIDI, AC97_CODEC
4152 includes PCM, and OPL3_LIB includes HWDEP. You don't need to give
4153 the lowlevel selections again.
4155 For the details of Kconfig script, refer to the kbuild documentation.
4157 Drivers with Several Source Files
4158 ---------------------------------
4160 Suppose that the driver snd-xyz have several source files. They are
4161 located in the new subdirectory, sound/pci/xyz.
4163 1. Add a new directory (``sound/pci/xyz``) in ``sound/pci/Makefile``
4168 obj-$(CONFIG_SND) += sound/pci/xyz/
4171 2. Under the directory ``sound/pci/xyz``, create a Makefile
4175 snd-xyz-objs := xyz.o abc.o def.o
4176 obj-$(CONFIG_SND_XYZ) += snd-xyz.o
4178 3. Create the Kconfig entry
4180 This procedure is as same as in the last section.
4186 :c:func:`snd_printk()` and friends
4187 ----------------------------------
4189 .. note:: This subsection describes a few helper functions for
4190 decorating a bit more on the standard :c:func:`printk()` & co.
4191 However, in general, the use of such helpers is no longer recommended.
4192 If possible, try to stick with the standard functions like
4193 :c:func:`dev_err()` or :c:func:`pr_err()`.
4195 ALSA provides a verbose version of the :c:func:`printk()` function.
4196 If a kernel config ``CONFIG_SND_VERBOSE_PRINTK`` is set, this function
4197 prints the given message together with the file name and the line of the
4198 caller. The ``KERN_XXX`` prefix is processed as well as the original
4199 :c:func:`printk()` does, so it's recommended to add this prefix,
4200 e.g. snd_printk(KERN_ERR "Oh my, sorry, it's extremely bad!\\n");
4202 There are also :c:func:`printk()`'s for debugging.
4203 :c:func:`snd_printd()` can be used for general debugging purposes.
4204 If ``CONFIG_SND_DEBUG`` is set, this function is compiled, and works
4205 just like :c:func:`snd_printk()`. If the ALSA is compiled without
4206 the debugging flag, it's ignored.
4208 :c:func:`snd_printdd()` is compiled in only when
4209 ``CONFIG_SND_DEBUG_VERBOSE`` is set.
4214 It shows the ``BUG?`` message and stack trace as well as
4215 :c:func:`snd_BUG_ON()` at the point. It's useful to show that a
4216 fatal error happens there.
4218 When no debug flag is set, this macro is ignored.
4220 :c:func:`snd_BUG_ON()`
4221 ----------------------
4223 :c:func:`snd_BUG_ON()` macro is similar with
4224 :c:func:`WARN_ON()` macro. For example, snd_BUG_ON(!pointer); or
4225 it can be used as the condition, if (snd_BUG_ON(non_zero_is_bug))
4228 The macro takes an conditional expression to evaluate. When
4229 ``CONFIG_SND_DEBUG``, is set, if the expression is non-zero, it shows
4230 the warning message such as ``BUG? (xxx)`` normally followed by stack
4231 trace. In both cases it returns the evaluated value.
4236 I would like to thank Phil Kerr for his help for improvement and
4237 corrections of this document.
4239 Kevin Conder reformatted the original plain-text to the DocBook format.
4241 Giuliano Pochini corrected typos and contributed the example codes in
4242 the hardware constraints section.
git.samba.org / sfrench / cifs-2.6.git / blob