1 .. SPDX-License-Identifier: GPL-2.0
9 The V4L2 control API seems simple enough, but quickly becomes very hard to
10 implement correctly in drivers. But much of the code needed to handle controls
11 is actually not driver specific and can be moved to the V4L core framework.
13 After all, the only part that a driver developer is interested in is:
15 1) How do I add a control?
16 2) How do I set the control's value? (i.e. s_ctrl)
20 3) How do I get the control's value? (i.e. g_volatile_ctrl)
21 4) How do I validate the user's proposed control value? (i.e. try_ctrl)
23 All the rest is something that can be done centrally.
25 The control framework was created in order to implement all the rules of the
26 V4L2 specification with respect to controls in a central place. And to make
27 life as easy as possible for the driver developer.
29 Note that the control framework relies on the presence of a struct v4l2_device
30 for V4L2 drivers and struct v4l2_subdev for sub-device drivers.
33 Objects in the framework
34 ------------------------
36 There are two main objects:
38 The v4l2_ctrl object describes the control properties and keeps track of the
39 control's value (both the current value and the proposed new value).
41 v4l2_ctrl_handler is the object that keeps track of controls. It maintains a
42 list of v4l2_ctrl objects that it owns and another list of references to
43 controls, possibly to controls owned by other handlers.
46 Basic usage for V4L2 and sub-device drivers
47 -------------------------------------------
49 1) Prepare the driver:
51 1.1) Add the handler to your driver's top-level struct:
57 struct v4l2_ctrl_handler ctrl_handler;
63 1.2) Initialize the handler:
67 v4l2_ctrl_handler_init(&foo->ctrl_handler, nr_of_controls);
69 The second argument is a hint telling the function how many controls this
70 handler is expected to handle. It will allocate a hashtable based on this
71 information. It is a hint only.
73 1.3) Hook the control handler into the driver:
75 1.3.1) For V4L2 drivers do this:
81 struct v4l2_device v4l2_dev;
83 struct v4l2_ctrl_handler ctrl_handler;
87 foo->v4l2_dev.ctrl_handler = &foo->ctrl_handler;
89 Where foo->v4l2_dev is of type struct v4l2_device.
91 Finally, remove all control functions from your v4l2_ioctl_ops (if any):
92 vidioc_queryctrl, vidioc_query_ext_ctrl, vidioc_querymenu, vidioc_g_ctrl,
93 vidioc_s_ctrl, vidioc_g_ext_ctrls, vidioc_try_ext_ctrls and vidioc_s_ext_ctrls.
94 Those are now no longer needed.
96 1.3.2) For sub-device drivers do this:
102 struct v4l2_subdev sd;
104 struct v4l2_ctrl_handler ctrl_handler;
108 foo->sd.ctrl_handler = &foo->ctrl_handler;
110 Where foo->sd is of type struct v4l2_subdev.
112 1.4) Clean up the handler at the end:
116 v4l2_ctrl_handler_free(&foo->ctrl_handler);
121 You add non-menu controls by calling v4l2_ctrl_new_std:
125 struct v4l2_ctrl *v4l2_ctrl_new_std(struct v4l2_ctrl_handler *hdl,
126 const struct v4l2_ctrl_ops *ops,
127 u32 id, s32 min, s32 max, u32 step, s32 def);
129 Menu and integer menu controls are added by calling v4l2_ctrl_new_std_menu:
133 struct v4l2_ctrl *v4l2_ctrl_new_std_menu(struct v4l2_ctrl_handler *hdl,
134 const struct v4l2_ctrl_ops *ops,
135 u32 id, s32 max, s32 skip_mask, s32 def);
137 Menu controls with a driver specific menu are added by calling
138 v4l2_ctrl_new_std_menu_items:
142 struct v4l2_ctrl *v4l2_ctrl_new_std_menu_items(
143 struct v4l2_ctrl_handler *hdl,
144 const struct v4l2_ctrl_ops *ops, u32 id, s32 max,
145 s32 skip_mask, s32 def, const char * const *qmenu);
147 Integer menu controls with a driver specific menu can be added by calling
148 v4l2_ctrl_new_int_menu:
152 struct v4l2_ctrl *v4l2_ctrl_new_int_menu(struct v4l2_ctrl_handler *hdl,
153 const struct v4l2_ctrl_ops *ops,
154 u32 id, s32 max, s32 def, const s64 *qmenu_int);
156 These functions are typically called right after the v4l2_ctrl_handler_init:
160 static const s64 exp_bias_qmenu[] = {
163 static const char * const test_pattern[] = {
170 v4l2_ctrl_handler_init(&foo->ctrl_handler, nr_of_controls);
171 v4l2_ctrl_new_std(&foo->ctrl_handler, &foo_ctrl_ops,
172 V4L2_CID_BRIGHTNESS, 0, 255, 1, 128);
173 v4l2_ctrl_new_std(&foo->ctrl_handler, &foo_ctrl_ops,
174 V4L2_CID_CONTRAST, 0, 255, 1, 128);
175 v4l2_ctrl_new_std_menu(&foo->ctrl_handler, &foo_ctrl_ops,
176 V4L2_CID_POWER_LINE_FREQUENCY,
177 V4L2_CID_POWER_LINE_FREQUENCY_60HZ, 0,
178 V4L2_CID_POWER_LINE_FREQUENCY_DISABLED);
179 v4l2_ctrl_new_int_menu(&foo->ctrl_handler, &foo_ctrl_ops,
180 V4L2_CID_EXPOSURE_BIAS,
181 ARRAY_SIZE(exp_bias_qmenu) - 1,
182 ARRAY_SIZE(exp_bias_qmenu) / 2 - 1,
184 v4l2_ctrl_new_std_menu_items(&foo->ctrl_handler, &foo_ctrl_ops,
185 V4L2_CID_TEST_PATTERN, ARRAY_SIZE(test_pattern) - 1, 0,
188 if (foo->ctrl_handler.error) {
189 int err = foo->ctrl_handler.error;
191 v4l2_ctrl_handler_free(&foo->ctrl_handler);
195 The v4l2_ctrl_new_std function returns the v4l2_ctrl pointer to the new
196 control, but if you do not need to access the pointer outside the control ops,
197 then there is no need to store it.
199 The v4l2_ctrl_new_std function will fill in most fields based on the control
200 ID except for the min, max, step and default values. These are passed in the
201 last four arguments. These values are driver specific while control attributes
202 like type, name, flags are all global. The control's current value will be set
203 to the default value.
205 The v4l2_ctrl_new_std_menu function is very similar but it is used for menu
206 controls. There is no min argument since that is always 0 for menu controls,
207 and instead of a step there is a skip_mask argument: if bit X is 1, then menu
210 The v4l2_ctrl_new_int_menu function creates a new standard integer menu
211 control with driver-specific items in the menu. It differs from
212 v4l2_ctrl_new_std_menu in that it doesn't have the mask argument and takes
213 as the last argument an array of signed 64-bit integers that form an exact
216 The v4l2_ctrl_new_std_menu_items function is very similar to
217 v4l2_ctrl_new_std_menu but takes an extra parameter qmenu, which is the driver
218 specific menu for an otherwise standard menu control. A good example for this
219 control is the test pattern control for capture/display/sensors devices that
220 have the capability to generate test patterns. These test patterns are hardware
221 specific, so the contents of the menu will vary from device to device.
223 Note that if something fails, the function will return NULL or an error and
224 set ctrl_handler->error to the error code. If ctrl_handler->error was already
225 set, then it will just return and do nothing. This is also true for
226 v4l2_ctrl_handler_init if it cannot allocate the internal data structure.
228 This makes it easy to init the handler and just add all controls and only check
229 the error code at the end. Saves a lot of repetitive error checking.
231 It is recommended to add controls in ascending control ID order: it will be
232 a bit faster that way.
234 3) Optionally force initial control setup:
238 v4l2_ctrl_handler_setup(&foo->ctrl_handler);
240 This will call s_ctrl for all controls unconditionally. Effectively this
241 initializes the hardware to the default control values. It is recommended
242 that you do this as this ensures that both the internal data structures and
243 the hardware are in sync.
245 4) Finally: implement the v4l2_ctrl_ops
249 static const struct v4l2_ctrl_ops foo_ctrl_ops = {
250 .s_ctrl = foo_s_ctrl,
253 Usually all you need is s_ctrl:
257 static int foo_s_ctrl(struct v4l2_ctrl *ctrl)
259 struct foo *state = container_of(ctrl->handler, struct foo, ctrl_handler);
262 case V4L2_CID_BRIGHTNESS:
263 write_reg(0x123, ctrl->val);
265 case V4L2_CID_CONTRAST:
266 write_reg(0x456, ctrl->val);
272 The control ops are called with the v4l2_ctrl pointer as argument.
273 The new control value has already been validated, so all you need to do is
274 to actually update the hardware registers.
276 You're done! And this is sufficient for most of the drivers we have. No need
277 to do any validation of control values, or implement QUERYCTRL, QUERY_EXT_CTRL
278 and QUERYMENU. And G/S_CTRL as well as G/TRY/S_EXT_CTRLS are automatically supported.
283 The remainder sections deal with more advanced controls topics and scenarios.
284 In practice the basic usage as described above is sufficient for most drivers.
290 When a sub-device is registered with a V4L2 driver by calling
291 v4l2_device_register_subdev() and the ctrl_handler fields of both v4l2_subdev
292 and v4l2_device are set, then the controls of the subdev will become
293 automatically available in the V4L2 driver as well. If the subdev driver
294 contains controls that already exist in the V4L2 driver, then those will be
295 skipped (so a V4L2 driver can always override a subdev control).
297 What happens here is that v4l2_device_register_subdev() calls
298 v4l2_ctrl_add_handler() adding the controls of the subdev to the controls
302 Accessing Control Values
303 ------------------------
305 The following union is used inside the control framework to access control
310 union v4l2_ctrl_ptr {
317 The v4l2_ctrl struct contains these fields that can be used to access both
318 current and new values:
328 union v4l2_ctrl_ptr p_new;
329 union v4l2_ctrl_ptr p_cur;
331 If the control has a simple s32 type type, then:
335 &ctrl->val == ctrl->p_new.p_s32
336 &ctrl->cur.val == ctrl->p_cur.p_s32
338 For all other types use ctrl->p_cur.p<something>. Basically the val
339 and cur.val fields can be considered an alias since these are used so often.
341 Within the control ops you can freely use these. The val and cur.val speak for
342 themselves. The p_char pointers point to character buffers of length
343 ctrl->maximum + 1, and are always 0-terminated.
345 Unless the control is marked volatile the p_cur field points to the the
346 current cached control value. When you create a new control this value is made
347 identical to the default value. After calling v4l2_ctrl_handler_setup() this
348 value is passed to the hardware. It is generally a good idea to call this
351 Whenever a new value is set that new value is automatically cached. This means
352 that most drivers do not need to implement the g_volatile_ctrl() op. The
353 exception is for controls that return a volatile register such as a signal
354 strength read-out that changes continuously. In that case you will need to
355 implement g_volatile_ctrl like this:
359 static int foo_g_volatile_ctrl(struct v4l2_ctrl *ctrl)
362 case V4L2_CID_BRIGHTNESS:
363 ctrl->val = read_reg(0x123);
368 Note that you use the 'new value' union as well in g_volatile_ctrl. In general
369 controls that need to implement g_volatile_ctrl are read-only controls. If they
370 are not, a V4L2_EVENT_CTRL_CH_VALUE will not be generated when the control
373 To mark a control as volatile you have to set V4L2_CTRL_FLAG_VOLATILE:
377 ctrl = v4l2_ctrl_new_std(&sd->ctrl_handler, ...);
379 ctrl->flags |= V4L2_CTRL_FLAG_VOLATILE;
381 For try/s_ctrl the new values (i.e. as passed by the user) are filled in and
382 you can modify them in try_ctrl or set them in s_ctrl. The 'cur' union
383 contains the current value, which you can use (but not change!) as well.
385 If s_ctrl returns 0 (OK), then the control framework will copy the new final
386 values to the 'cur' union.
388 While in g_volatile/s/try_ctrl you can access the value of all controls owned
389 by the same handler since the handler's lock is held. If you need to access
390 the value of controls owned by other handlers, then you have to be very careful
391 not to introduce deadlocks.
393 Outside of the control ops you have to go through to helper functions to get
394 or set a single control value safely in your driver:
398 s32 v4l2_ctrl_g_ctrl(struct v4l2_ctrl *ctrl);
399 int v4l2_ctrl_s_ctrl(struct v4l2_ctrl *ctrl, s32 val);
401 These functions go through the control framework just as VIDIOC_G/S_CTRL ioctls
402 do. Don't use these inside the control ops g_volatile/s/try_ctrl, though, that
403 will result in a deadlock since these helpers lock the handler as well.
405 You can also take the handler lock yourself:
409 mutex_lock(&state->ctrl_handler.lock);
410 pr_info("String value is '%s'\n", ctrl1->p_cur.p_char);
411 pr_info("Integer value is '%s'\n", ctrl2->cur.val);
412 mutex_unlock(&state->ctrl_handler.lock);
418 The v4l2_ctrl struct contains this union:
427 For menu controls menu_skip_mask is used. What it does is that it allows you
428 to easily exclude certain menu items. This is used in the VIDIOC_QUERYMENU
429 implementation where you can return -EINVAL if a certain menu item is not
430 present. Note that VIDIOC_QUERYCTRL always returns a step value of 1 for
433 A good example is the MPEG Audio Layer II Bitrate menu control where the
434 menu is a list of standardized possible bitrates. But in practice hardware
435 implementations will only support a subset of those. By setting the skip
436 mask you can tell the framework which menu items should be skipped. Setting
437 it to 0 means that all menu items are supported.
439 You set this mask either through the v4l2_ctrl_config struct for a custom
440 control, or by calling v4l2_ctrl_new_std_menu().
446 Driver specific controls can be created using v4l2_ctrl_new_custom():
450 static const struct v4l2_ctrl_config ctrl_filter = {
451 .ops = &ctrl_custom_ops,
452 .id = V4L2_CID_MPEG_CX2341X_VIDEO_SPATIAL_FILTER,
453 .name = "Spatial Filter",
454 .type = V4L2_CTRL_TYPE_INTEGER,
455 .flags = V4L2_CTRL_FLAG_SLIDER,
460 ctrl = v4l2_ctrl_new_custom(&foo->ctrl_handler, &ctrl_filter, NULL);
462 The last argument is the priv pointer which can be set to driver-specific
465 The v4l2_ctrl_config struct also has a field to set the is_private flag.
467 If the name field is not set, then the framework will assume this is a standard
468 control and will fill in the name, type and flags fields accordingly.
471 Active and Grabbed Controls
472 ---------------------------
474 If you get more complex relationships between controls, then you may have to
475 activate and deactivate controls. For example, if the Chroma AGC control is
476 on, then the Chroma Gain control is inactive. That is, you may set it, but
477 the value will not be used by the hardware as long as the automatic gain
478 control is on. Typically user interfaces can disable such input fields.
480 You can set the 'active' status using v4l2_ctrl_activate(). By default all
481 controls are active. Note that the framework does not check for this flag.
482 It is meant purely for GUIs. The function is typically called from within
485 The other flag is the 'grabbed' flag. A grabbed control means that you cannot
486 change it because it is in use by some resource. Typical examples are MPEG
487 bitrate controls that cannot be changed while capturing is in progress.
489 If a control is set to 'grabbed' using v4l2_ctrl_grab(), then the framework
490 will return -EBUSY if an attempt is made to set this control. The
491 v4l2_ctrl_grab() function is typically called from the driver when it
492 starts or stops streaming.
498 By default all controls are independent from the others. But in more
499 complex scenarios you can get dependencies from one control to another.
500 In that case you need to 'cluster' them:
505 struct v4l2_ctrl_handler ctrl_handler;
506 #define AUDIO_CL_VOLUME (0)
507 #define AUDIO_CL_MUTE (1)
508 struct v4l2_ctrl *audio_cluster[2];
512 state->audio_cluster[AUDIO_CL_VOLUME] =
513 v4l2_ctrl_new_std(&state->ctrl_handler, ...);
514 state->audio_cluster[AUDIO_CL_MUTE] =
515 v4l2_ctrl_new_std(&state->ctrl_handler, ...);
516 v4l2_ctrl_cluster(ARRAY_SIZE(state->audio_cluster), state->audio_cluster);
518 From now on whenever one or more of the controls belonging to the same
519 cluster is set (or 'gotten', or 'tried'), only the control ops of the first
520 control ('volume' in this example) is called. You effectively create a new
521 composite control. Similar to how a 'struct' works in C.
523 So when s_ctrl is called with V4L2_CID_AUDIO_VOLUME as argument, you should set
524 all two controls belonging to the audio_cluster:
528 static int foo_s_ctrl(struct v4l2_ctrl *ctrl)
530 struct foo *state = container_of(ctrl->handler, struct foo, ctrl_handler);
533 case V4L2_CID_AUDIO_VOLUME: {
534 struct v4l2_ctrl *mute = ctrl->cluster[AUDIO_CL_MUTE];
536 write_reg(0x123, mute->val ? 0 : ctrl->val);
539 case V4L2_CID_CONTRAST:
540 write_reg(0x456, ctrl->val);
546 In the example above the following are equivalent for the VOLUME case:
550 ctrl == ctrl->cluster[AUDIO_CL_VOLUME] == state->audio_cluster[AUDIO_CL_VOLUME]
551 ctrl->cluster[AUDIO_CL_MUTE] == state->audio_cluster[AUDIO_CL_MUTE]
553 In practice using cluster arrays like this becomes very tiresome. So instead
554 the following equivalent method is used:
560 struct v4l2_ctrl *volume;
561 struct v4l2_ctrl *mute;
564 The anonymous struct is used to clearly 'cluster' these two control pointers,
565 but it serves no other purpose. The effect is the same as creating an
566 array with two control pointers. So you can just do:
570 state->volume = v4l2_ctrl_new_std(&state->ctrl_handler, ...);
571 state->mute = v4l2_ctrl_new_std(&state->ctrl_handler, ...);
572 v4l2_ctrl_cluster(2, &state->volume);
574 And in foo_s_ctrl you can use these pointers directly: state->mute->val.
576 Note that controls in a cluster may be NULL. For example, if for some
577 reason mute was never added (because the hardware doesn't support that
578 particular feature), then mute will be NULL. So in that case we have a
579 cluster of 2 controls, of which only 1 is actually instantiated. The
580 only restriction is that the first control of the cluster must always be
581 present, since that is the 'master' control of the cluster. The master
582 control is the one that identifies the cluster and that provides the
583 pointer to the v4l2_ctrl_ops struct that is used for that cluster.
585 Obviously, all controls in the cluster array must be initialized to either
586 a valid control or to NULL.
588 In rare cases you might want to know which controls of a cluster actually
589 were set explicitly by the user. For this you can check the 'is_new' flag of
590 each control. For example, in the case of a volume/mute cluster the 'is_new'
591 flag of the mute control would be set if the user called VIDIOC_S_CTRL for
592 mute only. If the user would call VIDIOC_S_EXT_CTRLS for both mute and volume
593 controls, then the 'is_new' flag would be 1 for both controls.
595 The 'is_new' flag is always 1 when called from v4l2_ctrl_handler_setup().
598 Handling autogain/gain-type Controls with Auto Clusters
599 -------------------------------------------------------
601 A common type of control cluster is one that handles 'auto-foo/foo'-type
602 controls. Typical examples are autogain/gain, autoexposure/exposure,
603 autowhitebalance/red balance/blue balance. In all cases you have one control
604 that determines whether another control is handled automatically by the hardware,
605 or whether it is under manual control from the user.
607 If the cluster is in automatic mode, then the manual controls should be
608 marked inactive and volatile. When the volatile controls are read the
609 g_volatile_ctrl operation should return the value that the hardware's automatic
610 mode set up automatically.
612 If the cluster is put in manual mode, then the manual controls should become
613 active again and the volatile flag is cleared (so g_volatile_ctrl is no longer
614 called while in manual mode). In addition just before switching to manual mode
615 the current values as determined by the auto mode are copied as the new manual
618 Finally the V4L2_CTRL_FLAG_UPDATE should be set for the auto control since
619 changing that control affects the control flags of the manual controls.
621 In order to simplify this a special variation of v4l2_ctrl_cluster was
626 void v4l2_ctrl_auto_cluster(unsigned ncontrols, struct v4l2_ctrl **controls,
627 u8 manual_val, bool set_volatile);
629 The first two arguments are identical to v4l2_ctrl_cluster. The third argument
630 tells the framework which value switches the cluster into manual mode. The
631 last argument will optionally set V4L2_CTRL_FLAG_VOLATILE for the non-auto controls.
632 If it is false, then the manual controls are never volatile. You would typically
633 use that if the hardware does not give you the option to read back to values as
634 determined by the auto mode (e.g. if autogain is on, the hardware doesn't allow
635 you to obtain the current gain value).
637 The first control of the cluster is assumed to be the 'auto' control.
639 Using this function will ensure that you don't need to handle all the complex
640 flag and volatile handling.
643 VIDIOC_LOG_STATUS Support
644 -------------------------
646 This ioctl allow you to dump the current status of a driver to the kernel log.
647 The v4l2_ctrl_handler_log_status(ctrl_handler, prefix) can be used to dump the
648 value of the controls owned by the given handler to the log. You can supply a
649 prefix as well. If the prefix didn't end with a space, then ': ' will be added
653 Different Handlers for Different Video Nodes
654 --------------------------------------------
656 Usually the V4L2 driver has just one control handler that is global for
657 all video nodes. But you can also specify different control handlers for
658 different video nodes. You can do that by manually setting the ctrl_handler
659 field of struct video_device.
661 That is no problem if there are no subdevs involved but if there are, then
662 you need to block the automatic merging of subdev controls to the global
663 control handler. You do that by simply setting the ctrl_handler field in
664 struct v4l2_device to NULL. Now v4l2_device_register_subdev() will no longer
665 merge subdev controls.
667 After each subdev was added, you will then have to call v4l2_ctrl_add_handler
668 manually to add the subdev's control handler (sd->ctrl_handler) to the desired
669 control handler. This control handler may be specific to the video_device or
670 for a subset of video_device's. For example: the radio device nodes only have
671 audio controls, while the video and vbi device nodes share the same control
672 handler for the audio and video controls.
674 If you want to have one handler (e.g. for a radio device node) have a subset
675 of another handler (e.g. for a video device node), then you should first add
676 the controls to the first handler, add the other controls to the second
677 handler and finally add the first handler to the second. For example:
681 v4l2_ctrl_new_std(&radio_ctrl_handler, &radio_ops, V4L2_CID_AUDIO_VOLUME, ...);
682 v4l2_ctrl_new_std(&radio_ctrl_handler, &radio_ops, V4L2_CID_AUDIO_MUTE, ...);
683 v4l2_ctrl_new_std(&video_ctrl_handler, &video_ops, V4L2_CID_BRIGHTNESS, ...);
684 v4l2_ctrl_new_std(&video_ctrl_handler, &video_ops, V4L2_CID_CONTRAST, ...);
685 v4l2_ctrl_add_handler(&video_ctrl_handler, &radio_ctrl_handler, NULL);
687 The last argument to v4l2_ctrl_add_handler() is a filter function that allows
688 you to filter which controls will be added. Set it to NULL if you want to add
691 Or you can add specific controls to a handler:
695 volume = v4l2_ctrl_new_std(&video_ctrl_handler, &ops, V4L2_CID_AUDIO_VOLUME, ...);
696 v4l2_ctrl_new_std(&video_ctrl_handler, &ops, V4L2_CID_BRIGHTNESS, ...);
697 v4l2_ctrl_new_std(&video_ctrl_handler, &ops, V4L2_CID_CONTRAST, ...);
699 What you should not do is make two identical controls for two handlers.
704 v4l2_ctrl_new_std(&radio_ctrl_handler, &radio_ops, V4L2_CID_AUDIO_MUTE, ...);
705 v4l2_ctrl_new_std(&video_ctrl_handler, &video_ops, V4L2_CID_AUDIO_MUTE, ...);
707 This would be bad since muting the radio would not change the video mute
708 control. The rule is to have one control for each hardware 'knob' that you
715 Normally you have created the controls yourself and you can store the struct
716 v4l2_ctrl pointer into your own struct.
718 But sometimes you need to find a control from another handler that you do
719 not own. For example, if you have to find a volume control from a subdev.
721 You can do that by calling v4l2_ctrl_find:
725 struct v4l2_ctrl *volume;
727 volume = v4l2_ctrl_find(sd->ctrl_handler, V4L2_CID_AUDIO_VOLUME);
729 Since v4l2_ctrl_find will lock the handler you have to be careful where you
730 use it. For example, this is not a good idea:
734 struct v4l2_ctrl_handler ctrl_handler;
736 v4l2_ctrl_new_std(&ctrl_handler, &video_ops, V4L2_CID_BRIGHTNESS, ...);
737 v4l2_ctrl_new_std(&ctrl_handler, &video_ops, V4L2_CID_CONTRAST, ...);
739 ...and in video_ops.s_ctrl:
743 case V4L2_CID_BRIGHTNESS:
744 contrast = v4l2_find_ctrl(&ctrl_handler, V4L2_CID_CONTRAST);
747 When s_ctrl is called by the framework the ctrl_handler.lock is already taken, so
748 attempting to find another control from the same handler will deadlock.
750 It is recommended not to use this function from inside the control ops.
756 When one control handler is added to another using v4l2_ctrl_add_handler, then
757 by default all controls from one are merged to the other. But a subdev might
758 have low-level controls that make sense for some advanced embedded system, but
759 not when it is used in consumer-level hardware. In that case you want to keep
760 those low-level controls local to the subdev. You can do this by simply
761 setting the 'is_private' flag of the control to 1:
765 static const struct v4l2_ctrl_config ctrl_private = {
766 .ops = &ctrl_custom_ops,
768 .name = "Some Private Control",
769 .type = V4L2_CTRL_TYPE_INTEGER,
775 ctrl = v4l2_ctrl_new_custom(&foo->ctrl_handler, &ctrl_private, NULL);
777 These controls will now be skipped when v4l2_ctrl_add_handler is called.
780 V4L2_CTRL_TYPE_CTRL_CLASS Controls
781 ----------------------------------
783 Controls of this type can be used by GUIs to get the name of the control class.
784 A fully featured GUI can make a dialog with multiple tabs with each tab
785 containing the controls belonging to a particular control class. The name of
786 each tab can be found by querying a special control with ID <control class | 1>.
788 Drivers do not have to care about this. The framework will automatically add
789 a control of this type whenever the first control belonging to a new control
793 Adding Notify Callbacks
794 -----------------------
796 Sometimes the platform or bridge driver needs to be notified when a control
797 from a sub-device driver changes. You can set a notify callback by calling
802 void v4l2_ctrl_notify(struct v4l2_ctrl *ctrl,
803 void (*notify)(struct v4l2_ctrl *ctrl, void *priv), void *priv);
805 Whenever the give control changes value the notify callback will be called
806 with a pointer to the control and the priv pointer that was passed with
807 v4l2_ctrl_notify. Note that the control's handler lock is held when the
808 notify function is called.
810 There can be only one notify function per control handler. Any attempt
811 to set another notify function will cause a WARN_ON.
813 v4l2_ctrl functions and data structures
814 ---------------------------------------
816 .. kernel-doc:: include/media/v4l2-ctrls.h