6 This document is meant as a brief overview of some aspects of the new serial
7 driver. It is not complete, any questions you have should be directed to
10 The reference implementation is contained within amba_pl011.c.
14 Low Level Serial Hardware Driver
15 --------------------------------
17 The low level serial hardware driver is responsible for supplying port
18 information (defined by uart_port) and a set of control methods (defined
19 by uart_ops) to the core serial driver. The low level driver is also
20 responsible for handling interrupts for the port, and providing any
27 The serial core provides a few helper functions. This includes identifing
28 the correct port structure (via uart_get_console) and decoding command line
29 arguments (uart_parse_options).
35 It is the responsibility of the low level hardware driver to perform the
36 necessary locking using port->lock. There are some exceptions (which
37 are described in the uart_ops listing below.)
39 There are two locks. A per-port spinlock, and an overall semaphore.
41 From the core driver perspective, the port->lock locks the following
46 info->xmit.head (circ->head)
47 info->xmit.tail (circ->tail)
49 The low level driver is free to use this lock to provide any additional
52 The port_sem semaphore is used to protect against ports being added/
53 removed or reconfigured at inappropriate times. Since v2.6.27, this
54 semaphore has been the 'mutex' member of the tty_port struct, and
55 commonly referred to as the port mutex (or port->mutex).
61 The uart_ops structure is the main interface between serial_core and the
62 hardware specific driver. It contains all the methods to control the
66 This function tests whether the transmitter fifo and shifter
67 for the port described by 'port' is empty. If it is empty,
68 this function should return TIOCSER_TEMT, otherwise return 0.
69 If the port does not support this operation, then it should
73 Interrupts: caller dependent.
74 This call must not sleep
76 set_mctrl(port, mctrl)
77 This function sets the modem control lines for port described
78 by 'port' to the state described by mctrl. The relevant bits
80 - TIOCM_RTS RTS signal.
81 - TIOCM_DTR DTR signal.
82 - TIOCM_OUT1 OUT1 signal.
83 - TIOCM_OUT2 OUT2 signal.
84 - TIOCM_LOOP Set the port into loopback mode.
85 If the appropriate bit is set, the signal should be driven
86 active. If the bit is clear, the signal should be driven
89 Locking: port->lock taken.
90 Interrupts: locally disabled.
91 This call must not sleep
94 Returns the current state of modem control inputs. The state
95 of the outputs should not be returned, since the core keeps
96 track of their state. The state information should include:
97 - TIOCM_CAR state of DCD signal
98 - TIOCM_CTS state of CTS signal
99 - TIOCM_DSR state of DSR signal
100 - TIOCM_RI state of RI signal
101 The bit is set if the signal is currently driven active. If
102 the port does not support CTS, DCD or DSR, the driver should
103 indicate that the signal is permanently active. If RI is
104 not available, the signal should not be indicated as active.
106 Locking: port->lock taken.
107 Interrupts: locally disabled.
108 This call must not sleep
111 Stop transmitting characters. This might be due to the CTS
112 line becoming inactive or the tty layer indicating we want
113 to stop transmission due to an XOFF character.
115 The driver should stop transmitting characters as soon as
118 Locking: port->lock taken.
119 Interrupts: locally disabled.
120 This call must not sleep
123 Start transmitting characters.
125 Locking: port->lock taken.
126 Interrupts: locally disabled.
127 This call must not sleep
130 Notify the serial driver that input buffers for the line discipline are
131 close to full, and it should somehow signal that no more characters
132 should be sent to the serial port.
137 Notify the serial driver that characters can now be sent to the serial
138 port without fear of overrunning the input buffers of the line
144 Transmit a high priority character, even if the port is stopped.
145 This is used to implement XON/XOFF flow control and tcflow(). If
146 the serial driver does not implement this function, the tty core
147 will append the character to the circular buffer and then call
148 start_tx() / stop_tx() to flush the data out.
150 Do not transmit if ch == '\0' (__DISABLED_CHAR).
153 Interrupts: caller dependent.
156 Stop receiving characters; the port is in the process of
159 Locking: port->lock taken.
160 Interrupts: locally disabled.
161 This call must not sleep
164 Enable the modem status interrupts.
166 This method may be called multiple times. Modem status
167 interrupts should be disabled when the shutdown method is
170 Locking: port->lock taken.
171 Interrupts: locally disabled.
172 This call must not sleep
175 Control the transmission of a break signal. If ctl is
176 nonzero, the break signal should be transmitted. The signal
177 should be terminated when another call is made with a zero
181 Interrupts: caller dependent.
182 This call must not sleep
185 Grab any interrupt resources and initialise any low level driver
186 state. Enable the port for reception. It should not activate
187 RTS nor DTR; this will be done via a separate call to set_mctrl.
189 This method will only be called when the port is initially opened.
191 Locking: port_sem taken.
192 Interrupts: globally disabled.
195 Disable the port, disable any break condition that may be in
196 effect, and free any interrupt resources. It should not disable
197 RTS nor DTR; this will have already been done via a separate
200 Drivers must not access port->info once this call has completed.
202 This method will only be called when there are no more users of
205 Locking: port_sem taken.
206 Interrupts: caller dependent.
209 Flush any write buffers, reset any DMA state and stop any
210 ongoing DMA transfers.
212 This will be called whenever the port->info->xmit circular
215 Locking: port->lock taken.
216 Interrupts: locally disabled.
217 This call must not sleep
219 set_termios(port,termios,oldtermios)
220 Change the port parameters, including word length, parity, stop
221 bits. Update read_status_mask and ignore_status_mask to indicate
222 the types of events we are interested in receiving. Relevant
223 termios->c_cflag bits are:
226 PARENB - parity enable
227 PARODD - odd parity (when PARENB is in force)
228 CREAD - enable reception of characters (if not set,
229 still receive characters from the port, but
231 CRTSCTS - if set, enable CTS status change reporting
232 CLOCAL - if not set, enable modem status change
234 Relevant termios->c_iflag bits are:
235 INPCK - enable frame and parity error events to be
236 passed to the TTY layer.
238 PARMRK - both of these enable break events to be
239 passed to the TTY layer.
241 IGNPAR - ignore parity and framing errors
242 IGNBRK - ignore break errors, If IGNPAR is also
243 set, ignore overrun errors as well.
244 The interaction of the iflag bits is as follows (parity error
245 given as an example):
246 Parity error INPCK IGNPAR
247 n/a 0 n/a character received, marked as
249 None 1 n/a character received, marked as
251 Yes 1 0 character received, marked as
253 Yes 1 1 character discarded
255 Other flags may be used (eg, xon/xoff characters) if your
256 hardware supports hardware "soft" flow control.
258 Locking: caller holds port->mutex
259 Interrupts: caller dependent.
260 This call must not sleep
262 pm(port,state,oldstate)
263 Perform any power management related activities on the specified
264 port. State indicates the new state (defined by
265 enum uart_pm_state), oldstate indicates the previous state.
267 This function should not be used to grab any resources.
269 This will be called when the port is initially opened and finally
270 closed, except when the port is also the system console. This
271 will occur even if CONFIG_PM is not set.
274 Interrupts: caller dependent.
277 Return a pointer to a string constant describing the specified
278 port, or return NULL, in which case the string 'unknown' is
282 Interrupts: caller dependent.
285 Release any memory and IO region resources currently in use by
289 Interrupts: caller dependent.
292 Request any memory and IO region resources required by the port.
293 If any fail, no resources should be registered when this function
294 returns, and it should return -EBUSY on failure.
297 Interrupts: caller dependent.
299 config_port(port,type)
300 Perform any autoconfiguration steps required for the port. `type`
301 contains a bit mask of the required configuration. UART_CONFIG_TYPE
302 indicates that the port requires detection and identification.
303 port->type should be set to the type found, or PORT_UNKNOWN if
304 no port was detected.
306 UART_CONFIG_IRQ indicates autoconfiguration of the interrupt signal,
307 which should be probed using standard kernel autoprobing techniques.
308 This is not necessary on platforms where ports have interrupts
309 internally hard wired (eg, system on a chip implementations).
312 Interrupts: caller dependent.
314 verify_port(port,serinfo)
315 Verify the new serial port information contained within serinfo is
316 suitable for this port type.
319 Interrupts: caller dependent.
322 Perform any port specific IOCTLs. IOCTL commands must be defined
323 using the standard numbering system found in <asm/ioctl.h>
326 Interrupts: caller dependent.
329 Called by kgdb to perform the minimal hardware initialization needed
330 to support poll_put_char() and poll_get_char(). Unlike ->startup()
331 this should not request interrupts.
333 Locking: tty_mutex and tty_port->mutex taken.
336 poll_put_char(port,ch)
337 Called by kgdb to write a single character directly to the serial
338 port. It can and should block until there is space in the TX FIFO.
341 Interrupts: caller dependent.
342 This call must not sleep
345 Called by kgdb to read a single character directly from the serial
346 port. If data is available, it should be returned; otherwise
347 the function should return NO_POLL_CHAR immediately.
350 Interrupts: caller dependent.
351 This call must not sleep
356 uart_update_timeout(port,cflag,baud)
357 Update the FIFO drain timeout, port->timeout, according to the
358 number of bits, parity, stop bits and baud rate.
360 Locking: caller is expected to take port->lock
363 uart_get_baud_rate(port,termios,old,min,max)
364 Return the numeric baud rate for the specified termios, taking
365 account of the special 38400 baud "kludge". The B0 baud rate
366 is mapped to 9600 baud.
368 If the baud rate is not within min..max, then if old is non-NULL,
369 the original baud rate will be tried. If that exceeds the
370 min..max constraint, 9600 baud will be returned. termios will
371 be updated to the baud rate in use.
373 Note: min..max must always allow 9600 baud to be selected.
375 Locking: caller dependent.
378 uart_get_divisor(port,baud)
379 Return the divsor (baud_base / baud) for the specified baud
380 rate, appropriately rounded.
382 If 38400 baud and custom divisor is selected, return the
383 custom divisor instead.
385 Locking: caller dependent.
388 uart_match_port(port1,port2)
389 This utility function can be used to determine whether two
390 uart_port structures describe the same port.
395 uart_write_wakeup(port)
396 A driver is expected to call this function when the number of
397 characters in the transmit buffer have dropped below a threshold.
399 Locking: port->lock should be held.
402 uart_register_driver(drv)
403 Register a uart driver with the core driver. We in turn register
404 with the tty layer, and initialise the core driver per-port state.
406 drv->port should be NULL, and the per-port structures should be
407 registered using uart_add_one_port after this call has succeeded.
412 uart_unregister_driver()
413 Remove all references to a driver from the core driver. The low
414 level driver must have removed all its ports via the
415 uart_remove_one_port() if it registered them with uart_add_one_port().
426 uart_remove_one_port()
431 It is intended some day to drop the 'unused' entries from uart_port, and
432 allow low level drivers to register their own individual uart_port's with
433 the core. This will allow drivers to use uart_port as a pointer to a
434 structure containing both the uart_port entry with their own extensions,
438 struct uart_port port;
442 Modem control lines via GPIO
443 ----------------------------
445 Some helpers are provided in order to set/get modem control lines via GPIO.
447 mctrl_gpio_init(port, idx):
448 This will get the {cts,rts,...}-gpios from device tree if they are
449 present and request them, set direction etc, and return an
450 allocated structure. devm_* functions are used, so there's no need
451 to call mctrl_gpio_free().
452 As this sets up the irq handling make sure to not handle changes to the
453 gpio input lines in your driver, too.
455 mctrl_gpio_free(dev, gpios):
456 This will free the requested gpios in mctrl_gpio_init().
457 As devm_* function are used, there's generally no need to call
460 mctrl_gpio_to_gpiod(gpios, gidx)
461 This returns the gpio structure associated to the modem line index.
463 mctrl_gpio_set(gpios, mctrl):
464 This will sets the gpios according to the mctrl state.
466 mctrl_gpio_get(gpios, mctrl):
467 This will update mctrl with the gpios values.
469 mctrl_gpio_enable_ms(gpios):
470 Enables irqs and handling of changes to the ms lines.
472 mctrl_gpio_disable_ms(gpios):
473 Disables irqs and handling of changes to the ms lines.