1====================
2Low Level Serial API
3====================
4
5
6This document is meant as a brief overview of some aspects of the new serial
7driver.  It is not complete, any questions you have should be directed to
8<rmk@arm.linux.org.uk>
9
10The reference implementation is contained within amba-pl011.c.
11
12
13
14Low Level Serial Hardware Driver
15--------------------------------
16
17The low level serial hardware driver is responsible for supplying port
18information (defined by uart_port) and a set of control methods (defined
19by uart_ops) to the core serial driver.  The low level driver is also
20responsible for handling interrupts for the port, and providing any
21console support.
22
23
24Console Support
25---------------
26
27The serial core provides a few helper functions.  This includes identifing
28the correct port structure (via uart_get_console) and decoding command line
29arguments (uart_parse_options).
30
31There is also a helper function (uart_console_write) which performs a
32character by character write, translating newlines to CRLF sequences.
33Driver writers are recommended to use this function rather than implementing
34their own version.
35
36
37Locking
38-------
39
40It is the responsibility of the low level hardware driver to perform the
41necessary locking using port->lock.  There are some exceptions (which
42are described in the uart_ops listing below.)
43
44There are two locks.  A per-port spinlock, and an overall semaphore.
45
46From the core driver perspective, the port->lock locks the following
47data::
48
49	port->mctrl
50	port->icount
51	port->state->xmit.head (circ_buf->head)
52	port->state->xmit.tail (circ_buf->tail)
53
54The low level driver is free to use this lock to provide any additional
55locking.
56
57The port_sem semaphore is used to protect against ports being added/
58removed or reconfigured at inappropriate times. Since v2.6.27, this
59semaphore has been the 'mutex' member of the tty_port struct, and
60commonly referred to as the port mutex.
61
62
63uart_ops
64--------
65
66The uart_ops structure is the main interface between serial_core and the
67hardware specific driver.  It contains all the methods to control the
68hardware.
69
70  tx_empty(port)
71	This function tests whether the transmitter fifo and shifter
72	for the port described by 'port' is empty.  If it is empty,
73	this function should return TIOCSER_TEMT, otherwise return 0.
74	If the port does not support this operation, then it should
75	return TIOCSER_TEMT.
76
77	Locking: none.
78
79	Interrupts: caller dependent.
80
81	This call must not sleep
82
83  set_mctrl(port, mctrl)
84	This function sets the modem control lines for port described
85	by 'port' to the state described by mctrl.  The relevant bits
86	of mctrl are:
87
88		- TIOCM_RTS	RTS signal.
89		- TIOCM_DTR	DTR signal.
90		- TIOCM_OUT1	OUT1 signal.
91		- TIOCM_OUT2	OUT2 signal.
92		- TIOCM_LOOP	Set the port into loopback mode.
93
94	If the appropriate bit is set, the signal should be driven
95	active.  If the bit is clear, the signal should be driven
96	inactive.
97
98	Locking: port->lock taken.
99
100	Interrupts: locally disabled.
101
102	This call must not sleep
103
104  get_mctrl(port)
105	Returns the current state of modem control inputs.  The state
106	of the outputs should not be returned, since the core keeps
107	track of their state.  The state information should include:
108
109		- TIOCM_CAR	state of DCD signal
110		- TIOCM_CTS	state of CTS signal
111		- TIOCM_DSR	state of DSR signal
112		- TIOCM_RI	state of RI signal
113
114	The bit is set if the signal is currently driven active.  If
115	the port does not support CTS, DCD or DSR, the driver should
116	indicate that the signal is permanently active.  If RI is
117	not available, the signal should not be indicated as active.
118
119	Locking: port->lock taken.
120
121	Interrupts: locally disabled.
122
123	This call must not sleep
124
125  stop_tx(port)
126	Stop transmitting characters.  This might be due to the CTS
127	line becoming inactive or the tty layer indicating we want
128	to stop transmission due to an XOFF character.
129
130	The driver should stop transmitting characters as soon as
131	possible.
132
133	Locking: port->lock taken.
134
135	Interrupts: locally disabled.
136
137	This call must not sleep
138
139  start_tx(port)
140	Start transmitting characters.
141
142	Locking: port->lock taken.
143
144	Interrupts: locally disabled.
145
146	This call must not sleep
147
148  throttle(port)
149	Notify the serial driver that input buffers for the line discipline are
150	close to full, and it should somehow signal that no more characters
151	should be sent to the serial port.
152	This will be called only if hardware assisted flow control is enabled.
153
154	Locking: serialized with .unthrottle() and termios modification by the
155	tty layer.
156
157  unthrottle(port)
158	Notify the serial driver that characters can now be sent to the serial
159	port without fear of overrunning the input buffers of the line
160	disciplines.
161
162	This will be called only if hardware assisted flow control is enabled.
163
164	Locking: serialized with .throttle() and termios modification by the
165	tty layer.
166
167  send_xchar(port,ch)
168	Transmit a high priority character, even if the port is stopped.
169	This is used to implement XON/XOFF flow control and tcflow().  If
170	the serial driver does not implement this function, the tty core
171	will append the character to the circular buffer and then call
172	start_tx() / stop_tx() to flush the data out.
173
174	Do not transmit if ch == '\0' (__DISABLED_CHAR).
175
176	Locking: none.
177
178	Interrupts: caller dependent.
179
180  stop_rx(port)
181	Stop receiving characters; the port is in the process of
182	being closed.
183
184	Locking: port->lock taken.
185
186	Interrupts: locally disabled.
187
188	This call must not sleep
189
190  enable_ms(port)
191	Enable the modem status interrupts.
192
193	This method may be called multiple times.  Modem status
194	interrupts should be disabled when the shutdown method is
195	called.
196
197	Locking: port->lock taken.
198
199	Interrupts: locally disabled.
200
201	This call must not sleep
202
203  break_ctl(port,ctl)
204	Control the transmission of a break signal.  If ctl is
205	nonzero, the break signal should be transmitted.  The signal
206	should be terminated when another call is made with a zero
207	ctl.
208
209	Locking: caller holds tty_port->mutex
210
211  startup(port)
212	Grab any interrupt resources and initialise any low level driver
213	state.  Enable the port for reception.  It should not activate
214	RTS nor DTR; this will be done via a separate call to set_mctrl.
215
216	This method will only be called when the port is initially opened.
217
218	Locking: port_sem taken.
219
220	Interrupts: globally disabled.
221
222  shutdown(port)
223	Disable the port, disable any break condition that may be in
224	effect, and free any interrupt resources.  It should not disable
225	RTS nor DTR; this will have already been done via a separate
226	call to set_mctrl.
227
228	Drivers must not access port->state once this call has completed.
229
230	This method will only be called when there are no more users of
231	this port.
232
233	Locking: port_sem taken.
234
235	Interrupts: caller dependent.
236
237  flush_buffer(port)
238	Flush any write buffers, reset any DMA state and stop any
239	ongoing DMA transfers.
240
241	This will be called whenever the port->state->xmit circular
242	buffer is cleared.
243
244	Locking: port->lock taken.
245
246	Interrupts: locally disabled.
247
248	This call must not sleep
249
250  set_termios(port,termios,oldtermios)
251	Change the port parameters, including word length, parity, stop
252	bits.  Update read_status_mask and ignore_status_mask to indicate
253	the types of events we are interested in receiving.  Relevant
254	termios->c_cflag bits are:
255
256		CSIZE
257			- word size
258		CSTOPB
259			- 2 stop bits
260		PARENB
261			- parity enable
262		PARODD
263			- odd parity (when PARENB is in force)
264		CREAD
265			- enable reception of characters (if not set,
266			  still receive characters from the port, but
267			  throw them away.
268		CRTSCTS
269			- if set, enable CTS status change reporting
270		CLOCAL
271			- if not set, enable modem status change
272			  reporting.
273
274	Relevant termios->c_iflag bits are:
275
276		INPCK
277			- enable frame and parity error events to be
278			  passed to the TTY layer.
279		BRKINT / PARMRK
280			- both of these enable break events to be
281			  passed to the TTY layer.
282
283		IGNPAR
284			- ignore parity and framing errors
285		IGNBRK
286			- ignore break errors,  If IGNPAR is also
287			  set, ignore overrun errors as well.
288
289	The interaction of the iflag bits is as follows (parity error
290	given as an example):
291
292	=============== ======= ======  =============================
293	Parity error	INPCK	IGNPAR
294	=============== ======= ======  =============================
295	n/a		0	n/a	character received, marked as
296					TTY_NORMAL
297	None		1	n/a	character received, marked as
298					TTY_NORMAL
299	Yes		1	0	character received, marked as
300					TTY_PARITY
301	Yes		1	1	character discarded
302	=============== ======= ======  =============================
303
304	Other flags may be used (eg, xon/xoff characters) if your
305	hardware supports hardware "soft" flow control.
306
307	Locking: caller holds tty_port->mutex
308
309	Interrupts: caller dependent.
310
311	This call must not sleep
312
313  set_ldisc(port,termios)
314	Notifier for discipline change. See Documentation/tty/tty_ldisc.rst.
315
316	Locking: caller holds tty_port->mutex
317
318  pm(port,state,oldstate)
319	Perform any power management related activities on the specified
320	port.  State indicates the new state (defined by
321	enum uart_pm_state), oldstate indicates the previous state.
322
323	This function should not be used to grab any resources.
324
325	This will be called when the port is initially opened and finally
326	closed, except when the port is also the system console.  This
327	will occur even if CONFIG_PM is not set.
328
329	Locking: none.
330
331	Interrupts: caller dependent.
332
333  type(port)
334	Return a pointer to a string constant describing the specified
335	port, or return NULL, in which case the string 'unknown' is
336	substituted.
337
338	Locking: none.
339
340	Interrupts: caller dependent.
341
342  release_port(port)
343	Release any memory and IO region resources currently in use by
344	the port.
345
346	Locking: none.
347
348	Interrupts: caller dependent.
349
350  request_port(port)
351	Request any memory and IO region resources required by the port.
352	If any fail, no resources should be registered when this function
353	returns, and it should return -EBUSY on failure.
354
355	Locking: none.
356
357	Interrupts: caller dependent.
358
359  config_port(port,type)
360	Perform any autoconfiguration steps required for the port.  `type`
361	contains a bit mask of the required configuration.  UART_CONFIG_TYPE
362	indicates that the port requires detection and identification.
363	port->type should be set to the type found, or PORT_UNKNOWN if
364	no port was detected.
365
366	UART_CONFIG_IRQ indicates autoconfiguration of the interrupt signal,
367	which should be probed using standard kernel autoprobing techniques.
368	This is not necessary on platforms where ports have interrupts
369	internally hard wired (eg, system on a chip implementations).
370
371	Locking: none.
372
373	Interrupts: caller dependent.
374
375  verify_port(port,serinfo)
376	Verify the new serial port information contained within serinfo is
377	suitable for this port type.
378
379	Locking: none.
380
381	Interrupts: caller dependent.
382
383  ioctl(port,cmd,arg)
384	Perform any port specific IOCTLs.  IOCTL commands must be defined
385	using the standard numbering system found in <asm/ioctl.h>
386
387	Locking: none.
388
389	Interrupts: caller dependent.
390
391  poll_init(port)
392	Called by kgdb to perform the minimal hardware initialization needed
393	to support poll_put_char() and poll_get_char().  Unlike ->startup()
394	this should not request interrupts.
395
396	Locking: tty_mutex and tty_port->mutex taken.
397
398	Interrupts: n/a.
399
400  poll_put_char(port,ch)
401	Called by kgdb to write a single character directly to the serial
402	port.  It can and should block until there is space in the TX FIFO.
403
404	Locking: none.
405
406	Interrupts: caller dependent.
407
408	This call must not sleep
409
410  poll_get_char(port)
411	Called by kgdb to read a single character directly from the serial
412	port.  If data is available, it should be returned; otherwise
413	the function should return NO_POLL_CHAR immediately.
414
415	Locking: none.
416
417	Interrupts: caller dependent.
418
419	This call must not sleep
420
421Other functions
422---------------
423
424uart_update_timeout(port,cflag,baud)
425	Update the FIFO drain timeout, port->timeout, according to the
426	number of bits, parity, stop bits and baud rate.
427
428	Locking: caller is expected to take port->lock
429
430	Interrupts: n/a
431
432uart_get_baud_rate(port,termios,old,min,max)
433	Return the numeric baud rate for the specified termios, taking
434	account of the special 38400 baud "kludge".  The B0 baud rate
435	is mapped to 9600 baud.
436
437	If the baud rate is not within min..max, then if old is non-NULL,
438	the original baud rate will be tried.  If that exceeds the
439	min..max constraint, 9600 baud will be returned.  termios will
440	be updated to the baud rate in use.
441
442	Note: min..max must always allow 9600 baud to be selected.
443
444	Locking: caller dependent.
445
446	Interrupts: n/a
447
448uart_get_divisor(port,baud)
449	Return the divisor (baud_base / baud) for the specified baud
450	rate, appropriately rounded.
451
452	If 38400 baud and custom divisor is selected, return the
453	custom divisor instead.
454
455	Locking: caller dependent.
456
457	Interrupts: n/a
458
459uart_match_port(port1,port2)
460	This utility function can be used to determine whether two
461	uart_port structures describe the same port.
462
463	Locking: n/a
464
465	Interrupts: n/a
466
467uart_write_wakeup(port)
468	A driver is expected to call this function when the number of
469	characters in the transmit buffer have dropped below a threshold.
470
471	Locking: port->lock should be held.
472
473	Interrupts: n/a
474
475uart_register_driver(drv)
476	Register a uart driver with the core driver.  We in turn register
477	with the tty layer, and initialise the core driver per-port state.
478
479	drv->port should be NULL, and the per-port structures should be
480	registered using uart_add_one_port after this call has succeeded.
481
482	Locking: none
483
484	Interrupts: enabled
485
486uart_unregister_driver()
487	Remove all references to a driver from the core driver.  The low
488	level driver must have removed all its ports via the
489	uart_remove_one_port() if it registered them with uart_add_one_port().
490
491	Locking: none
492
493	Interrupts: enabled
494
495**uart_suspend_port()**
496
497**uart_resume_port()**
498
499**uart_add_one_port()**
500
501**uart_remove_one_port()**
502
503Other notes
504-----------
505
506It is intended some day to drop the 'unused' entries from uart_port, and
507allow low level drivers to register their own individual uart_port's with
508the core.  This will allow drivers to use uart_port as a pointer to a
509structure containing both the uart_port entry with their own extensions,
510thus::
511
512	struct my_port {
513		struct uart_port	port;
514		int			my_stuff;
515	};
516
517Modem control lines via GPIO
518----------------------------
519
520Some helpers are provided in order to set/get modem control lines via GPIO.
521
522mctrl_gpio_init(port, idx):
523	This will get the {cts,rts,...}-gpios from device tree if they are
524	present and request them, set direction etc, and return an
525	allocated structure. `devm_*` functions are used, so there's no need
526	to call mctrl_gpio_free().
527	As this sets up the irq handling make sure to not handle changes to the
528	gpio input lines in your driver, too.
529
530mctrl_gpio_free(dev, gpios):
531	This will free the requested gpios in mctrl_gpio_init().
532	As `devm_*` functions are used, there's generally no need to call
533	this function.
534
535mctrl_gpio_to_gpiod(gpios, gidx)
536	This returns the gpio_desc structure associated to the modem line
537	index.
538
539mctrl_gpio_set(gpios, mctrl):
540	This will sets the gpios according to the mctrl state.
541
542mctrl_gpio_get(gpios, mctrl):
543	This will update mctrl with the gpios values.
544
545mctrl_gpio_enable_ms(gpios):
546	Enables irqs and handling of changes to the ms lines.
547
548mctrl_gpio_disable_ms(gpios):
549	Disables irqs and handling of changes to the ms lines.
550