[linux.git] / Documentation / serial / driver


			Low Level Serial API
			--------------------


This document is meant as a brief overview of some aspects of the new serial
driver.  It is not complete, any questions you have should be directed to
<[email protected]>

The reference implementation is contained within amba_pl011.c.


Low Level Serial Hardware Driver
--------------------------------

The low level serial hardware driver is responsible for supplying port
information (defined by uart_port) and a set of control methods (defined
by uart_ops) to the core serial driver.  The low level driver is also
responsible for handling interrupts for the port, and providing any
console support.


Console Support
---------------

The serial core provides a few helper functions.  This includes identifing
the correct port structure (via uart_get_console) and decoding command line
arguments (uart_parse_options).

There is also a helper function (uart_write_console) which performs a
character by character write, translating newlines to CRLF sequences.
Driver writers are recommended to use this function rather than implementing
their own version.


Locking
-------

It is the responsibility of the low level hardware driver to perform the
necessary locking using port->lock.  There are some exceptions (which
are described in the uart_ops listing below.)

There are three locks.  A per-port spinlock, a per-port tmpbuf semaphore,
and an overall semaphore.

From the core driver perspective, the port->lock locks the following
data:

	port->mctrl
	port->icount
	info->xmit.head (circ->head)
	info->xmit.tail (circ->tail)

The low level driver is free to use this lock to provide any additional
locking.

The core driver uses the info->tmpbuf_sem lock to prevent multi-threaded
access to the info->tmpbuf bouncebuffer used for port writes.

The port_sem semaphore is used to protect against ports being added/
removed or reconfigured at inappropriate times. Since v2.6.27, this
semaphore has been the 'mutex' member of the tty_port struct, and
commonly referred to as the port mutex (or port->mutex).


uart_ops
--------

The uart_ops structure is the main interface between serial_core and the
hardware specific driver.  It contains all the methods to control the
hardware.

  tx_empty(port)
	This function tests whether the transmitter fifo and shifter
	for the port described by 'port' is empty.  If it is empty,
	this function should return TIOCSER_TEMT, otherwise return 0.
	If the port does not support this operation, then it should
	return TIOCSER_TEMT.

	Locking: none.
	Interrupts: caller dependent.
	This call must not sleep

  set_mctrl(port, mctrl)
	This function sets the modem control lines for port described
	by 'port' to the state described by mctrl.  The relevant bits
	of mctrl are:
		- TIOCM_RTS	RTS signal.
		- TIOCM_DTR	DTR signal.
		- TIOCM_OUT1	OUT1 signal.
		- TIOCM_OUT2	OUT2 signal.
		- TIOCM_LOOP	Set the port into loopback mode.
	If the appropriate bit is set, the signal should be driven
	active.  If the bit is clear, the signal should be driven
	inactive.

	Locking: port->lock taken.
	Interrupts: locally disabled.
	This call must not sleep

  get_mctrl(port)
	Returns the current state of modem control inputs.  The state
	of the outputs should not be returned, since the core keeps
	track of their state.  The state information should include:
		- TIOCM_CAR	state of DCD signal
		- TIOCM_CTS	state of CTS signal
		- TIOCM_DSR	state of DSR signal
		- TIOCM_RI	state of RI signal
	The bit is set if the signal is currently driven active.  If
	the port does not support CTS, DCD or DSR, the driver should
	indicate that the signal is permanently active.  If RI is
	not available, the signal should not be indicated as active.

	Locking: port->lock taken.
	Interrupts: locally disabled.
	This call must not sleep

  stop_tx(port)
	Stop transmitting characters.  This might be due to the CTS
	line becoming inactive or the tty layer indicating we want
	to stop transmission due to an XOFF character.

	The driver should stop transmitting characters as soon as
	possible.

	Locking: port->lock taken.
	Interrupts: locally disabled.
	This call must not sleep

  start_tx(port)
	Start transmitting characters.

	Locking: port->lock taken.
	Interrupts: locally disabled.
	This call must not sleep

  send_xchar(port,ch)
	Transmit a high priority character, even if the port is stopped.
	This is used to implement XON/XOFF flow control and tcflow().  If
	the serial driver does not implement this function, the tty core
	will append the character to the circular buffer and then call
	start_tx() / stop_tx() to flush the data out.

	Do not transmit if ch == '\0' (__DISABLED_CHAR).

	Locking: none.
	Interrupts: caller dependent.

  stop_rx(port)
	Stop receiving characters; the port is in the process of
	being closed.

	Locking: port->lock taken.
	Interrupts: locally disabled.
	This call must not sleep

  enable_ms(port)
	Enable the modem status interrupts.

	This method may be called multiple times.  Modem status
	interrupts should be disabled when the shutdown method is
	called.

	Locking: port->lock taken.
	Interrupts: locally disabled.
	This call must not sleep

  break_ctl(port,ctl)
	Control the transmission of a break signal.  If ctl is
	nonzero, the break signal should be transmitted.  The signal
	should be terminated when another call is made with a zero
	ctl.

	Locking: none.
	Interrupts: caller dependent.
	This call must not sleep

  startup(port)
	Grab any interrupt resources and initialise any low level driver
	state.  Enable the port for reception.  It should not activate
	RTS nor DTR; this will be done via a separate call to set_mctrl.

	This method will only be called when the port is initially opened.

	Locking: port_sem taken.
	Interrupts: globally disabled.

  shutdown(port)
	Disable the port, disable any break condition that may be in
	effect, and free any interrupt resources.  It should not disable
	RTS nor DTR; this will have already been done via a separate
	call to set_mctrl.

	Drivers must not access port->info once this call has completed.

	This method will only be called when there are no more users of
	this port.

	Locking: port_sem taken.
	Interrupts: caller dependent.

  flush_buffer(port)
	Flush any write buffers, reset any DMA state and stop any
	ongoing DMA transfers.

	This will be called whenever the port->info->xmit circular
	buffer is cleared.

	Locking: port->lock taken.
	Interrupts: locally disabled.
	This call must not sleep

  set_termios(port,termios,oldtermios)
	Change the port parameters, including word length, parity, stop
	bits.  Update read_status_mask and ignore_status_mask to indicate
	the types of events we are interested in receiving.  Relevant
	termios->c_cflag bits are:
		CSIZE	- word size
		CSTOPB	- 2 stop bits
		PARENB	- parity enable
		PARODD	- odd parity (when PARENB is in force)
		CREAD	- enable reception of characters (if not set,
			  still receive characters from the port, but
			  throw them away.
		CRTSCTS	- if set, enable CTS status change reporting
		CLOCAL	- if not set, enable modem status change
			  reporting.
	Relevant termios->c_iflag bits are:
		INPCK	- enable frame and parity error events to be
			  passed to the TTY layer.
		BRKINT
		PARMRK	- both of these enable break events to be
			  passed to the TTY layer.

		IGNPAR	- ignore parity and framing errors
		IGNBRK	- ignore break errors,  If IGNPAR is also
			  set, ignore overrun errors as well.
	The interaction of the iflag bits is as follows (parity error
	given as an example):
	Parity error	INPCK	IGNPAR
	n/a		0	n/a	character received, marked as
					TTY_NORMAL
	None		1	n/a	character received, marked as
					TTY_NORMAL
	Yes		1	0	character received, marked as
					TTY_PARITY
	Yes		1	1	character discarded

	Other flags may be used (eg, xon/xoff characters) if your
	hardware supports hardware "soft" flow control.

	Locking: caller holds port->mutex
	Interrupts: caller dependent.
	This call must not sleep

  pm(port,state,oldstate)
	Perform any power management related activities on the specified
	port.  State indicates the new state (defined by
	enum uart_pm_state), oldstate indicates the previous state.

	This function should not be used to grab any resources.

	This will be called when the port is initially opened and finally
	closed, except when the port is also the system console.  This
	will occur even if CONFIG_PM is not set.

	Locking: none.
	Interrupts: caller dependent.

  type(port)
	Return a pointer to a string constant describing the specified
	port, or return NULL, in which case the string 'unknown' is
	substituted.

	Locking: none.
	Interrupts: caller dependent.

  release_port(port)
	Release any memory and IO region resources currently in use by
	the port.

	Locking: none.
	Interrupts: caller dependent.

  request_port(port)
	Request any memory and IO region resources required by the port.
	If any fail, no resources should be registered when this function
	returns, and it should return -EBUSY on failure.

	Locking: none.
	Interrupts: caller dependent.

  config_port(port,type)
	Perform any autoconfiguration steps required for the port.  `type`
	contains a bit mask of the required configuration.  UART_CONFIG_TYPE
	indicates that the port requires detection and identification.
	port->type should be set to the type found, or PORT_UNKNOWN if
	no port was detected.

	UART_CONFIG_IRQ indicates autoconfiguration of the interrupt signal,
	which should be probed using standard kernel autoprobing techniques.
	This is not necessary on platforms where ports have interrupts
	internally hard wired (eg, system on a chip implementations).

	Locking: none.
	Interrupts: caller dependent.

  verify_port(port,serinfo)
	Verify the new serial port information contained within serinfo is
	suitable for this port type.

	Locking: none.
	Interrupts: caller dependent.

  ioctl(port,cmd,arg)
	Perform any port specific IOCTLs.  IOCTL commands must be defined
	using the standard numbering system found in <asm/ioctl.h>

	Locking: none.
	Interrupts: caller dependent.

  poll_init(port)
	Called by kgdb to perform the minimal hardware initialization needed
	to support poll_put_char() and poll_get_char().  Unlike ->startup()
	this should not request interrupts.

	Locking: tty_mutex and tty_port->mutex taken.
	Interrupts: n/a.

  poll_put_char(port,ch)
	Called by kgdb to write a single character directly to the serial
	port.  It can and should block until there is space in the TX FIFO.

	Locking: none.
	Interrupts: caller dependent.
	This call must not sleep

  poll_get_char(port)
	Called by kgdb to read a single character directly from the serial
	port.  If data is available, it should be returned; otherwise
	the function should return NO_POLL_CHAR immediately.

	Locking: none.
	Interrupts: caller dependent.
	This call must not sleep

Other functions
---------------

uart_update_timeout(port,cflag,baud)
	Update the FIFO drain timeout, port->timeout, according to the
	number of bits, parity, stop bits and baud rate.

	Locking: caller is expected to take port->lock
	Interrupts: n/a

uart_get_baud_rate(port,termios,old,min,max)
	Return the numeric baud rate for the specified termios, taking
	account of the special 38400 baud "kludge".  The B0 baud rate
	is mapped to 9600 baud.

	If the baud rate is not within min..max, then if old is non-NULL,
	the original baud rate will be tried.  If that exceeds the
	min..max constraint, 9600 baud will be returned.  termios will
	be updated to the baud rate in use.

	Note: min..max must always allow 9600 baud to be selected.

	Locking: caller dependent.
	Interrupts: n/a

uart_get_divisor(port,baud)
	Return the divsor (baud_base / baud) for the specified baud
	rate, appropriately rounded.

	If 38400 baud and custom divisor is selected, return the
	custom divisor instead.

	Locking: caller dependent.
	Interrupts: n/a

uart_match_port(port1,port2)
	This utility function can be used to determine whether two
	uart_port structures describe the same port.

	Locking: n/a
	Interrupts: n/a

uart_write_wakeup(port)
	A driver is expected to call this function when the number of
	characters in the transmit buffer have dropped below a threshold.

	Locking: port->lock should be held.
	Interrupts: n/a

uart_register_driver(drv)
	Register a uart driver with the core driver.  We in turn register
	with the tty layer, and initialise the core driver per-port state.

	drv->port should be NULL, and the per-port structures should be
	registered using uart_add_one_port after this call has succeeded.

	Locking: none
	Interrupts: enabled

uart_unregister_driver()
	Remove all references to a driver from the core driver.  The low
	level driver must have removed all its ports via the
	uart_remove_one_port() if it registered them with uart_add_one_port().

	Locking: none
	Interrupts: enabled

uart_suspend_port()

uart_resume_port()

uart_add_one_port()

uart_remove_one_port()

Other notes
-----------

It is intended some day to drop the 'unused' entries from uart_port, and
allow low level drivers to register their own individual uart_port's with
the core.  This will allow drivers to use uart_port as a pointer to a
structure containing both the uart_port entry with their own extensions,
thus:

	struct my_port {
		struct uart_port	port;
		int			my_stuff;
	};

Modem control lines via GPIO
----------------------------

Some helpers are provided in order to set/get modem control lines via GPIO.

mctrl_gpio_init(dev, idx):
	This will get the {cts,rts,...}-gpios from device tree if they are
	present and request them, set direction etc, and return an
	allocated structure. devm_* functions are used, so there's no need
	to call mctrl_gpio_free().

mctrl_gpio_free(dev, gpios):
	This will free the requested gpios in mctrl_gpio_init().
	As devm_* function are used, there's generally no need to call
	this function.

mctrl_gpio_to_gpiod(gpios, gidx)
	This returns the gpio structure associated to the modem line index.

mctrl_gpio_set(gpios, mctrl):
	This will sets the gpios according to the mctrl state.

mctrl_gpio_get(gpios, mctrl):
	This will update mctrl with the gpios values.
Commit	Line	Data
1da177e4 LT	1
	2	Low Level Serial API
	3	--------------------
	4
	5
1da177e4 LT	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
	8	<[email protected]>
	9
67ab7f59	10	The reference implementation is contained within amba_pl011.c.
1da177e4 LT	11
	12
	13
	14	Low Level Serial Hardware Driver
	15	--------------------------------
	16
	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
	21	console support.
	22
	23
	24	Console Support
	25	---------------
	26
	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).
	30
67ab7f59 RK	31	There is also a helper function (uart_write_console) which performs a
	32	character by character write, translating newlines to CRLF sequences.
	33	Driver writers are recommended to use this function rather than implementing
	34	their own version.
	35
1da177e4 LT	36
	37	Locking
	38	-------
	39
	40	It is the responsibility of the low level hardware driver to perform the
	41	necessary locking using port->lock. There are some exceptions (which
	42	are described in the uart_ops listing below.)
	43
	44	There are three locks. A per-port spinlock, a per-port tmpbuf semaphore,
	45	and an overall semaphore.
	46
	47	From the core driver perspective, the port->lock locks the following
	48	data:
	49
	50	port->mctrl
	51	port->icount
	52	info->xmit.head (circ->head)
	53	info->xmit.tail (circ->tail)
	54
	55	The low level driver is free to use this lock to provide any additional
	56	locking.
	57
	58	The core driver uses the info->tmpbuf_sem lock to prevent multi-threaded
	59	access to the info->tmpbuf bouncebuffer used for port writes.
	60
	61	The port_sem semaphore is used to protect against ports being added/
7c8ab967 PH	62	removed or reconfigured at inappropriate times. Since v2.6.27, this
	63	semaphore has been the 'mutex' member of the tty_port struct, and
	64	commonly referred to as the port mutex (or port->mutex).
1da177e4 LT	65
	66
	67	uart_ops
	68	--------
	69
	70	The uart_ops structure is the main interface between serial_core and the
	71	hardware specific driver. It contains all the methods to control the
	72	hardware.
	73
	74	tx_empty(port)
	75	This function tests whether the transmitter fifo and shifter
	76	for the port described by 'port' is empty. If it is empty,
	77	this function should return TIOCSER_TEMT, otherwise return 0.
	78	If the port does not support this operation, then it should
	79	return TIOCSER_TEMT.
	80
	81	Locking: none.
	82	Interrupts: caller dependent.
	83	This call must not sleep
	84
	85	set_mctrl(port, mctrl)
	86	This function sets the modem control lines for port described
	87	by 'port' to the state described by mctrl. The relevant bits
	88	of mctrl are:
	89	- TIOCM_RTS RTS signal.
	90	- TIOCM_DTR DTR signal.
	91	- TIOCM_OUT1 OUT1 signal.
	92	- TIOCM_OUT2 OUT2 signal.
67ab7f59	93	- TIOCM_LOOP Set the port into loopback mode.
1da177e4 LT	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	Interrupts: locally disabled.
	100	This call must not sleep
	101
	102	get_mctrl(port)
	103	Returns the current state of modem control inputs. The state
	104	of the outputs should not be returned, since the core keeps
	105	track of their state. The state information should include:
343fe407	106	- TIOCM_CAR state of DCD signal
1da177e4 LT	107	- TIOCM_CTS state of CTS signal
	108	- TIOCM_DSR state of DSR signal
	109	- TIOCM_RI state of RI signal
	110	The bit is set if the signal is currently driven active. If
	111	the port does not support CTS, DCD or DSR, the driver should
	112	indicate that the signal is permanently active. If RI is
	113	not available, the signal should not be indicated as active.
	114
c5f4644e RK	115	Locking: port->lock taken.
c5f4644e RK	116	Interrupts: locally disabled.
1da177e4 LT	117	This call must not sleep
1da177e4 LT	118
b129a8cc	119	stop_tx(port)
1da177e4 LT	120	Stop transmitting characters. This might be due to the CTS
1da177e4 LT	121	line becoming inactive or the tty layer indicating we want
b129a8cc	122	to stop transmission due to an XOFF character.
1da177e4	123
6a8f8d72 RK	124	The driver should stop transmitting characters as soon as
	125	possible.
	126
1da177e4 LT	127	Locking: port->lock taken.
	128	Interrupts: locally disabled.
	129	This call must not sleep
	130
b129a8cc	131	start_tx(port)
6a8f8d72	132	Start transmitting characters.
1da177e4 LT	133
	134	Locking: port->lock taken.
	135	Interrupts: locally disabled.
	136	This call must not sleep
	137
e759d7c5 KC	138	send_xchar(port,ch)
	139	Transmit a high priority character, even if the port is stopped.
	140	This is used to implement XON/XOFF flow control and tcflow(). If
	141	the serial driver does not implement this function, the tty core
	142	will append the character to the circular buffer and then call
	143	start_tx() / stop_tx() to flush the data out.
	144
db106df3 PH	145	Do not transmit if ch == '\0' (__DISABLED_CHAR).
db106df3 PH	146
e759d7c5 KC	147	Locking: none.
	148	Interrupts: caller dependent.
	149
1da177e4 LT	150	stop_rx(port)
	151	Stop receiving characters; the port is in the process of
	152	being closed.
	153
	154	Locking: port->lock taken.
	155	Interrupts: locally disabled.
	156	This call must not sleep
	157
	158	enable_ms(port)
	159	Enable the modem status interrupts.
	160
67ab7f59 RK	161	This method may be called multiple times. Modem status
	162	interrupts should be disabled when the shutdown method is
	163	called.
	164
1da177e4 LT	165	Locking: port->lock taken.
	166	Interrupts: locally disabled.
	167	This call must not sleep
	168
	169	break_ctl(port,ctl)
	170	Control the transmission of a break signal. If ctl is
	171	nonzero, the break signal should be transmitted. The signal
	172	should be terminated when another call is made with a zero
	173	ctl.
	174
	175	Locking: none.
	176	Interrupts: caller dependent.
	177	This call must not sleep
	178
	179	startup(port)
	180	Grab any interrupt resources and initialise any low level driver
	181	state. Enable the port for reception. It should not activate
	182	RTS nor DTR; this will be done via a separate call to set_mctrl.
	183
67ab7f59 RK	184	This method will only be called when the port is initially opened.
67ab7f59 RK	185
1da177e4 LT	186	Locking: port_sem taken.
	187	Interrupts: globally disabled.
	188
	189	shutdown(port)
	190	Disable the port, disable any break condition that may be in
	191	effect, and free any interrupt resources. It should not disable
	192	RTS nor DTR; this will have already been done via a separate
	193	call to set_mctrl.
	194
67ab7f59 RK	195	Drivers must not access port->info once this call has completed.
	196
	197	This method will only be called when there are no more users of
	198	this port.
	199
1da177e4 LT	200	Locking: port_sem taken.
	201	Interrupts: caller dependent.
	202
6bb0e3a5 HS	203	flush_buffer(port)
	204	Flush any write buffers, reset any DMA state and stop any
	205	ongoing DMA transfers.
	206
	207	This will be called whenever the port->info->xmit circular
	208	buffer is cleared.
	209
	210	Locking: port->lock taken.
	211	Interrupts: locally disabled.
	212	This call must not sleep
	213
1da177e4 LT	214	set_termios(port,termios,oldtermios)
	215	Change the port parameters, including word length, parity, stop
	216	bits. Update read_status_mask and ignore_status_mask to indicate
	217	the types of events we are interested in receiving. Relevant
	218	termios->c_cflag bits are:
	219	CSIZE - word size
	220	CSTOPB - 2 stop bits
	221	PARENB - parity enable
	222	PARODD - odd parity (when PARENB is in force)
	223	CREAD - enable reception of characters (if not set,
	224	still receive characters from the port, but
	225	throw them away.
	226	CRTSCTS - if set, enable CTS status change reporting
	227	CLOCAL - if not set, enable modem status change
	228	reporting.
	229	Relevant termios->c_iflag bits are:
	230	INPCK - enable frame and parity error events to be
	231	passed to the TTY layer.
	232	BRKINT
	233	PARMRK - both of these enable break events to be
	234	passed to the TTY layer.
	235
	236	IGNPAR - ignore parity and framing errors
	237	IGNBRK - ignore break errors, If IGNPAR is also
	238	set, ignore overrun errors as well.
	239	The interaction of the iflag bits is as follows (parity error
	240	given as an example):
	241	Parity error INPCK IGNPAR
89f3da3e	242	n/a 0 n/a character received, marked as
1da177e4	243	TTY_NORMAL
89f3da3e PK	244	None 1 n/a character received, marked as
	245	TTY_NORMAL
	246	Yes 1 0 character received, marked as
1da177e4	247	TTY_PARITY
89f3da3e	248	Yes 1 1 character discarded
1da177e4 LT	249
	250	Other flags may be used (eg, xon/xoff characters) if your
	251	hardware supports hardware "soft" flow control.
	252
7c8ab967	253	Locking: caller holds port->mutex
1da177e4 LT	254	Interrupts: caller dependent.
	255	This call must not sleep
	256
	257	pm(port,state,oldstate)
	258	Perform any power management related activities on the specified
6f538fe3 LW	259	port. State indicates the new state (defined by
6f538fe3 LW	260	enum uart_pm_state), oldstate indicates the previous state.
1da177e4 LT	261
	262	This function should not be used to grab any resources.
	263
	264	This will be called when the port is initially opened and finally
	265	closed, except when the port is also the system console. This
	266	will occur even if CONFIG_PM is not set.
	267
	268	Locking: none.
	269	Interrupts: caller dependent.
	270
	271	type(port)
	272	Return a pointer to a string constant describing the specified
	273	port, or return NULL, in which case the string 'unknown' is
	274	substituted.
	275
	276	Locking: none.
	277	Interrupts: caller dependent.
	278
	279	release_port(port)
	280	Release any memory and IO region resources currently in use by
	281	the port.
	282
	283	Locking: none.
	284	Interrupts: caller dependent.
	285
	286	request_port(port)
	287	Request any memory and IO region resources required by the port.
	288	If any fail, no resources should be registered when this function
	289	returns, and it should return -EBUSY on failure.
	290
	291	Locking: none.
	292	Interrupts: caller dependent.
	293
	294	config_port(port,type)
	295	Perform any autoconfiguration steps required for the port. `type`
	296	contains a bit mask of the required configuration. UART_CONFIG_TYPE
	297	indicates that the port requires detection and identification.
	298	port->type should be set to the type found, or PORT_UNKNOWN if
	299	no port was detected.
	300
	301	UART_CONFIG_IRQ indicates autoconfiguration of the interrupt signal,
	302	which should be probed using standard kernel autoprobing techniques.
	303	This is not necessary on platforms where ports have interrupts
	304	internally hard wired (eg, system on a chip implementations).
	305
	306	Locking: none.
	307	Interrupts: caller dependent.
	308
	309	verify_port(port,serinfo)
	310	Verify the new serial port information contained within serinfo is
	311	suitable for this port type.
	312
	313	Locking: none.
	314	Interrupts: caller dependent.
	315
	316	ioctl(port,cmd,arg)
	317	Perform any port specific IOCTLs. IOCTL commands must be defined
	318	using the standard numbering system found in <asm/ioctl.h>
	319
	320	Locking: none.
	321	Interrupts: caller dependent.
	322
e759d7c5 KC	323	poll_init(port)
	324	Called by kgdb to perform the minimal hardware initialization needed
	325	to support poll_put_char() and poll_get_char(). Unlike ->startup()
	326	this should not request interrupts.
	327
	328	Locking: tty_mutex and tty_port->mutex taken.
	329	Interrupts: n/a.
	330
	331	poll_put_char(port,ch)
	332	Called by kgdb to write a single character directly to the serial
	333	port. It can and should block until there is space in the TX FIFO.
	334
	335	Locking: none.
	336	Interrupts: caller dependent.
	337	This call must not sleep
	338
	339	poll_get_char(port)
	340	Called by kgdb to read a single character directly from the serial
	341	port. If data is available, it should be returned; otherwise
	342	the function should return NO_POLL_CHAR immediately.
	343
	344	Locking: none.
	345	Interrupts: caller dependent.
	346	This call must not sleep
	347
1da177e4 LT	348	Other functions
	349	---------------
	350
6a8f8d72	351	uart_update_timeout(port,cflag,baud)
1da177e4	352	Update the FIFO drain timeout, port->timeout, according to the
6a8f8d72	353	number of bits, parity, stop bits and baud rate.
1da177e4 LT	354
	355	Locking: caller is expected to take port->lock
	356	Interrupts: n/a
	357
6a8f8d72	358	uart_get_baud_rate(port,termios,old,min,max)
1da177e4 LT	359	Return the numeric baud rate for the specified termios, taking
	360	account of the special 38400 baud "kludge". The B0 baud rate
	361	is mapped to 9600 baud.
	362
6a8f8d72 RK	363	If the baud rate is not within min..max, then if old is non-NULL,
	364	the original baud rate will be tried. If that exceeds the
	365	min..max constraint, 9600 baud will be returned. termios will
	366	be updated to the baud rate in use.
	367
	368	Note: min..max must always allow 9600 baud to be selected.
	369
1da177e4 LT	370	Locking: caller dependent.
	371	Interrupts: n/a
	372
6a8f8d72 RK	373	uart_get_divisor(port,baud)
	374	Return the divsor (baud_base / baud) for the specified baud
	375	rate, appropriately rounded.
1da177e4 LT	376
	377	If 38400 baud and custom divisor is selected, return the
	378	custom divisor instead.
	379
	380	Locking: caller dependent.
	381	Interrupts: n/a
	382
6a8f8d72 RK	383	uart_match_port(port1,port2)
	384	This utility function can be used to determine whether two
	385	uart_port structures describe the same port.
	386
	387	Locking: n/a
	388	Interrupts: n/a
	389
	390	uart_write_wakeup(port)
	391	A driver is expected to call this function when the number of
	392	characters in the transmit buffer have dropped below a threshold.
	393
	394	Locking: port->lock should be held.
	395	Interrupts: n/a
	396
	397	uart_register_driver(drv)
	398	Register a uart driver with the core driver. We in turn register
	399	with the tty layer, and initialise the core driver per-port state.
	400
	401	drv->port should be NULL, and the per-port structures should be
	402	registered using uart_add_one_port after this call has succeeded.
	403
	404	Locking: none
	405	Interrupts: enabled
	406
	407	uart_unregister_driver()
	408	Remove all references to a driver from the core driver. The low
	409	level driver must have removed all its ports via the
	410	uart_remove_one_port() if it registered them with uart_add_one_port().
	411
	412	Locking: none
	413	Interrupts: enabled
	414
	415	uart_suspend_port()
	416
	417	uart_resume_port()
	418
	419	uart_add_one_port()
	420
	421	uart_remove_one_port()
	422
1da177e4 LT	423	Other notes
	424	-----------
	425
	426	It is intended some day to drop the 'unused' entries from uart_port, and
	427	allow low level drivers to register their own individual uart_port's with
	428	the core. This will allow drivers to use uart_port as a pointer to a
	429	structure containing both the uart_port entry with their own extensions,
	430	thus:
	431
	432	struct my_port {
	433	struct uart_port port;
	434	int my_stuff;
	435	};
84130aac RG	436
	437	Modem control lines via GPIO
	438	----------------------------
	439
	440	Some helpers are provided in order to set/get modem control lines via GPIO.
	441
	442	mctrl_gpio_init(dev, idx):
	443	This will get the {cts,rts,...}-gpios from device tree if they are
	444	present and request them, set direction etc, and return an
	445	allocated structure. devm_* functions are used, so there's no need
	446	to call mctrl_gpio_free().
	447
	448	mctrl_gpio_free(dev, gpios):
	449	This will free the requested gpios in mctrl_gpio_init().
	450	As devm_* function are used, there's generally no need to call
	451	this function.
	452
	453	mctrl_gpio_to_gpiod(gpios, gidx)
	454	This returns the gpio structure associated to the modem line index.
	455
	456	mctrl_gpio_set(gpios, mctrl):
	457	This will sets the gpios according to the mctrl state.
	458
	459	mctrl_gpio_get(gpios, mctrl):
	460	This will update mctrl with the gpios values.