[deliverable/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
<rmk@arm.linux.org.uk>

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.


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_DCD	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

  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.

  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
	None		n/a	n/a	character received
	Yes		n/a	0	character discarded
	Yes		0	1	character received, marked as
					TTY_NORMAL
	Yes		1	1	character received, marked as
					TTY_PARITY

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

	Locking: none.
	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 ACPI D0-D3),
	oldstate indicates the previous state.  Essentially, D0 means
	fully on, D3 means powered down.

	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.

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;
	};
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	<rmk@arm.linux.org.uk>
	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/
	62	removed or reconfigured at inappropriate times.
	63
	64
	65	uart_ops
	66	--------
	67
	68	The uart_ops structure is the main interface between serial_core and the
	69	hardware specific driver. It contains all the methods to control the
	70	hardware.
	71
	72	tx_empty(port)
	73	This function tests whether the transmitter fifo and shifter
	74	for the port described by 'port' is empty. If it is empty,
	75	this function should return TIOCSER_TEMT, otherwise return 0.
	76	If the port does not support this operation, then it should
	77	return TIOCSER_TEMT.
	78
	79	Locking: none.
	80	Interrupts: caller dependent.
	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	- TIOCM_RTS RTS signal.
	88	- TIOCM_DTR DTR signal.
	89	- TIOCM_OUT1 OUT1 signal.
	90	- TIOCM_OUT2 OUT2 signal.
67ab7f59	91	- TIOCM_LOOP Set the port into loopback mode.
1da177e4 LT	92	If the appropriate bit is set, the signal should be driven
	93	active. If the bit is clear, the signal should be driven
	94	inactive.
	95
	96	Locking: port->lock taken.
	97	Interrupts: locally disabled.
	98	This call must not sleep
	99
	100	get_mctrl(port)
	101	Returns the current state of modem control inputs. The state
	102	of the outputs should not be returned, since the core keeps
	103	track of their state. The state information should include:
	104	- TIOCM_DCD state of DCD signal
	105	- TIOCM_CTS state of CTS signal
	106	- TIOCM_DSR state of DSR signal
	107	- TIOCM_RI state of RI signal
	108	The bit is set if the signal is currently driven active. If
	109	the port does not support CTS, DCD or DSR, the driver should
	110	indicate that the signal is permanently active. If RI is
	111	not available, the signal should not be indicated as active.
	112
c5f4644e RK	113	Locking: port->lock taken.
c5f4644e RK	114	Interrupts: locally disabled.
1da177e4 LT	115	This call must not sleep
1da177e4 LT	116
b129a8cc	117	stop_tx(port)
1da177e4 LT	118	Stop transmitting characters. This might be due to the CTS
1da177e4 LT	119	line becoming inactive or the tty layer indicating we want
b129a8cc	120	to stop transmission due to an XOFF character.
1da177e4	121
6a8f8d72 RK	122	The driver should stop transmitting characters as soon as
	123	possible.
	124
1da177e4 LT	125	Locking: port->lock taken.
	126	Interrupts: locally disabled.
	127	This call must not sleep
	128
b129a8cc	129	start_tx(port)
6a8f8d72	130	Start transmitting characters.
1da177e4 LT	131
	132	Locking: port->lock taken.
	133	Interrupts: locally disabled.
	134	This call must not sleep
	135
	136	stop_rx(port)
	137	Stop receiving characters; the port is in the process of
	138	being closed.
	139
	140	Locking: port->lock taken.
	141	Interrupts: locally disabled.
	142	This call must not sleep
	143
	144	enable_ms(port)
	145	Enable the modem status interrupts.
	146
67ab7f59 RK	147	This method may be called multiple times. Modem status
	148	interrupts should be disabled when the shutdown method is
	149	called.
	150
1da177e4 LT	151	Locking: port->lock taken.
	152	Interrupts: locally disabled.
	153	This call must not sleep
	154
	155	break_ctl(port,ctl)
	156	Control the transmission of a break signal. If ctl is
	157	nonzero, the break signal should be transmitted. The signal
	158	should be terminated when another call is made with a zero
	159	ctl.
	160
	161	Locking: none.
	162	Interrupts: caller dependent.
	163	This call must not sleep
	164
	165	startup(port)
	166	Grab any interrupt resources and initialise any low level driver
	167	state. Enable the port for reception. It should not activate
	168	RTS nor DTR; this will be done via a separate call to set_mctrl.
	169
67ab7f59 RK	170	This method will only be called when the port is initially opened.
67ab7f59 RK	171
1da177e4 LT	172	Locking: port_sem taken.
	173	Interrupts: globally disabled.
	174
	175	shutdown(port)
	176	Disable the port, disable any break condition that may be in
	177	effect, and free any interrupt resources. It should not disable
	178	RTS nor DTR; this will have already been done via a separate
	179	call to set_mctrl.
	180
67ab7f59 RK	181	Drivers must not access port->info once this call has completed.
	182
	183	This method will only be called when there are no more users of
	184	this port.
	185
1da177e4 LT	186	Locking: port_sem taken.
	187	Interrupts: caller dependent.
	188
	189	set_termios(port,termios,oldtermios)
	190	Change the port parameters, including word length, parity, stop
	191	bits. Update read_status_mask and ignore_status_mask to indicate
	192	the types of events we are interested in receiving. Relevant
	193	termios->c_cflag bits are:
	194	CSIZE - word size
	195	CSTOPB - 2 stop bits
	196	PARENB - parity enable
	197	PARODD - odd parity (when PARENB is in force)
	198	CREAD - enable reception of characters (if not set,
	199	still receive characters from the port, but
	200	throw them away.
	201	CRTSCTS - if set, enable CTS status change reporting
	202	CLOCAL - if not set, enable modem status change
	203	reporting.
	204	Relevant termios->c_iflag bits are:
	205	INPCK - enable frame and parity error events to be
	206	passed to the TTY layer.
	207	BRKINT
	208	PARMRK - both of these enable break events to be
	209	passed to the TTY layer.
	210
	211	IGNPAR - ignore parity and framing errors
	212	IGNBRK - ignore break errors, If IGNPAR is also
	213	set, ignore overrun errors as well.
	214	The interaction of the iflag bits is as follows (parity error
	215	given as an example):
	216	Parity error INPCK IGNPAR
	217	None n/a n/a character received
	218	Yes n/a 0 character discarded
	219	Yes 0 1 character received, marked as
	220	TTY_NORMAL
	221	Yes 1 1 character received, marked as
	222	TTY_PARITY
	223
	224	Other flags may be used (eg, xon/xoff characters) if your
	225	hardware supports hardware "soft" flow control.
	226
	227	Locking: none.
	228	Interrupts: caller dependent.
	229	This call must not sleep
	230
	231	pm(port,state,oldstate)
	232	Perform any power management related activities on the specified
	233	port. State indicates the new state (defined by ACPI D0-D3),
	234	oldstate indicates the previous state. Essentially, D0 means
	235	fully on, D3 means powered down.
	236
	237	This function should not be used to grab any resources.
	238
	239	This will be called when the port is initially opened and finally
	240	closed, except when the port is also the system console. This
	241	will occur even if CONFIG_PM is not set.
	242
	243	Locking: none.
	244	Interrupts: caller dependent.
	245
	246	type(port)
	247	Return a pointer to a string constant describing the specified
	248	port, or return NULL, in which case the string 'unknown' is
	249	substituted.
250
251	Locking: none.
252	Interrupts: caller dependent.
253
254	release_port(port)
255	Release any memory and IO region resources currently in use by
256	the port.
257
258	Locking: none.
259	Interrupts: caller dependent.
260
261	request_port(port)
262	Request any memory and IO region resources required by the port.
263	If any fail, no resources should be registered when this function
264	returns, and it should return -EBUSY on failure.
265
266	Locking: none.
267	Interrupts: caller dependent.
268
269	config_port(port,type)
270	Perform any autoconfiguration steps required for the port. `type`
271	contains a bit mask of the required configuration. UART_CONFIG_TYPE
272	indicates that the port requires detection and identification.
273	port->type should be set to the type found, or PORT_UNKNOWN if
274	no port was detected.
275
276	UART_CONFIG_IRQ indicates autoconfiguration of the interrupt signal,
277	which should be probed using standard kernel autoprobing techniques.
278	This is not necessary on platforms where ports have interrupts
279	internally hard wired (eg, system on a chip implementations).
280
281	Locking: none.
282	Interrupts: caller dependent.
283
284	verify_port(port,serinfo)
285	Verify the new serial port information contained within serinfo is
286	suitable for this port type.
287
288	Locking: none.
289	Interrupts: caller dependent.
290
291	ioctl(port,cmd,arg)
292	Perform any port specific IOCTLs. IOCTL commands must be defined
293	using the standard numbering system found in <asm/ioctl.h>
294
295	Locking: none.
296	Interrupts: caller dependent.
297
298	Other functions
299	---------------
300
6a8f8d72	301	uart_update_timeout(port,cflag,baud)
1da177e4	302	Update the FIFO drain timeout, port->timeout, according to the
6a8f8d72	303	number of bits, parity, stop bits and baud rate.
1da177e4 LT	304
	305	Locking: caller is expected to take port->lock
	306	Interrupts: n/a
	307
6a8f8d72	308	uart_get_baud_rate(port,termios,old,min,max)
1da177e4 LT	309	Return the numeric baud rate for the specified termios, taking
	310	account of the special 38400 baud "kludge". The B0 baud rate
	311	is mapped to 9600 baud.
	312
6a8f8d72 RK	313	If the baud rate is not within min..max, then if old is non-NULL,
	314	the original baud rate will be tried. If that exceeds the
	315	min..max constraint, 9600 baud will be returned. termios will
	316	be updated to the baud rate in use.
	317
	318	Note: min..max must always allow 9600 baud to be selected.
	319
1da177e4 LT	320	Locking: caller dependent.
	321	Interrupts: n/a
	322
6a8f8d72 RK	323	uart_get_divisor(port,baud)
	324	Return the divsor (baud_base / baud) for the specified baud
	325	rate, appropriately rounded.
1da177e4 LT	326
	327	If 38400 baud and custom divisor is selected, return the
	328	custom divisor instead.
	329
	330	Locking: caller dependent.
	331	Interrupts: n/a
	332
6a8f8d72 RK	333	uart_match_port(port1,port2)
	334	This utility function can be used to determine whether two
	335	uart_port structures describe the same port.
	336
	337	Locking: n/a
	338	Interrupts: n/a
	339
	340	uart_write_wakeup(port)
	341	A driver is expected to call this function when the number of
	342	characters in the transmit buffer have dropped below a threshold.
	343
	344	Locking: port->lock should be held.
	345	Interrupts: n/a
	346
	347	uart_register_driver(drv)
	348	Register a uart driver with the core driver. We in turn register
	349	with the tty layer, and initialise the core driver per-port state.
	350
	351	drv->port should be NULL, and the per-port structures should be
	352	registered using uart_add_one_port after this call has succeeded.
	353
	354	Locking: none
	355	Interrupts: enabled
	356
	357	uart_unregister_driver()
	358	Remove all references to a driver from the core driver. The low
	359	level driver must have removed all its ports via the
	360	uart_remove_one_port() if it registered them with uart_add_one_port().
	361
	362	Locking: none
	363	Interrupts: enabled
	364
	365	uart_suspend_port()
	366
	367	uart_resume_port()
	368
	369	uart_add_one_port()
	370
	371	uart_remove_one_port()
	372
1da177e4 LT	373	Other notes
	374	-----------
	375
	376	It is intended some day to drop the 'unused' entries from uart_port, and
	377	allow low level drivers to register their own individual uart_port's with
	378	the core. This will allow drivers to use uart_port as a pointer to a
	379	structure containing both the uart_port entry with their own extensions,
	380	thus:
	381
	382	struct my_port {
	383	struct uart_port port;
	384	int my_stuff;
	385	};