Documentation/tty.txt - kernel/msm - Gitiles


 			The Lockronomicon

 Your guide to the ancient and twisted locking policies of the tty layer and
 the warped logic behind them. Beware all ye who read on.

 FIXME: still need to work out the full set of BKL assumptions and document
 them so they can eventually be killed off.


 Line Discipline
 ---------------

 Line disciplines are registered with tty_register_ldisc() passing the
 discipline number and the ldisc structure. At the point of registration the
 discipline must be ready to use and it is possible it will get used before
 the call returns success. If the call returns an error then it won't get
 called. Do not re-use ldisc numbers as they are part of the userspace ABI
 and writing over an existing ldisc will cause demons to eat your computer.
 After the return the ldisc data has been copied so you may free your own
 copy of the structure. You must not re-register over the top of the line
 discipline even with the same data or your computer again will be eaten by
 demons.

 In order to remove a line discipline call tty_unregister_ldisc().
 In ancient times this always worked. In modern times the function will
 return -EBUSY if the ldisc is currently in use. Since the ldisc referencing
 code manages the module counts this should not usually be a concern.

 Heed this warning: the reference count field of the registered copies of the
 tty_ldisc structure in the ldisc table counts the number of lines using this
 discipline. The reference count of the tty_ldisc structure within a tty
 counts the number of active users of the ldisc at this instant. In effect it
 counts the number of threads of execution within an ldisc method (plus those
 about to enter and exit although this detail matters not).

 Line Discipline Methods
 -----------------------

 TTY side interfaces:

 close()		-	This is called on a terminal when the line
 			discipline is being unplugged. At the point of
 			execution no further users will enter the
 			ldisc code for this tty. Can sleep.

 open()		-	Called when the line discipline is attached to
 			the terminal. No other call into the line
 			discipline for this tty will occur until it
 			completes successfully. Can sleep.

 write()		-	A process is writing data through the line
 			discipline.  Multiple write calls are serialized
 			by the tty layer for the ldisc.  May sleep.

 flush_buffer()	-	May be called at any point between open and close.

 chars_in_buffer() -	Report the number of bytes in the buffer.

 set_termios()	-	Called on termios structure changes. The caller
 			passes the old termios data and the current data
 			is in the tty. Called under the termios semaphore so
 			allowed to sleep. Serialized against itself only.

 read()		-	Move data from the line discipline to the user.
 			Multiple read calls may occur in parallel and the
 			ldisc must deal with serialization issues. May
 			sleep.

 poll()		-	Check the status for the poll/select calls. Multiple
 			poll calls may occur in parallel. May sleep.

 ioctl()		-	Called when an ioctl is handed to the tty layer
 			that might be for the ldisc. Multiple ioctl calls
 			may occur in parallel. May sleep.

 Driver Side Interfaces:

 receive_buf()	-	Hand buffers of bytes from the driver to the ldisc
 			for processing. Semantics currently rather
 			mysterious 8(

 receive_room()	-	Can be called by the driver layer at any time when
 			the ldisc is opened. The ldisc must be able to
 			handle the reported amount of data at that instant.
 			Synchronization between active receive_buf and
 			receive_room calls is down to the driver not the
 			ldisc. Must not sleep.

 write_wakeup()	-	May be called at any point between open and close.
 			The TTY_DO_WRITE_WAKEUP flag indicates if a call
 			is needed but always races versus calls. Thus the
 			ldisc must be careful about setting order and to
 			handle unexpected calls. Must not sleep.

 			The driver is forbidden from calling this directly
 			from the ->write call from the ldisc as the ldisc
 			is permitted to call the driver write method from
 			this function. In such a situation defer it.


 Locking

 Callers to the line discipline functions from the tty layer are required to
 take line discipline locks. The same is true of calls from the driver side
 but not yet enforced.

 Three calls are now provided

 	ldisc = tty_ldisc_ref(tty);

 takes a handle to the line discipline in the tty and returns it. If no ldisc
 is currently attached or the ldisc is being closed and re-opened at this
 point then NULL is returned. While this handle is held the ldisc will not
 change or go away.

 	tty_ldisc_deref(ldisc)

 Returns the ldisc reference and allows the ldisc to be closed. Returning the
 reference takes away your right to call the ldisc functions until you take
 a new reference.

 	ldisc = tty_ldisc_ref_wait(tty);

 Performs the same function as tty_ldisc_ref except that it will wait for an
 ldisc change to complete and then return a reference to the new ldisc.

 While these functions are slightly slower than the old code they should have
 minimal impact as most receive logic uses the flip buffers and they only
 need to take a reference when they push bits up through the driver.

 A caution: The ldisc->open(), ldisc->close() and driver->set_ldisc
 functions are called with the ldisc unavailable. Thus tty_ldisc_ref will
 fail in this situation if used within these functions. Ldisc and driver
 code calling its own functions must be careful in this case.


 Driver Interface
 ----------------

 open()		-	Called when a device is opened. May sleep

 close()		-	Called when a device is closed. At the point of
 			return from this call the driver must make no
 			further ldisc calls of any kind. May sleep

 write()		-	Called to write bytes to the device. May not
 			sleep. May occur in parallel in special cases.
 			Because this includes panic paths drivers generally
 			shouldn't try and do clever locking here.

 put_char()	-	Stuff a single character onto the queue. The
 			driver is guaranteed following up calls to
 			flush_chars.

 flush_chars()	-	Ask the kernel to write put_char queue

 write_room()	-	Return the number of characters tht can be stuffed
 			into the port buffers without overflow (or less).
 			The ldisc is responsible for being intelligent
  			about multi-threading of write_room/write calls

 ioctl()		-	Called when an ioctl may be for the driver

 set_termios()	-	Called on termios change, serialized against
 			itself by a semaphore. May sleep.

 set_ldisc()	-	Notifier for discipline change. At the point this
 			is done the discipline is not yet usable. Can now
 			sleep (I think)

 throttle()	-	Called by the ldisc to ask the driver to do flow
 			control.  Serialization including with unthrottle
 			is the job of the ldisc layer.

 unthrottle()	-	Called by the ldisc to ask the driver to stop flow
 			control.

 stop()		-	Ldisc notifier to the driver to stop output. As with
 			throttle the serializations with start() are down
 			to the ldisc layer.

 start()		-	Ldisc notifier to the driver to start output.

 hangup()	-	Ask the tty driver to cause a hangup initiated
 			from the host side. [Can sleep ??]

 break_ctl()	-	Send RS232 break. Can sleep. Can get called in
 			parallel, driver must serialize (for now), and
 			with write calls.

 wait_until_sent() -	Wait for characters to exit the hardware queue
 			of the driver. Can sleep

 send_xchar()	  -	Send XON/XOFF and if possible jump the queue with
 			it in order to get fast flow control responses.
 			Cannot sleep ??

	The Lockronomicon

	Your guide to the ancient and twisted locking policies of the tty layer and
	the warped logic behind them. Beware all ye who read on.

	FIXME: still need to work out the full set of BKL assumptions and document
	them so they can eventually be killed off.


	Line Discipline
	---------------

	Line disciplines are registered with tty_register_ldisc() passing the
	discipline number and the ldisc structure. At the point of registration the
	discipline must be ready to use and it is possible it will get used before
	the call returns success. If the call returns an error then it won't get
	called. Do not re-use ldisc numbers as they are part of the userspace ABI
	and writing over an existing ldisc will cause demons to eat your computer.
	After the return the ldisc data has been copied so you may free your own
	copy of the structure. You must not re-register over the top of the line
	discipline even with the same data or your computer again will be eaten by
	demons.

	In order to remove a line discipline call tty_unregister_ldisc().
	In ancient times this always worked. In modern times the function will
	return -EBUSY if the ldisc is currently in use. Since the ldisc referencing
	code manages the module counts this should not usually be a concern.

	Heed this warning: the reference count field of the registered copies of the
	tty_ldisc structure in the ldisc table counts the number of lines using this
	discipline. The reference count of the tty_ldisc structure within a tty
	counts the number of active users of the ldisc at this instant. In effect it
	counts the number of threads of execution within an ldisc method (plus those
	about to enter and exit although this detail matters not).

	Line Discipline Methods
	-----------------------

	TTY side interfaces:

	close() - This is called on a terminal when the line
	discipline is being unplugged. At the point of
	execution no further users will enter the
	ldisc code for this tty. Can sleep.

	open() - Called when the line discipline is attached to
	the terminal. No other call into the line
	discipline for this tty will occur until it
	completes successfully. Can sleep.

	write() - A process is writing data through the line
	discipline. Multiple write calls are serialized
	by the tty layer for the ldisc. May sleep.

	flush_buffer() - May be called at any point between open and close.

	chars_in_buffer() - Report the number of bytes in the buffer.

	set_termios() - Called on termios structure changes. The caller
	passes the old termios data and the current data
	is in the tty. Called under the termios semaphore so
	allowed to sleep. Serialized against itself only.

	read() - Move data from the line discipline to the user.
	Multiple read calls may occur in parallel and the
	ldisc must deal with serialization issues. May
	sleep.

	poll() - Check the status for the poll/select calls. Multiple
	poll calls may occur in parallel. May sleep.

	ioctl() - Called when an ioctl is handed to the tty layer
	that might be for the ldisc. Multiple ioctl calls
	may occur in parallel. May sleep.

	Driver Side Interfaces:

	receive_buf() - Hand buffers of bytes from the driver to the ldisc
	for processing. Semantics currently rather
	mysterious 8(

	receive_room() - Can be called by the driver layer at any time when
	the ldisc is opened. The ldisc must be able to
	handle the reported amount of data at that instant.
	Synchronization between active receive_buf and
	receive_room calls is down to the driver not the
	ldisc. Must not sleep.

	write_wakeup() - May be called at any point between open and close.
	The TTY_DO_WRITE_WAKEUP flag indicates if a call
	is needed but always races versus calls. Thus the
	ldisc must be careful about setting order and to
	handle unexpected calls. Must not sleep.

	The driver is forbidden from calling this directly
	from the ->write call from the ldisc as the ldisc
	is permitted to call the driver write method from
	this function. In such a situation defer it.


	Locking

	Callers to the line discipline functions from the tty layer are required to
	take line discipline locks. The same is true of calls from the driver side
	but not yet enforced.

	Three calls are now provided

	ldisc = tty_ldisc_ref(tty);

	takes a handle to the line discipline in the tty and returns it. If no ldisc
	is currently attached or the ldisc is being closed and re-opened at this
	point then NULL is returned. While this handle is held the ldisc will not
	change or go away.

	tty_ldisc_deref(ldisc)

	Returns the ldisc reference and allows the ldisc to be closed. Returning the
	reference takes away your right to call the ldisc functions until you take
	a new reference.

	ldisc = tty_ldisc_ref_wait(tty);

	Performs the same function as tty_ldisc_ref except that it will wait for an
	ldisc change to complete and then return a reference to the new ldisc.

	While these functions are slightly slower than the old code they should have
	minimal impact as most receive logic uses the flip buffers and they only
	need to take a reference when they push bits up through the driver.

	A caution: The ldisc->open(), ldisc->close() and driver->set_ldisc
	functions are called with the ldisc unavailable. Thus tty_ldisc_ref will
	fail in this situation if used within these functions. Ldisc and driver
	code calling its own functions must be careful in this case.


	Driver Interface
	----------------

	open() - Called when a device is opened. May sleep

	close() - Called when a device is closed. At the point of
	return from this call the driver must make no
	further ldisc calls of any kind. May sleep

	write() - Called to write bytes to the device. May not
	sleep. May occur in parallel in special cases.
	Because this includes panic paths drivers generally
	shouldn't try and do clever locking here.

	put_char() - Stuff a single character onto the queue. The
	driver is guaranteed following up calls to
	flush_chars.

	flush_chars() - Ask the kernel to write put_char queue

	write_room() - Return the number of characters tht can be stuffed
	into the port buffers without overflow (or less).
	The ldisc is responsible for being intelligent
	about multi-threading of write_room/write calls

	ioctl() - Called when an ioctl may be for the driver

	set_termios() - Called on termios change, serialized against
	itself by a semaphore. May sleep.

	set_ldisc() - Notifier for discipline change. At the point this
	is done the discipline is not yet usable. Can now
	sleep (I think)

	throttle() - Called by the ldisc to ask the driver to do flow
	control. Serialization including with unthrottle
	is the job of the ldisc layer.

	unthrottle() - Called by the ldisc to ask the driver to stop flow
	control.

	stop() - Ldisc notifier to the driver to stop output. As with
	throttle the serializations with start() are down
	to the ldisc layer.

	start() - Ldisc notifier to the driver to start output.

	hangup() - Ask the tty driver to cause a hangup initiated
	from the host side. [Can sleep ??]

	break_ctl() - Send RS232 break. Can sleep. Can get called in
	parallel, driver must serialize (for now), and
	with write calls.

	wait_until_sent() - Wait for characters to exit the hardware queue
	of the driver. Can sleep

	send_xchar() - Send XON/XOFF and if possible jump the queue with
	it in order to get fast flow control responses.
	Cannot sleep ??