linux_dsm_epyc7002/Documentation/tty.txt

			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 ??