[PATCH] Update to Documentation/tty.txt on line disciplines

While trying to develop a line discipline I found a couple of things worth
mentioning in Documentation/tty.txt which weren't, so I decided to add
them.  It would be nice if someone more knowledgeable than me in that area
would look over them, in case I got something wrong.

Signed-off-by: Tilman Schmidt <tilman@imap.cc>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 101 insertions, 10 deletions

diff --git a/Documentation/tty.txt b/Documentation/tty.txt
index dab56604745d..5f799e612e03 100644
--- a/Documentation/tty.txt
+++ b/Documentation/tty.txt
@@ -39,28 +39,37 @@ Line Discipline Methods
 
 TTY side interfaces:
 
+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.
+
 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.
+hangup()        -       Called when the tty line is hung up.
+                        The line discipline should cease I/O to the tty.
+                        No further calls into the ldisc code will occur.
+                        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.
+flush_buffer()  -       (optional) May be called at any point between
+                        open and close, and instructs the line discipline
+                        to empty its input buffer.
 
-chars_in_buffer() -     Report the number of bytes in the buffer.
+chars_in_buffer() -     (optional) Report the number of bytes in the input
+                        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.
+set_termios()   -       (optional) 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
@@ -92,6 +101,88 @@ write_wakeup()	-	May be called at any point between open and close.
                         this function. In such a situation defer it.
 
 
+Driver Access
+
+Line discipline methods can call the following methods of the underlying
+hardware driver through the function pointers within the tty->driver
+structure:
+
+write()                 Write a block of characters to the tty device.
+                        Returns the number of characters accepted.
+
+put_char()              Queues a character for writing to the tty device.
+                        If there is no room in the queue, the character is
+                        ignored.
+
+flush_chars()           (Optional) If defined, must be called after
+                        queueing characters with put_char() in order to
+                        start transmission.
+
+write_room()            Returns the numbers of characters the tty driver
+                        will accept for queueing to be written.
+
+ioctl()                 Invoke device specific ioctl.
+                        Expects data pointers to refer to userspace.
+                        Returns ENOIOCTLCMD for unrecognized ioctl numbers.
+
+set_termios()           Notify the tty driver that the device's termios
+                        settings have changed. New settings are in
+                        tty->termios. Previous settings should be passed in
+                        the "old" argument.
+
+throttle()              Notify the tty driver that input buffers for the
+                        line discipline are close to full, and it should
+                        somehow signal that no more characters should be
+                        sent to the tty.
+
+unthrottle()            Notify the tty driver that characters can now be
+                        sent to the tty without fear of overrunning the
+                        input buffers of the line disciplines.
+
+stop()                  Ask the tty driver to stop outputting characters
+                        to the tty device.
+
+start()                 Ask the tty driver to resume sending characters
+                        to the tty device.
+
+hangup()                Ask the tty driver to hang up the tty device.
+
+break_ctl()             (Optional) Ask the tty driver to turn on or off
+                        BREAK status on the RS-232 port.  If state is -1,
+                        then the BREAK status should be turned on; if
+                        state is 0, then BREAK should be turned off.
+                        If this routine is not implemented, use ioctls
+                        TIOCSBRK / TIOCCBRK instead.
+
+wait_until_sent()       Waits until the device has written out all of the
+                        characters in its transmitter FIFO.
+
+send_xchar()            Send a high-priority XON/XOFF character to the device.
+
+
+Flags
+
+Line discipline methods have access to tty->flags field containing the
+following interesting flags:
+
+TTY_THROTTLED           Driver input is throttled. The ldisc should call
+                        tty->driver->unthrottle() in order to resume
+                        reception when it is ready to process more data.
+
+TTY_DO_WRITE_WAKEUP     If set, causes the driver to call the ldisc's
+                        write_wakeup() method in order to resume
+                        transmission when it can accept more data
+                        to transmit.
+
+TTY_IO_ERROR            If set, causes all subsequent userspace read/write
+                        calls on the tty to fail, returning -EIO.
+
+TTY_OTHER_CLOSED        Device is a pty and the other side has closed.
+
+TTY_NO_WRITE_SPLIT      Prevent driver from splitting up writes into
+                        smaller chunks.
+
+
 Locking
 
 Callers to the line discipline functions from the tty layer are required to