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+
+                        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:
+
+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.
+
+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()  -       (optional) May be called at any point between
+                        open and close, and instructs the line discipline
+                        to empty its input buffer.
+
+chars_in_buffer() -     (optional) Report the number of bytes in the input
+                        buffer.
+
+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
+                        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(
+
+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.
+
+
+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. The
+                        character buffer passed to this method is already
+                        in kernel space.
+
+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.
+
+                        The API is defined such that the driver should return
+                        the actual modes selected. This means that the
+                        driver function is responsible for modifying any
+                        bits in the request it cannot fulfill to indicate
+                        the actual modes being used. A device with no
+                        hardware capability for change (eg a USB dongle or
+                        virtual port) can provide NULL for this method.
+
+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
+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 ??
+