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. Returning an error will
  			prevent the ldisc from being attached. 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.
  			The return value is ignored. 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. 
  
  compat_ioctl()	-	Called when a 32 bit 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.
  
  dcd_change()	-	Report to the tty line the current DCD pin status
  			changes and the relative timestamp. The timestamp
  			cannot be NULL.
  
  
  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 ??