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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_register_ldisc passing NULL.
+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 ??
+