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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
+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.
+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 ??