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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. 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 ??
-