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diff --git a/Documentation/tty.txt b/Documentation/tty.txt deleted file mode 100644 index dab56604745..00000000000 --- a/Documentation/tty.txt +++ /dev/null @@ -1,191 +0,0 @@ - - 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: - -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( - -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 ?? - |