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Diffstat (limited to 'Documentation/serial/tty.txt')
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diff --git a/Documentation/serial/tty.txt b/Documentation/serial/tty.txt deleted file mode 100644 index b48780977a68..000000000000 --- a/Documentation/serial/tty.txt +++ /dev/null @@ -1,313 +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. - - -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. Should initialize any - state needed by the ldisc, and set receive_room - in the tty_struct to the maximum amount of data - the line discipline is willing to accept from the - driver with a single call to receive_buf(). - 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. - -read() - (optional) A process requests reading data from - the line. Multiple read calls may occur in parallel - and the ldisc must deal with serialization issues. - If not defined, the process will receive an EIO - error. May sleep. - -write() - (optional) A process requests writing data to the - line. Multiple write calls are serialized by the - tty layer for the ldisc. If not defined, the - process will receive an EIO error. 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. - -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. - -poll() - (optional) Check the status for the poll/select - calls. Multiple poll calls may occur in parallel. - May sleep. - -ioctl() - (optional) 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() - (optional) 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() - (optional) Called by the low-level driver to hand - a buffer of received bytes to the ldisc for - processing. The number of bytes is guaranteed not - to exceed the current value of tty->receive_room. - All bytes must be processed. - -receive_buf2() - (optional) Called by the low-level driver to hand - a buffer of received bytes to the ldisc for - processing. Returns the number of bytes processed. - - If both receive_buf() and receive_buf2() are - defined, receive_buf2() should be preferred. - -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 (e.g. 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 that 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 ?? - |