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diff --git a/Documentation/tty.txt b/Documentation/tty.txt new file mode 100644 index 00000000000..3958cf746dd --- /dev/null +++ b/Documentation/tty.txt @@ -0,0 +1,198 @@ + + 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 ?? + |