317 lines
11 KiB
Plaintext
317 lines
11 KiB
Plaintext
|
|
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_OTHER_DONE Device is a pty and the other side has closed and
|
|
all pending input processing has been completed.
|
|
|
|
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 ??
|
|
|