tty: move tty_ldisc docs to new Documentation/tty/

Create a new directory in Documentation/ called tty. We will create more
documents in it in the next patches, so let's have this one in the very
same place.

Change title accordingly and all the headers. This is the way what other
documents look like in this directory in the next patches. So make this
unified.

And add a TOC.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20211126081611.11001-20-jslaby@suse.cz
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 84 insertions, 0 deletions

diff --git a/Documentation/tty/tty_ldisc.rst b/Documentation/tty/tty_ldisc.rst
new file mode 100644
index 000000000000..8e48ab72f965
--- /dev/null
+++ b/Documentation/tty/tty_ldisc.rst
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+.. SPDX-License-Identifier: GPL-2.0
+
+===================
+TTY Line Discipline
+===================
+
+.. contents:: :local:
+
+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
+=======================
+
+.. kernel-doc:: include/linux/tty_ldisc.h
+   :identifiers: tty_ldisc_ops
+
+Driver Access
+=============
+
+Line discipline methods can call the methods of the underlying hardware driver.
+These are documented as a part of struct tty_operations.
+
+TTY Flags
+=========
+
+Line discipline methods have access to :c:member:`tty_struct.flags` field. See
+:doc:`tty_struct`.
+
+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.