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.. SPDX-License-Identifier: GPL-2.0
===
TTY
===
Teletypewriter (TTY) layer takes care of all those serial devices. Including
the virtual ones like pseudoterminal (PTY).
TTY structures
==============
There are several major TTY structures. Every TTY device in a system has a
corresponding struct tty_port. These devices are maintained by a TTY driver
which is struct tty_driver. This structure describes the driver but also
contains a reference to operations which could be performed on the TTYs. It is
struct tty_operations. Then, upon open, a struct tty_struct is allocated and
lives until the final close. During this time, several callbacks from struct
tty_operations are invoked by the TTY layer.
Every character received by the kernel (both from devices and users) is passed
through a preselected :doc:`tty_ldisc` (in
short ldisc; in C, struct tty_ldisc_ops). Its task is to transform characters
as defined by a particular ldisc or by user too. The default one is n_tty,
implementing echoes, signal handling, jobs control, special characters
processing, and more. The transformed characters are passed further to
user/device, depending on the source.
In-detail description of the named TTY structures is in separate documents:
.. toctree::
:maxdepth: 2
tty_driver
tty_port
tty_struct
tty_ldisc
tty_buffer
n_tty
tty_internals
Writing TTY Driver
==================
Before one starts writing a TTY driver, they must consider
:doc:`Serial <../driver-api/serial/driver>` and :doc:`USB Serial
<../usb/usb-serial>` layers
first. Drivers for serial devices can often use one of these specific layers to
implement a serial driver. Only special devices should be handled directly by
the TTY Layer. If you are about to write such a driver, read on.
A *typical* sequence a TTY driver performs is as follows:
#. Allocate and register a TTY driver (module init)
#. Create and register TTY devices as they are probed (probe function)
#. Handle TTY operations and events like interrupts (TTY core invokes the
former, the device the latter)
#. Remove devices as they are going away (remove function)
#. Unregister and free the TTY driver (module exit)
Steps regarding driver, i.e. 1., 3., and 5. are described in detail in
:doc:`tty_driver`. For the other two (devices handling), look into
:doc:`tty_port`.
|
