.. 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`.