64 lines
2.3 KiB
ReStructuredText
64 lines
2.3 KiB
ReStructuredText
|
.. 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`.
|