xref: /kernel/linux/linux-6.6/Documentation/driver-api/tty/tty_driver.rst

.. SPDX-License-Identifier: GPL-2.0

=============================
TTY Driver and TTY Operations
=============================

.. contents:: :local:

Allocation
==========

The first thing a driver needs to do is to allocate a struct tty_driver. This
is done by tty_alloc_driver() (or __tty_alloc_driver()). Next, the newly
allocated structure is filled with information. See `TTY Driver Reference`_ at
the end of this document on what actually shall be filled in.

The allocation routines expect a number of devices the driver can handle at
most and flags. Flags are those starting ``TTY_DRIVER_`` listed and described
in `TTY Driver Flags`_ below.

When the driver is about to be freed, tty_driver_kref_put() is called on that.
It will decrements the reference count and if it reaches zero, the driver is
freed.

For reference, both allocation and deallocation functions are explained here in
detail:

.. kernel-doc:: drivers/tty/tty_io.c
   :identifiers: __tty_alloc_driver tty_driver_kref_put

TTY Driver Flags
----------------

Here comes the documentation of flags accepted by tty_alloc_driver() (or
__tty_alloc_driver()):

.. kernel-doc:: include/linux/tty_driver.h
   :doc: TTY Driver Flags

----

Registration
============

When a struct tty_driver is allocated and filled in, it can be registered using
tty_register_driver(). It is recommended to pass ``TTY_DRIVER_DYNAMIC_DEV`` in
flags of tty_alloc_driver(). If it is not passed, *all* devices are also
registered during tty_register_driver() and the following paragraph of
registering devices can be skipped for such drivers. However, the struct
tty_port part in `Registering Devices`_ is still relevant there.

.. kernel-doc:: drivers/tty/tty_io.c
   :identifiers: tty_register_driver tty_unregister_driver

Registering Devices
-------------------

Every TTY device shall be backed by a struct tty_port. Usually, TTY drivers
embed tty_port into device's private structures. Further details about handling
tty_port can be found in :doc:`tty_port`. The driver is also recommended to use
tty_port's reference counting by tty_port_get() and tty_port_put(). The final
put is supposed to free the tty_port including the device's private struct.

Unless ``TTY_DRIVER_DYNAMIC_DEV`` was passed as flags to tty_alloc_driver(),
TTY driver is supposed to register every device discovered in the system
(the latter is preferred). This is performed by tty_register_device(). Or by
tty_register_device_attr() if the driver wants to expose some information
through struct attribute_group. Both of them register ``index``'th device and
upon return, the device can be opened. There are also preferred tty_port
variants described in `Linking Devices to Ports`_ later. It is up to driver to
manage free indices and choosing the right one. The TTY layer only refuses to
register more devices than passed to tty_alloc_driver().

When the device is opened, the TTY layer allocates struct tty_struct and starts
calling operations from :c:member:`tty_driver.ops`, see `TTY Operations
Reference`_.

The registration routines are documented as follows:

.. kernel-doc:: drivers/tty/tty_io.c
   :identifiers: tty_register_device tty_register_device_attr
        tty_unregister_device

----

Linking Devices to Ports
------------------------
As stated earlier, every TTY device shall have a struct tty_port assigned to
it. It must be known to the TTY layer at :c:member:`tty_driver.ops.install()`
at latest.  There are few helpers to *link* the two. Ideally, the driver uses
tty_port_register_device() or tty_port_register_device_attr() instead of
tty_register_device() and tty_register_device_attr() at the registration time.
This way, the driver needs not care about linking later on.

If that is not possible, the driver still can link the tty_port to a specific
index *before* the actual registration by tty_port_link_device(). If it still
does not fit, tty_port_install() can be used from the
:c:member:`tty_driver.ops.install` hook as a last resort. The last one is
dedicated mostly for in-memory devices like PTY where tty_ports are allocated
on demand.

The linking routines are documented here:

.. kernel-doc::  drivers/tty/tty_port.c
   :identifiers: tty_port_link_device tty_port_register_device
        tty_port_register_device_attr

----

TTY Driver Reference
====================

All members of struct tty_driver are documented here. The required members are
noted at the end. struct tty_operations are documented next.

.. kernel-doc:: include/linux/tty_driver.h
   :identifiers: tty_driver

----

TTY Operations Reference
========================

When a TTY is registered, these driver hooks can be invoked by the TTY layer:

.. kernel-doc:: include/linux/tty_driver.h
   :identifiers: tty_operations