Documentation/driver-api/tty/tty_driver.rst
.. SPDX-License-Identifier: GPL-2.0
.. contents:: :local:
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
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
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
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
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
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
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