.. 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. Thisis done by tty_alloc_driver() (or __tty_alloc_driver()). Next, the newlyallocated structure is filled with information. See `TTY Driver Reference`_ atthe end of this document on what actually shall be filled in. The allocation routines expect a number of devices the driver can handle atmost and flags. Flags are those starting ``TTY_DRIVER_`` listed and describedin `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 isfreed. For reference, both allocation and deallocation functions are explained here indetail: .. kernel-doc:: include/linux/tty_driver.h   :identifiers: tty_alloc_driver.. 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   :identifiers: tty_driver_flag ---- Registration============ When a struct tty_driver is allocated and filled in, it can be registered usingtty_register_driver(). It is recommended to pass ``TTY_DRIVER_DYNAMIC_DEV`` inflags of tty_alloc_driver(). If it is not passed, *all* devices are alsoregistered during tty_register_driver() and the following paragraph ofregistering devices can be skipped for such drivers. However, the structtty_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 driversembed tty_port into device's private structures. Further details about handlingtty_port can be found in :doc:`tty_port`. The driver is also recommended to usetty_port's reference counting by tty_port_get() and tty_port_put(). The finalput 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 bytty_register_device_attr() if the driver wants to expose some informationthrough struct attribute_group. Both of them register ``index``'th device andupon return, the device can be opened. There are also preferred tty_portvariants described in `Linking Devices to Ports`_ later. It is up to driver tomanage free indices and choosing the right one. The TTY layer only refuses toregister more devices than passed to tty_alloc_driver(). When the device is opened, the TTY layer allocates struct tty_struct and startscalling operations from :c:member:`tty_driver.ops`, see `TTY OperationsReference`_. 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 toit. 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 usestty_port_register_device() or tty_port_register_device_attr() instead oftty_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 specificindex *before* the actual registration by tty_port_link_device(). If it stilldoes not fit, tty_port_install() can be used from the:c:member:`tty_driver.ops.install` hook as a last resort. The last one isdedicated mostly for in-memory devices like PTY where tty_ports are allocatedon 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 arenoted 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