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1.. SPDX-License-Identifier: GPL-2.02 3=============================4TTY Driver and TTY Operations5=============================6 7.. contents:: :local:8 9Allocation10==========11 12The first thing a driver needs to do is to allocate a struct tty_driver. This13is done by tty_alloc_driver() (or __tty_alloc_driver()). Next, the newly14allocated structure is filled with information. See `TTY Driver Reference`_ at15the end of this document on what actually shall be filled in.16 17The allocation routines expect a number of devices the driver can handle at18most and flags. Flags are those starting ``TTY_DRIVER_`` listed and described19in `TTY Driver Flags`_ below.20 21When the driver is about to be freed, tty_driver_kref_put() is called on that.22It will decrements the reference count and if it reaches zero, the driver is23freed.24 25For reference, both allocation and deallocation functions are explained here in26detail:27 28.. kernel-doc:: drivers/tty/tty_io.c29 :identifiers: __tty_alloc_driver tty_driver_kref_put30 31TTY Driver Flags32----------------33 34Here comes the documentation of flags accepted by tty_alloc_driver() (or35__tty_alloc_driver()):36 37.. kernel-doc:: include/linux/tty_driver.h38 :doc: TTY Driver Flags39 40----41 42Registration43============44 45When a struct tty_driver is allocated and filled in, it can be registered using46tty_register_driver(). It is recommended to pass ``TTY_DRIVER_DYNAMIC_DEV`` in47flags of tty_alloc_driver(). If it is not passed, *all* devices are also48registered during tty_register_driver() and the following paragraph of49registering devices can be skipped for such drivers. However, the struct50tty_port part in `Registering Devices`_ is still relevant there.51 52.. kernel-doc:: drivers/tty/tty_io.c53 :identifiers: tty_register_driver tty_unregister_driver54 55Registering Devices56-------------------57 58Every TTY device shall be backed by a struct tty_port. Usually, TTY drivers59embed tty_port into device's private structures. Further details about handling60tty_port can be found in :doc:`tty_port`. The driver is also recommended to use61tty_port's reference counting by tty_port_get() and tty_port_put(). The final62put is supposed to free the tty_port including the device's private struct.63 64Unless ``TTY_DRIVER_DYNAMIC_DEV`` was passed as flags to tty_alloc_driver(),65TTY driver is supposed to register every device discovered in the system66(the latter is preferred). This is performed by tty_register_device(). Or by67tty_register_device_attr() if the driver wants to expose some information68through struct attribute_group. Both of them register ``index``'th device and69upon return, the device can be opened. There are also preferred tty_port70variants described in `Linking Devices to Ports`_ later. It is up to driver to71manage free indices and choosing the right one. The TTY layer only refuses to72register more devices than passed to tty_alloc_driver().73 74When the device is opened, the TTY layer allocates struct tty_struct and starts75calling operations from :c:member:`tty_driver.ops`, see `TTY Operations76Reference`_.77 78The registration routines are documented as follows:79 80.. kernel-doc:: drivers/tty/tty_io.c81 :identifiers: tty_register_device tty_register_device_attr82 tty_unregister_device83 84----85 86Linking Devices to Ports87------------------------88As stated earlier, every TTY device shall have a struct tty_port assigned to89it. It must be known to the TTY layer at :c:member:`tty_driver.ops.install()`90at latest. There are few helpers to *link* the two. Ideally, the driver uses91tty_port_register_device() or tty_port_register_device_attr() instead of92tty_register_device() and tty_register_device_attr() at the registration time.93This way, the driver needs not care about linking later on.94 95If that is not possible, the driver still can link the tty_port to a specific96index *before* the actual registration by tty_port_link_device(). If it still97does not fit, tty_port_install() can be used from the98:c:member:`tty_driver.ops.install` hook as a last resort. The last one is99dedicated mostly for in-memory devices like PTY where tty_ports are allocated100on demand.101 102The linking routines are documented here:103 104.. kernel-doc:: drivers/tty/tty_port.c105 :identifiers: tty_port_link_device tty_port_register_device106 tty_port_register_device_attr107 108----109 110TTY Driver Reference111====================112 113All members of struct tty_driver are documented here. The required members are114noted at the end. struct tty_operations are documented next.115 116.. kernel-doc:: include/linux/tty_driver.h117 :identifiers: tty_driver118 119----120 121TTY Operations Reference122========================123 124When a TTY is registered, these driver hooks can be invoked by the TTY layer:125 126.. kernel-doc:: include/linux/tty_driver.h127 :identifiers: tty_operations128 129