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Diffstat (limited to 'Documentation/driver-api/tty/tty_driver.rst')
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diff --git a/Documentation/driver-api/tty/tty_driver.rst b/Documentation/driver-api/tty/tty_driver.rst new file mode 100644 index 000000000000..cc529f863406 --- /dev/null +++ b/Documentation/driver-api/tty/tty_driver.rst @@ -0,0 +1,128 @@ +.. 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 + |