129 lines
4.6 KiB
ReStructuredText
129 lines
4.6 KiB
ReStructuredText
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.. SPDX-License-Identifier: GPL-2.0
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=============================
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TTY Driver and TTY Operations
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=============================
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.. contents:: :local:
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Allocation
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==========
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The first thing a driver needs to do is to allocate a struct tty_driver. This
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is done by tty_alloc_driver() (or __tty_alloc_driver()). Next, the newly
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allocated structure is filled with information. See `TTY Driver Reference`_ at
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the end of this document on what actually shall be filled in.
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The allocation routines expect a number of devices the driver can handle at
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most and flags. Flags are those starting ``TTY_DRIVER_`` listed and described
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in `TTY Driver Flags`_ below.
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When the driver is about to be freed, tty_driver_kref_put() is called on that.
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It will decrements the reference count and if it reaches zero, the driver is
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freed.
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For reference, both allocation and deallocation functions are explained here in
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detail:
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.. kernel-doc:: drivers/tty/tty_io.c
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:identifiers: __tty_alloc_driver tty_driver_kref_put
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TTY Driver Flags
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----------------
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Here comes the documentation of flags accepted by tty_alloc_driver() (or
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__tty_alloc_driver()):
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.. kernel-doc:: include/linux/tty_driver.h
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:doc: TTY Driver Flags
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----
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Registration
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============
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When a struct tty_driver is allocated and filled in, it can be registered using
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tty_register_driver(). It is recommended to pass ``TTY_DRIVER_DYNAMIC_DEV`` in
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flags of tty_alloc_driver(). If it is not passed, *all* devices are also
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registered during tty_register_driver() and the following paragraph of
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registering devices can be skipped for such drivers. However, the struct
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tty_port part in `Registering Devices`_ is still relevant there.
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.. kernel-doc:: drivers/tty/tty_io.c
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:identifiers: tty_register_driver tty_unregister_driver
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Registering Devices
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-------------------
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Every TTY device shall be backed by a struct tty_port. Usually, TTY drivers
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embed tty_port into device's private structures. Further details about handling
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tty_port can be found in :doc:`tty_port`. The driver is also recommended to use
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tty_port's reference counting by tty_port_get() and tty_port_put(). The final
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put is supposed to free the tty_port including the device's private struct.
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Unless ``TTY_DRIVER_DYNAMIC_DEV`` was passed as flags to tty_alloc_driver(),
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TTY driver is supposed to register every device discovered in the system
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(the latter is preferred). This is performed by tty_register_device(). Or by
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tty_register_device_attr() if the driver wants to expose some information
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through struct attribute_group. Both of them register ``index``'th device and
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upon return, the device can be opened. There are also preferred tty_port
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variants described in `Linking Devices to Ports`_ later. It is up to driver to
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manage free indices and choosing the right one. The TTY layer only refuses to
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register more devices than passed to tty_alloc_driver().
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When the device is opened, the TTY layer allocates struct tty_struct and starts
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calling operations from :c:member:`tty_driver.ops`, see `TTY Operations
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Reference`_.
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The registration routines are documented as follows:
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.. kernel-doc:: drivers/tty/tty_io.c
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:identifiers: tty_register_device tty_register_device_attr
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tty_unregister_device
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----
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Linking Devices to Ports
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------------------------
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As stated earlier, every TTY device shall have a struct tty_port assigned to
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it. It must be known to the TTY layer at :c:member:`tty_driver.ops.install()`
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at latest. There are few helpers to *link* the two. Ideally, the driver uses
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tty_port_register_device() or tty_port_register_device_attr() instead of
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tty_register_device() and tty_register_device_attr() at the registration time.
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This way, the driver needs not care about linking later on.
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If that is not possible, the driver still can link the tty_port to a specific
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index *before* the actual registration by tty_port_link_device(). If it still
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does not fit, tty_port_install() can be used from the
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:c:member:`tty_driver.ops.install` hook as a last resort. The last one is
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dedicated mostly for in-memory devices like PTY where tty_ports are allocated
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on demand.
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The linking routines are documented here:
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.. kernel-doc:: drivers/tty/tty_port.c
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:identifiers: tty_port_link_device tty_port_register_device
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tty_port_register_device_attr
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----
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TTY Driver Reference
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====================
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All members of struct tty_driver are documented here. The required members are
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noted at the end. struct tty_operations are documented next.
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.. kernel-doc:: include/linux/tty_driver.h
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:identifiers: tty_driver
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----
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TTY Operations Reference
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========================
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When a TTY is registered, these driver hooks can be invoked by the TTY layer:
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.. kernel-doc:: include/linux/tty_driver.h
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:identifiers: tty_operations
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