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Documentation: add TTY chapter
We now have all the kernel-doc comments in the code ready. So add a couple of documents dragging those into generated docs from Documentation/. There is only some sugar text around included kernel-docs here. It's a complete chapter, to be extended later as desired. This is a solid cornerstone for the time being, I believe. Signed-off-by: Jiri Slaby <jslaby@suse.cz> Link: https://lore.kernel.org/r/20211126081611.11001-24-jslaby@suse.cz Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Jiri Slaby
Greg Kroah-Hartman
parent
31bc35d334
commit
3f19fed8d0
@ -137,6 +137,7 @@ needed).
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misc-devices/index
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scheduler/index
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mhi/index
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tty/index
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Architecture-agnostic documentation
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-----------------------------------
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63
Documentation/tty/index.rst
Normal file
63
Documentation/tty/index.rst
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@ -0,0 +1,63 @@
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.. SPDX-License-Identifier: GPL-2.0
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===
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TTY
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===
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Teletypewriter (TTY) layer takes care of all those serial devices. Including
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the virtual ones like pseudoterminal (PTY).
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TTY structures
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==============
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There are several major TTY structures. Every TTY device in a system has a
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corresponding struct tty_port. These devices are maintained by a TTY driver
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which is struct tty_driver. This structure describes the driver but also
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contains a reference to operations which could be performed on the TTYs. It is
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struct tty_operations. Then, upon open, a struct tty_struct is allocated and
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lives until the final close. During this time, several callbacks from struct
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tty_operations are invoked by the TTY layer.
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Every character received by the kernel (both from devices and users) is passed
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through a preselected :doc:`tty_ldisc` (in
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short ldisc; in C, struct tty_ldisc_ops). Its task is to transform characters
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as defined by a particular ldisc or by user too. The default one is n_tty,
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implementing echoes, signal handling, jobs control, special characters
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processing, and more. The transformed characters are passed further to
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user/device, depending on the source.
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In-detail description of the named TTY structures is in separate documents:
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.. toctree::
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:maxdepth: 2
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tty_driver
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tty_port
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tty_struct
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tty_ldisc
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tty_buffer
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n_tty
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tty_internals
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Writing TTY Driver
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==================
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Before one starts writing a TTY driver, they must consider
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:doc:`Serial <../driver-api/serial/driver>` and :doc:`USB Serial
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<../usb/usb-serial>` layers
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first. Drivers for serial devices can often use one of these specific layers to
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implement a serial driver. Only special devices should be handled directly by
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the TTY Layer. If you are about to write such a driver, read on.
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A *typical* sequence a TTY driver performs is as follows:
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#. Allocate and register a TTY driver (module init)
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#. Create and register TTY devices as they are probed (probe function)
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#. Handle TTY operations and events like interrupts (TTY core invokes the
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former, the device the latter)
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#. Remove devices as they are going away (remove function)
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#. Unregister and free the TTY driver (module exit)
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Steps regarding driver, i.e. 1., 3., and 5. are described in detail in
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:doc:`tty_driver`. For the other two (devices handling), look into
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:doc:`tty_port`.
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22
Documentation/tty/n_tty.rst
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22
Documentation/tty/n_tty.rst
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@ -0,0 +1,22 @@
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.. SPDX-License-Identifier: GPL-2.0
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=====
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N_TTY
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=====
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.. contents:: :local:
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The default (and fallback) :doc:`TTY line discipline <tty_ldisc>`. It tries to
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handle characters as per POSIX.
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External Functions
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==================
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.. kernel-doc:: drivers/tty/n_tty.c
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:export:
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Internal Functions
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==================
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.. kernel-doc:: drivers/tty/n_tty.c
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:internal:
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46
Documentation/tty/tty_buffer.rst
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46
Documentation/tty/tty_buffer.rst
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@ -0,0 +1,46 @@
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.. SPDX-License-Identifier: GPL-2.0
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==========
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TTY Buffer
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==========
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.. contents:: :local:
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Here, we document functions for taking care of tty buffer and their flipping.
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Drivers are supposed to fill the buffer by one of those functions below and
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then flip the buffer, so that the data are passed to :doc:`line discipline
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<tty_ldisc>` for further processing.
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Flip Buffer Management
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======================
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.. kernel-doc:: drivers/tty/tty_buffer.c
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:identifiers: tty_prepare_flip_string tty_insert_flip_string_fixed_flag
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tty_insert_flip_string_flags __tty_insert_flip_char
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tty_flip_buffer_push tty_ldisc_receive_buf
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----
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Other Functions
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===============
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.. kernel-doc:: drivers/tty/tty_buffer.c
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:identifiers: tty_buffer_space_avail tty_buffer_set_limit
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----
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Buffer Locking
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==============
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These are used only in special circumstances. Avoid them.
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.. kernel-doc:: drivers/tty/tty_buffer.c
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:identifiers: tty_buffer_lock_exclusive tty_buffer_unlock_exclusive
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----
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Internal Functions
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==================
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.. kernel-doc:: drivers/tty/tty_buffer.c
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:internal:
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128
Documentation/tty/tty_driver.rst
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128
Documentation/tty/tty_driver.rst
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@ -0,0 +1,128 @@
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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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31
Documentation/tty/tty_internals.rst
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31
Documentation/tty/tty_internals.rst
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@ -0,0 +1,31 @@
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.. SPDX-License-Identifier: GPL-2.0
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=============
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TTY Internals
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=============
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.. contents:: :local:
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Kopen
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=====
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These functions serve for opening a TTY from the kernelspace:
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.. kernel-doc:: drivers/tty/tty_io.c
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:identifiers: tty_kopen_exclusive tty_kopen_shared tty_kclose
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----
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Exported Internal Functions
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===========================
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.. kernel-doc:: drivers/tty/tty_io.c
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:identifiers: tty_release_struct tty_dev_name_to_number tty_get_icount
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----
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Internal Functions
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==================
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.. kernel-doc:: drivers/tty/tty_io.c
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:internal:
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@ -6,6 +6,12 @@ TTY Line Discipline
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.. contents:: :local:
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TTY line discipline process all incoming and outgoing character from/to a tty
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device. The default line discipline is :doc:`N_TTY <n_tty>`. It is also a
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fallback if establishing any other discipline for a tty fails. If even N_TTY
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fails, N_NULL takes over. That never fails, but also does not process any
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characters -- it throws them away.
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Registration
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============
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70
Documentation/tty/tty_port.rst
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70
Documentation/tty/tty_port.rst
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@ -0,0 +1,70 @@
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.. SPDX-License-Identifier: GPL-2.0
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========
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TTY Port
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========
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.. contents:: :local:
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The TTY drivers are advised to use struct tty_port helpers as much as possible.
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If the drivers implement :c:member:`tty_port.ops.activate()` and
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:c:member:`tty_port.ops.shutdown()`, they can use tty_port_open(),
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tty_port_close(), and tty_port_hangup() in respective
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:c:member:`tty_struct.ops` hooks.
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The reference and details are contained in the `TTY Port Reference`_ and `TTY
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Port Operations Reference`_ sections at the bottom.
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TTY Port Functions
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==================
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Init & Destroy
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--------------
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.. kernel-doc:: drivers/tty/tty_port.c
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:identifiers: tty_port_init tty_port_destroy
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tty_port_get tty_port_put
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Open/Close/Hangup Helpers
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-------------------------
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.. kernel-doc:: drivers/tty/tty_port.c
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:identifiers: tty_port_install tty_port_open tty_port_block_til_ready
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tty_port_close tty_port_close_start tty_port_close_end tty_port_hangup
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tty_port_shutdown
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TTY Refcounting
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---------------
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.. kernel-doc:: drivers/tty/tty_port.c
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:identifiers: tty_port_tty_get tty_port_tty_set
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TTY Helpers
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-----------
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.. kernel-doc:: drivers/tty/tty_port.c
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:identifiers: tty_port_tty_hangup tty_port_tty_wakeup
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Modem Signals
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-------------
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.. kernel-doc:: drivers/tty/tty_port.c
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:identifiers: tty_port_carrier_raised tty_port_raise_dtr_rts
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tty_port_lower_dtr_rts
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----
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TTY Port Reference
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==================
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.. kernel-doc:: include/linux/tty_port.h
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:identifiers: tty_port
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----
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TTY Port Operations Reference
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=============================
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.. kernel-doc:: include/linux/tty_port.h
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:identifiers: tty_port_operations
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81
Documentation/tty/tty_struct.rst
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81
Documentation/tty/tty_struct.rst
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.. SPDX-License-Identifier: GPL-2.0
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==========
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TTY Struct
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==========
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.. contents:: :local:
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struct tty_struct is allocated by the TTY layer upon the first open of the TTY
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device and released after the last close. The TTY layer passes this structure
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to most of struct tty_operation's hooks. Members of tty_struct are documented
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in `TTY Struct Reference`_ at the bottom.
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Initialization
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==============
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.. kernel-doc:: drivers/tty/tty_io.c
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:identifiers: tty_init_termios
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Name
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====
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.. kernel-doc:: drivers/tty/tty_io.c
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:identifiers: tty_name
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Reference counting
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==================
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.. kernel-doc:: include/linux/tty.h
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:identifiers: tty_kref_get
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.. kernel-doc:: drivers/tty/tty_io.c
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:identifiers: tty_kref_put
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Install
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=======
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.. kernel-doc:: drivers/tty/tty_io.c
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:identifiers: tty_standard_install
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Read & Write
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============
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.. kernel-doc:: drivers/tty/tty_io.c
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:identifiers: tty_put_char
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Start & Stop
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============
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.. kernel-doc:: drivers/tty/tty_io.c
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:identifiers: start_tty stop_tty
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Wakeup
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======
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.. kernel-doc:: drivers/tty/tty_io.c
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:identifiers: tty_wakeup
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Hangup
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======
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.. kernel-doc:: drivers/tty/tty_io.c
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:identifiers: tty_hangup tty_vhangup tty_hung_up_p
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Misc
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====
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.. kernel-doc:: drivers/tty/tty_io.c
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:identifiers: tty_do_resize
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TTY Struct Flags
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================
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.. kernel-doc:: include/linux/tty.h
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:doc: TTY Struct Flags
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TTY Struct Reference
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====================
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.. kernel-doc:: include/linux/tty.h
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:identifiers: tty_struct
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