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+.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
+
+.. _lirc_dev_intro:
+
+************
+Introduction
+************
+
+LIRC stands for Linux Infrared Remote Control. The LIRC device interface is
+a bi-directional interface for transporting raw IR and decoded scancodes
+data between userspace and kernelspace. Fundamentally, it is just a chardev
+(/dev/lircX, for X = 0, 1, 2, ...), with a number of standard struct
+file_operations defined on it. With respect to transporting raw IR and
+decoded scancodes to and fro, the essential fops are read, write and ioctl.
+
+It is also possible to attach a BPF program to a LIRC device for decoding
+raw IR into scancodes.
+
+Example dmesg output upon a driver registering w/LIRC:
+
+.. code-block:: none
+
+    $ dmesg |grep lirc_dev
+    rc rc0: lirc_dev: driver mceusb registered at minor = 0, raw IR receiver, raw IR transmitter
+
+What you should see for a chardev:
+
+.. code-block:: none
+
+    $ ls -l /dev/lirc*
+    crw-rw---- 1 root root 248, 0 Jul 2 22:20 /dev/lirc0
+
+Note that the package `v4l-utils <https://git.linuxtv.org/v4l-utils.git/>`_
+contains tools for working with LIRC devices:
+
+ - ir-ctl: can receive raw IR and transmit IR, as well as query LIRC
+   device features.
+
+ - ir-keytable: can load keymaps; allows you to set IR kernel protocols; load
+   BPF IR decoders and test IR decoding. Some BPF IR decoders are also
+   provided.
+
+.. _lirc_modes:
+
+**********
+LIRC modes
+**********
+
+LIRC supports some modes of receiving and sending IR codes, as shown
+on the following table.
+
+.. _lirc-mode-scancode:
+.. _lirc-scancode-flag-toggle:
+.. _lirc-scancode-flag-repeat:
+
+``LIRC_MODE_SCANCODE``
+
+    This mode is for both sending and receiving IR.
+
+    For transmitting (aka sending), create a ``struct lirc_scancode`` with
+    the desired scancode set in the ``scancode`` member, :c:type:`rc_proto`
+    set to the :ref:`IR protocol <Remote_controllers_Protocols>`, and all other
+    members set to 0. Write this struct to the lirc device.
+
+    For receiving, you read ``struct lirc_scancode`` from the LIRC device.
+    The ``scancode`` field is set to the received scancode and the
+    :ref:`IR protocol <Remote_controllers_Protocols>` is set in
+    :c:type:`rc_proto`. If the scancode maps to a valid key code, this is set
+    in the ``keycode`` field, else it is set to ``KEY_RESERVED``.
+
+    The ``flags`` can have ``LIRC_SCANCODE_FLAG_TOGGLE`` set if the toggle
+    bit is set in protocols that support it (e.g. rc-5 and rc-6), or
+    ``LIRC_SCANCODE_FLAG_REPEAT`` for when a repeat is received for protocols
+    that support it (e.g. nec).
+
+    In the Sanyo and NEC protocol, if you hold a button on remote, rather than
+    repeating the entire scancode, the remote sends a shorter message with
+    no scancode, which just means button is held, a "repeat". When this is
+    received, the ``LIRC_SCANCODE_FLAG_REPEAT`` is set and the scancode and
+    keycode is repeated.
+
+    With nec, there is no way to distinguish "button hold" from "repeatedly
+    pressing the same button". The rc-5 and rc-6 protocols have a toggle bit.
+    When a button is released and pressed again, the toggle bit is inverted.
+    If the toggle bit is set, the ``LIRC_SCANCODE_FLAG_TOGGLE`` is set.
+
+    The ``timestamp`` field is filled with the time nanoseconds
+    (in ``CLOCK_MONOTONIC``) when the scancode was decoded.
+
+.. _lirc-mode-mode2:
+
+``LIRC_MODE_MODE2``
+
+    The driver returns a sequence of pulse and space codes to userspace,
+    as a series of u32 values.
+
+    This mode is used only for IR receive.
+
+    The upper 8 bits determine the packet type, and the lower 24 bits
+    the payload. Use ``LIRC_VALUE()`` macro to get the payload, and
+    the macro ``LIRC_MODE2()`` will give you the type, which
+    is one of:
+
+    ``LIRC_MODE2_PULSE``
+
+        Signifies the presence of IR in microseconds.
+
+    ``LIRC_MODE2_SPACE``
+
+        Signifies absence of IR in microseconds.
+
+    ``LIRC_MODE2_FREQUENCY``
+
+        If measurement of the carrier frequency was enabled with
+        :ref:`lirc_set_measure_carrier_mode` then this packet gives you
+        the carrier frequency in Hertz.
+
+    ``LIRC_MODE2_TIMEOUT``
+
+        If timeout reports are enabled with
+        :ref:`lirc_set_rec_timeout_reports`, when the timeout set with
+        :ref:`lirc_set_rec_timeout` expires due to no IR being detected,
+        this packet will be sent, with the number of microseconds with
+        no IR.
+
+.. _lirc-mode-pulse:
+
+``LIRC_MODE_PULSE``
+
+    In pulse mode, a sequence of pulse/space integer values are written to the
+    lirc device using :ref:`lirc-write`.
+
+    The values are alternating pulse and space lengths, in microseconds. The
+    first and last entry must be a pulse, so there must be an odd number
+    of entries.
+
+    This mode is used only for IR send.
+
+********************
+BPF based IR decoder
+********************
+
+The kernel has support for decoding the most common
+:ref:`IR protocols <Remote_controllers_Protocols>`, but there
+are many protocols which are not supported. To support these, it is possible
+to load an BPF program which does the decoding. This can only be done on
+LIRC devices which support reading raw IR.
+
+First, using the `bpf(2)`_ syscall with the ``BPF_LOAD_PROG`` argument,
+program must be loaded of type ``BPF_PROG_TYPE_LIRC_MODE2``. Once attached
+to the LIRC device, this program will be called for each pulse, space or
+timeout event on the LIRC device. The context for the BPF program is a
+pointer to a unsigned int, which is a :ref:`LIRC_MODE_MODE2 <lirc-mode-mode2>`
+value. When the program has decoded the scancode, it can be submitted using
+the BPF functions ``bpf_rc_keydown()`` or ``bpf_rc_repeat()``. Mouse or pointer
+movements can be reported using ``bpf_rc_pointer_rel()``.
+
+Once you have the file descriptor for the ``BPF_PROG_TYPE_LIRC_MODE2`` BPF
+program, it can be attached to the LIRC device using the `bpf(2)`_ syscall.
+The target must be the file descriptor for the LIRC device, and the
+attach type must be ``BPF_LIRC_MODE2``. No more than 64 BPF programs can be
+attached to a single LIRC device at a time.
+
+.. _bpf(2): http://man7.org/linux/man-pages/man2/bpf.2.html