linux/Documentation/media/uapi/rc/lirc-dev-intro.rst

.. -*- coding: utf-8; mode: rst -*-

.. _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.

Example dmesg output upon a driver registering w/LIRC:

.. code-block:: none

    $ dmesg |grep lirc_dev
    lirc_dev: IR Remote Control driver registered, major 248
    rc rc0: lirc_dev: driver mceusb registered at minor = 0

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

.. _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, ``rc_proto`` set
    the IR protocol, 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,
    with ``scancode`` set to the received scancode and the IR protocol
    ``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.

    An ``enum rc_proto`` in the :ref:`lirc_header` lists all the supported
    IR protocols.

.. _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.