177 lines
6.4 KiB
ReStructuredText
177 lines
6.4 KiB
ReStructuredText
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.. SPDX-License-Identifier: GPL-2.0 OR GFDL-1.1-no-invariants-or-later
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.. _lirc_dev_intro:
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************
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Introduction
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************
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LIRC stands for Linux Infrared Remote Control. The LIRC device interface is
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a bi-directional interface for transporting raw IR and decoded scancodes
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data between userspace and kernelspace. Fundamentally, it is just a chardev
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(/dev/lircX, for X = 0, 1, 2, ...), with a number of standard struct
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file_operations defined on it. With respect to transporting raw IR and
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decoded scancodes to and fro, the essential fops are read, write and ioctl.
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It is also possible to attach a BPF program to a LIRC device for decoding
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raw IR into scancodes.
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Example dmesg output upon a driver registering w/LIRC:
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.. code-block:: none
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$ dmesg |grep lirc_dev
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rc rc0: lirc_dev: driver mceusb registered at minor = 0, raw IR receiver, raw IR transmitter
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What you should see for a chardev:
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.. code-block:: none
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$ ls -l /dev/lirc*
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crw-rw---- 1 root root 248, 0 Jul 2 22:20 /dev/lirc0
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Note that the package `v4l-utils <https://git.linuxtv.org/v4l-utils.git/>`_
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contains tools for working with LIRC devices:
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- ir-ctl: can receive raw IR and transmit IR, as well as query LIRC
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device features.
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- ir-keytable: can load keymaps; allows you to set IR kernel protocols; load
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BPF IR decoders and test IR decoding. Some BPF IR decoders are also
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provided.
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.. _lirc_modes:
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**********
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LIRC modes
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**********
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LIRC supports some modes of receiving and sending IR codes, as shown
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on the following table.
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.. _lirc-mode-scancode:
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.. _lirc-scancode-flag-toggle:
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.. _lirc-scancode-flag-repeat:
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``LIRC_MODE_SCANCODE``
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This mode is for both sending and receiving IR.
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For transmitting (aka sending), create a struct lirc_scancode with
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the desired scancode set in the ``scancode`` member, :c:type:`rc_proto`
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set to the :ref:`IR protocol <Remote_controllers_Protocols>`, and all other
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members set to 0. Write this struct to the lirc device.
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For receiving, you read struct lirc_scancode from the LIRC device.
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The ``scancode`` field is set to the received scancode and the
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:ref:`IR protocol <Remote_controllers_Protocols>` is set in
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:c:type:`rc_proto`. If the scancode maps to a valid key code, this is set
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in the ``keycode`` field, else it is set to ``KEY_RESERVED``.
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The ``flags`` can have ``LIRC_SCANCODE_FLAG_TOGGLE`` set if the toggle
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bit is set in protocols that support it (e.g. rc-5 and rc-6), or
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``LIRC_SCANCODE_FLAG_REPEAT`` for when a repeat is received for protocols
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that support it (e.g. nec).
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In the Sanyo and NEC protocol, if you hold a button on remote, rather than
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repeating the entire scancode, the remote sends a shorter message with
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no scancode, which just means button is held, a "repeat". When this is
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received, the ``LIRC_SCANCODE_FLAG_REPEAT`` is set and the scancode and
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keycode is repeated.
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With nec, there is no way to distinguish "button hold" from "repeatedly
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pressing the same button". The rc-5 and rc-6 protocols have a toggle bit.
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When a button is released and pressed again, the toggle bit is inverted.
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If the toggle bit is set, the ``LIRC_SCANCODE_FLAG_TOGGLE`` is set.
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The ``timestamp`` field is filled with the time nanoseconds
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(in ``CLOCK_MONOTONIC``) when the scancode was decoded.
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.. _lirc-mode-mode2:
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``LIRC_MODE_MODE2``
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The driver returns a sequence of pulse and space codes to userspace,
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as a series of u32 values.
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This mode is used only for IR receive.
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The upper 8 bits determine the packet type, and the lower 24 bits
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the payload. Use ``LIRC_VALUE()`` macro to get the payload, and
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the macro ``LIRC_MODE2()`` will give you the type, which
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is one of:
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``LIRC_MODE2_PULSE``
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Signifies the presence of IR in microseconds, also known as *flash*.
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``LIRC_MODE2_SPACE``
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Signifies absence of IR in microseconds, also known as *gap*.
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``LIRC_MODE2_FREQUENCY``
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If measurement of the carrier frequency was enabled with
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:ref:`lirc_set_measure_carrier_mode` then this packet gives you
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the carrier frequency in Hertz.
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``LIRC_MODE2_TIMEOUT``
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When the timeout set with :ref:`lirc_set_rec_timeout` expires due
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to no IR being detected, this packet will be sent, with the number
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of microseconds with no IR.
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``LIRC_MODE2_OVERFLOW``
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Signifies that the IR receiver encounter an overflow, and some IR
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is missing. The IR data after this should be correct again. The
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actual value is not important, but this is set to 0xffffff by the
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kernel for compatibility with lircd.
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.. _lirc-mode-pulse:
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``LIRC_MODE_PULSE``
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In pulse mode, a sequence of pulse/space integer values are written to the
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lirc device using :ref:`lirc-write`.
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The values are alternating pulse and space lengths, in microseconds. The
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first and last entry must be a pulse, so there must be an odd number
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of entries.
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This mode is used only for IR send.
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*************************************
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Data types used by LIRC_MODE_SCANCODE
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*************************************
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.. kernel-doc:: include/uapi/linux/lirc.h
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:identifiers: lirc_scancode rc_proto
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********************
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BPF based IR decoder
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********************
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The kernel has support for decoding the most common
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:ref:`IR protocols <Remote_controllers_Protocols>`, but there
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are many protocols which are not supported. To support these, it is possible
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to load an BPF program which does the decoding. This can only be done on
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LIRC devices which support reading raw IR.
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First, using the `bpf(2)`_ syscall with the ``BPF_LOAD_PROG`` argument,
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program must be loaded of type ``BPF_PROG_TYPE_LIRC_MODE2``. Once attached
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to the LIRC device, this program will be called for each pulse, space or
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timeout event on the LIRC device. The context for the BPF program is a
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pointer to a unsigned int, which is a :ref:`LIRC_MODE_MODE2 <lirc-mode-mode2>`
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value. When the program has decoded the scancode, it can be submitted using
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the BPF functions ``bpf_rc_keydown()`` or ``bpf_rc_repeat()``. Mouse or pointer
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movements can be reported using ``bpf_rc_pointer_rel()``.
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Once you have the file descriptor for the ``BPF_PROG_TYPE_LIRC_MODE2`` BPF
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program, it can be attached to the LIRC device using the `bpf(2)`_ syscall.
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The target must be the file descriptor for the LIRC device, and the
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attach type must be ``BPF_LIRC_MODE2``. No more than 64 BPF programs can be
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attached to a single LIRC device at a time.
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.. _bpf(2): http://man7.org/linux/man-pages/man2/bpf.2.html
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