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media: lirc: document BPF IR decoding
This is just a start. Signed-off-by: Sean Young <sean@mess.org> Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
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@ -20,6 +20,9 @@ data between userspace and kernelspace. Fundamentally, it is just a chardev
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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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@ -34,6 +37,16 @@ What you should see for a chardev:
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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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@ -129,12 +142,28 @@ on the following table.
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This mode is used only for IR send.
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********************
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BPF based IR decoder
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********************
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**************************
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Remote Controller protocol
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**************************
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The kernel has support for decoding the most common IR 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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An enum :c:type:`rc_proto` in the :ref:`lirc_header` lists all the
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supported IR protocols:
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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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.. kernel-doc:: include/uapi/linux/lirc.h
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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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