V4L/DVB: IR/lirc: add docbook info covering lirc device interface

<title>LIRC Device Interface</title>

<section id="lirc_dev_intro">

<title>Introduction</title>

<para>The LIRC device interface is a bi-directional interface for

transporting raw IR 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 data to and fro, the essential fops are read, write

and ioctl.</para>

<para>Example dmesg output upon a driver registering w/LIRC:</para>

  <blockquote>

    <para>$ dmesg |grep lirc_dev</para>

    <para>lirc_dev: IR Remote Control driver registered, major 248</para>

    <para>rc rc0: lirc_dev: driver ir-lirc-codec (mceusb) registered at minor = 0</para>

  </blockquote>

<para>

<para>What you should see for a chardev:</para>

  <blockquote>

    <para>$ ls -l /dev/lirc*</para>

    <para>crw-rw---- 1 root root 248, 0 Jul  2 22:20 /dev/lirc0</para>

  </blockquote>

</para>

<section id="lirc_read">

<title>LIRC read fop</title>

<para>The lircd userspace daemon reads raw IR data from the LIRC chardev. The

exact format of the data depends on what modes a driver supports, and what

mode has been selected. lircd obtains supported modes and sets the active mode

via the ioctl interface, detailed at <xref linkend="lirc_ioctl">. The generally

preferred mode is LIRC_MODE_MODE2, in which packets containing an int value

describing an IR signal are read from the chardev.</para>

<para>See also <ulink url="http://www.lirc.org/html/technical.html">http://www.lirc.org/html/technical.html</> for more info.</para>

<section id="lirc_write">

<title>LIRC write fop</title>

<para>The data written to the chardev is a pulse/space sequence of integer

values. Pulses and spaces are only marked implicitly by their position. The

data must start and end with a pulse, therefore, the data must always include

an unevent number of samples. The write function must block until the data has

been transmitted by the hardware.</para>

<section id="lirc_ioctl">

<title> LIRC ioctl fop</title>

<para>The LIRC device's ioctl definition is bound by the ioctl function

definition of struct file_operations, leaving us with an unsigned int

for the ioctl command and an unsigned long for the arg. For the purposes

of ioctl portability across 32-bit and 64-bit, these values are capped

to their 32-bit sizes.</para>

<para>The following ioctls can be used to change specific hardware settings.

In general each driver should have a default set of settings. The driver

implementation is expected to re-apply the default settings when the device

is closed by user-space, so that every application opening the device can rely

on working with the default settings initially.</para>

<variablelist>

  <varlistentry>

    <term>LIRC_GET_FEATURES</term>

    <listitem>

      <to>Obviously, get the underlying hardware device's features. If a driver

      does not announce support of certain features, calling of the corresponding

      ioctls is undefined.</to>

    </listitem>

  </varlistentry>

  <varlistentry>

    <term>LIRC_GET_SEND_MODE</term>

    <listitem>

      <to>Get supported transmit mode. Only LIRC_MODE_PULSE is supported by lircd.</to>

    </listitem>

  </varlistentry>

  <varlistentry>

    <term>LIRC_GET_REC_MODE</term>

    <listitem>

      <to>Get supported receive modes. Only LIRC_MODE_MODE2 and LIRC_MODE_LIRCCODE

      are supported by lircd.</to>

    </listitem>

  </varlistentry>

  <varlistentry>

    <term>LIRC_GET_SEND_CARRIER</term>

    <listitem>

      <to>Get carrier frequency (in Hz) currently used for transmit.</to>

    </listitem>

  </varlistentry>

  <varlistentry>

    <term>LIRC_GET_REC_CARRIER</term>

    <listitem>

      <to>Get carrier frequency (in Hz) currently used for IR reception.</to>

    </listitem>

  </varlistentry>

  <varlistentry>

    <term>LIRC_{G,S}ET_{SEND,REC}_DUTY_CYCLE</term>

    <listitem>

      <to>Get/set the duty cycle (from 0 to 100) of the carrier signal. Currently,

      no special meaning is defined for 0 or 100, but this could be used to switch

      off carrier generation in the future, so these values should be reserved.</to>

    </listitem>

  </varlistentry>

  <varlistentry>

    <term>LIRC_GET_REC_RESOLUTION</term>

    <listitem>

      <to>Some receiver have maximum resolution which is defined by internal

      sample rate or data format limitations. E.g. it's common that signals can

      only be reported in 50 microsecond steps. This integer value is used by

      lircd to automatically adjust the aeps tolerance value in the lircd

      config file.</to>

    </listitem>

  </varlistentry>

  <varlistentry>

    <term>LIRC_GET_M{IN,AX}_TIMEOUT</term>

    <listitem>

      <to>Some devices have internal timers that can be used to detect when

      there's no IR activity for a long time. This can help lircd in detecting

      that a IR signal is finished and can speed up the decoding process.

      Returns an integer value with the minimum/maximum timeout that can be

      set. Some devices have a fixed timeout, in that case both ioctls will

      return the same value even though the timeout cannot be changed.</to>

    </listitem>

  </varlistentry>

  <varlistentry>

    <term>LIRC_GET_M{IN,AX}_FILTER_{PULSE,SPACE}</term>

    <listitem>

      <to>Some devices are able to filter out spikes in the incoming signal

      using given filter rules. These ioctls return the hardware capabilities

      that describe the bounds of the possible filters. Filter settings depend

      on the IR protocols that are expected. lircd derives the settings from

      all protocols definitions found in its config file.</to>

    </listitem>

  </varlistentry>

  <varlistentry>

    <term>LIRC_GET_LENGTH</term>

    <listitem>

      <to>Retrieves the code length in bits (only for LIRC_MODE_LIRCCODE).

      Reads on the device must be done in blocks matching the bit count.

      The bit could should be rounded up so that it matches full bytes.</to>

    </listitem>

  </varlistentry>

  <varlistentry>

    <term>LIRC_SET_{SEND,REC}_MODE</term>

    <listitem>

      <to>Set send/receive mode. Largely obsolete for send, as only

      LIRC_MODE_PULSE is supported.</to>

    </listitem>

  </varlistentry>

  <varlistentry>

    <term>LIRC_SET_{SEND,REC}_CARRIER</term>

    <listitem>

      <to>Set send/receive carrier (in Hz).</to>

    </listitem>

  </varlistentry>

  <varlistentry>

    <term>LIRC_SET_TRANSMITTER_MASK</term>

    <listitem>

      <to>This enables the given set of transmitters. The first transmitter

      is encoded by the least significant bit, etc. When an invalid bit mask

      is given, i.e. a bit is set, even though the device does not have so many

      transitters, then this ioctl returns the number of available transitters

      and does nothing otherwise.</to>

    </listitem>

  </varlistentry>

  <varlistentry>

    <term>LIRC_SET_REC_TIMEOUT</term>

    <listitem>

      <to>Sets the integer value for IR inactivity timeout (cf.

      LIRC_GET_MIN_TIMEOUT and LIRC_GET_MAX_TIMEOUT). A value of 0 (if

      supported by the hardware) disables all hardware timeouts and data should

      be reported as soon as possible. If the exact value cannot be set, then

      the next possible value _greater_ than the given value should be set.</to>

    </listitem>

  </varlistentry>

  <varlistentry>

    <term>LIRC_SET_REC_TIMEOUT_REPORTS</term>

    <listitem>

      <to>Enable (1) or disable (0) timeout reports in LIRC_MODE_MODE2. By

      default, timeout reports should be turned off.</to>

    </listitem>

  </varlistentry>

  <varlistentry>

    <term>LIRC_SET_REC_FILTER_{,PULSE,SPACE}</term>

    <listitem>

      <to>Pulses/spaces shorter than this are filtered out by hardware. If

      filters cannot be set independently for pulse/space, the corresponding

      ioctls must return an error and LIRC_SET_REC_FILTER shall be used instead.</to>

    </listitem>

  </varlistentry>

  <varlistentry>

    <term>LIRC_SET_MEASURE_CARRIER_MODE</term>

    <listitem>

      <to>Enable (1)/disable (0) measure mode. If enabled, from the next key

      press on, the driver will send LIRC_MODE2_FREQUENCY packets. By default

      this should be turned off.</to>

    </listitem>

  </varlistentry>

  <varlistentry>

    <term>LIRC_SET_REC_{DUTY_CYCLE,CARRIER}_RANGE</term>

    <listitem>

      <to>To set a range use LIRC_SET_REC_DUTY_CYCLE_RANGE/LIRC_SET_REC_CARRIER_RANGE

      with the lower bound first and later LIRC_SET_REC_DUTY_CYCLE/LIRC_SET_REC_CARRIER

      with the upper bound.</to>

    </listitem>

  </varlistentry>

  <varlistentry>

    <term>LIRC_NOTIFY_DECODE</term>

    <listitem>

      <to>This ioctl is called by lircd whenever a successful decoding of an

      incoming IR signal could be done. This can be used by supporting hardware

      to give visual feedback to the user e.g. by flashing a LED.</to>

    </listitem>

  </varlistentry>

  <varlistentry>

    <term>LIRC_SETUP_{START,END}</term>

    <listitem>

      <to>Setting of several driver parameters can be optimized by encapsulating

      the according ioctl calls with LIRC_SETUP_START/LIRC_SETUP_END. When a

      driver receives a LIRC_SETUP_START ioctl it can choose to not commit

      further setting changes to the hardware until a LIRC_SETUP_END is received.

      But this is open to the driver implementation and every driver must also

      handle parameter changes which are not encapsulated by LIRC_SETUP_START

      and LIRC_SETUP_END. Drivers can also choose to ignore these ioctls.</to>

    </listitem>

  </varlistentry>

</variablelist>

</section>