linux-stable-rt/Documentation/DocBook/media/v4l/media-controller.xml

<partinfo>
  <authorgroup>
    <author>
      <firstname>Laurent</firstname>
      <surname>Pinchart</surname>
      <affiliation><address><email>laurent.pinchart@ideasonboard.com</email></address></affiliation>
      <contrib>Initial version.</contrib>
    </author>
  </authorgroup>
  <copyright>
    <year>2010</year>
    <holder>Laurent Pinchart</holder>
  </copyright>

  <revhistory>
    <!-- Put document revisions here, newest first. -->
    <revision>
      <revnumber>1.0.0</revnumber>
      <date>2010-11-10</date>
      <authorinitials>lp</authorinitials>
      <revremark>Initial revision</revremark>
    </revision>
  </revhistory>
</partinfo>

<title>Media Controller API</title>

<chapter id="media_controller">
  <title>Media Controller</title>

  <section id="media-controller-intro">
    <title>Introduction</title>
    <para>Media devices increasingly handle multiple related functions. Many USB
    cameras include microphones, video capture hardware can also output video,
    or SoC camera interfaces also perform memory-to-memory operations similar to
    video codecs.</para>
    <para>Independent functions, even when implemented in the same hardware, can
    be modelled as separate devices. A USB camera with a microphone will be
    presented to userspace applications as V4L2 and ALSA capture devices. The
    devices' relationships (when using a webcam, end-users shouldn't have to
    manually select the associated USB microphone), while not made available
    directly to applications by the drivers, can usually be retrieved from
    sysfs.</para>
    <para>With more and more advanced SoC devices being introduced, the current
    approach will not scale. Device topologies are getting increasingly complex
    and can't always be represented by a tree structure. Hardware blocks are
    shared between different functions, creating dependencies between seemingly
    unrelated devices.</para>
    <para>Kernel abstraction APIs such as V4L2 and ALSA provide means for
    applications to access hardware parameters. As newer hardware expose an
    increasingly high number of those parameters, drivers need to guess what
    applications really require based on limited information, thereby
    implementing policies that belong to userspace.</para>
    <para>The media controller API aims at solving those problems.</para>
  </section>

  <section id="media-controller-model">
    <title>Media device model</title>
    <para>Discovering a device internal topology, and configuring it at runtime,
    is one of the goals of the media controller API. To achieve this, hardware
    devices are modelled as an oriented graph of building blocks called entities
    connected through pads.</para>
    <para>An entity is a basic media hardware or software building block. It can
    correspond to a large variety of logical blocks such as physical hardware
    devices (CMOS sensor for instance), logical hardware devices (a building
    block in a System-on-Chip image processing pipeline), DMA channels or
    physical connectors.</para>
    <para>A pad is a connection endpoint through which an entity can interact
    with other entities. Data (not restricted to video) produced by an entity
    flows from the entity's output to one or more entity inputs. Pads should not
    be confused with physical pins at chip boundaries.</para>
    <para>A link is a point-to-point oriented connection between two pads,
    either on the same entity or on different entities. Data flows from a source
    pad to a sink pad.</para>
  </section>
</chapter>

<appendix id="media-user-func">
  <title>Function Reference</title>
  <!-- Keep this alphabetically sorted. -->
  &sub-media-func-open;
  &sub-media-func-close;
  &sub-media-func-ioctl;
  <!-- All ioctls go here. -->
  &sub-media-ioc-device-info;
  &sub-media-ioc-enum-entities;
  &sub-media-ioc-enum-links;
  &sub-media-ioc-setup-link;
</appendix>