Documentation/DocBook/v4l/media-controller.xml

   1 <partinfo>
   2   <authorgroup>
   3     <author>
   4       <firstname>Laurent</firstname>
   5       <surname>Pinchart</surname>
   6       <affiliation><address><email>laurent.pinchart@ideasonboard.com</email></address></affiliation>
   7       <contrib>Initial version.</contrib>
   8     </author>
   9   </authorgroup>
  10   <copyright>
  11     <year>2010</year>
  12     <holder>Laurent Pinchart</holder>
  13   </copyright>
  14
  15   <revhistory>
  16     <!-- Put document revisions here, newest first. -->
  17     <revision>
  18       <revnumber>1.0.0</revnumber>
  19       <date>2010-11-10</date>
  20       <authorinitials>lp</authorinitials>
  21       <revremark>Initial revision</revremark>
  22     </revision>
  23   </revhistory>
  24 </partinfo>
  25
  26 <title>Media Controller API</title>
  27
  28 <chapter id="media_controller">
  29   <title>Media Controller</title>
  30
  31   <section id="media-controller-intro">
  32     <title>Introduction</title>
  33     <para>Media devices increasingly handle multiple related functions. Many USB
  34     cameras include microphones, video capture hardware can also output video,
  35     or SoC camera interfaces also perform memory-to-memory operations similar to
  36     video codecs.</para>
  37     <para>Independent functions, even when implemented in the same hardware, can
  38     be modelled as separate devices. A USB camera with a microphone will be
  39     presented to userspace applications as V4L2 and ALSA capture devices. The
  40     devices' relationships (when using a webcam, end-users shouldn't have to
  41     manually select the associated USB microphone), while not made available
  42     directly to applications by the drivers, can usually be retrieved from
  43     sysfs.</para>
  44     <para>With more and more advanced SoC devices being introduced, the current
  45     approach will not scale. Device topologies are getting increasingly complex
  46     and can't always be represented by a tree structure. Hardware blocks are
  47     shared between different functions, creating dependencies between seemingly
  48     unrelated devices.</para>
  49     <para>Kernel abstraction APIs such as V4L2 and ALSA provide means for
  50     applications to access hardware parameters. As newer hardware expose an
  51     increasingly high number of those parameters, drivers need to guess what
  52     applications really require based on limited information, thereby
  53     implementing policies that belong to userspace.</para>
  54     <para>The media controller API aims at solving those problems.</para>
  55   </section>
  56
  57   <section id="media-controller-model">
  58     <title>Media device model</title>
  59     <para>Discovering a device internal topology, and configuring it at runtime,
  60     is one of the goals of the media controller API. To achieve this, hardware
  61     devices are modelled as an oriented graph of building blocks called entities
  62     connected through pads.</para>
  63     <para>An entity is a basic media hardware or software building block. It can
  64     correspond to a large variety of logical blocks such as physical hardware
  65     devices (CMOS sensor for instance), logical hardware devices (a building
  66     block in a System-on-Chip image processing pipeline), DMA channels or
  67     physical connectors.</para>
  68     <para>A pad is a connection endpoint through which an entity can interact
  69     with other entities. Data (not restricted to video) produced by an entity
  70     flows from the entity's output to one or more entity inputs. Pads should not
  71     be confused with physical pins at chip boundaries.</para>
  72     <para>A link is a point-to-point oriented connection between two pads,
  73     either on the same entity or on different entities. Data flows from a source
  74     pad to a sink pad.</para>
  75   </section>
  76 </chapter>
  77
  78 <appendix id="media-user-func">
  79   <title>Function Reference</title>
  80   <!-- Keep this alphabetically sorted. -->
  81   &sub-media-open;
  82   &sub-media-close;
  83   &sub-media-ioctl;
  84   <!-- All ioctls go here. -->
  85   &sub-media-ioc-device-info;
  86   &sub-media-ioc-enum-entities;
  87   &sub-media-ioc-enum-links;
  88   &sub-media-ioc-setup-link;
  89 </appendix>