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- <authorgroup>
- <author>
- <firstname>Laurent</firstname>
- <surname>Pinchart</surname>
- <affiliation><address><email></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>
-<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>
-<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;