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<contrib>Initial version.</contrib>
<holder>Laurent Pinchart</holder>
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<revremark>Initial revision</revremark>
<title>Media Controller API</title>
<chapter id="media_controller">
<title>Media Controller</title>
<section id="media-controller-intro">
<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
<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 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 and Linux Kernel interfaces are modelled as graph objects on
an oriented graph. The object types that constitute the graph are:</para>
<listitem><para>An <emphasis role="bold">entity</emphasis>
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
<listitem><para>An <emphasis role="bold">interface</emphasis>
is a graph representation of a Linux Kernel userspace API interface,
like a device node or a sysfs file that controls one or more entities
in the graph.</para></listitem>
<listitem><para>A <emphasis role="bold">pad</emphasis>
is a data 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></listitem>
<listitem><para>A <emphasis role="bold">data link</emphasis>
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></listitem>
<listitem><para>An <emphasis role="bold">interface link</emphasis>
is a point-to-point bidirectional control connection between a Linux
Kernel interface and an entity.m</para></listitem>
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<appendix id="media-user-func">
<title>Function Reference</title>
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