[media] DocBook: MC: add the concept of interfaces

    <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>

    devices and Linux Kernel interfaces are modelled as graph objects on

    an oriented graph. The object types that constitute the graph are:</para>

    <itemizedlist>

    <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

    connectors.</para></listitem>

    <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>

    </itemizedlist>

  </section>

</chapter>