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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 and Linux Kernel interfaces are modelled as graph objects on 62 an oriented graph. The object types that constitute the graph are:</para> 63 <itemizedlist> 64 <listitem><para>An <emphasis role="bold">entity</emphasis> 65 is a basic media hardware or software building block. It can correspond to 66 a large variety of logical blocks such as physical hardware devices 67 (CMOS sensor for instance), logical hardware devices (a building block in 68 a System-on-Chip image processing pipeline), DMA channels or physical 69 connectors.</para></listitem> 70 <listitem><para>An <emphasis role="bold">interface</emphasis> 71 is a graph representation of a Linux Kernel userspace API interface, 72 like a device node or a sysfs file that controls one or more entities 73 in the graph.</para></listitem> 74 <listitem><para>A <emphasis role="bold">pad</emphasis> 75 is a data connection endpoint through which an entity can interact with 76 other entities. Data (not restricted to video) produced by an entity 77 flows from the entity's output to one or more entity inputs. Pads should 78 not be confused with physical pins at chip boundaries.</para></listitem> 79 <listitem><para>A <emphasis role="bold">data link</emphasis> 80 is a point-to-point oriented connection between two pads, either on the 81 same entity or on different entities. Data flows from a source pad to a 82 sink pad.</para></listitem> 83 <listitem><para>An <emphasis role="bold">interface link</emphasis> 84 is a point-to-point bidirectional control connection between a Linux 85 Kernel interface and an entity.m</para></listitem> 86 </itemizedlist> 87 </section> 88 89 <!-- All non-ioctl specific data types go here. --> 90 &sub-media-types; 91 </chapter> 92 93 <appendix id="media-user-func"> 94 <title>Function Reference</title> 95 <!-- Keep this alphabetically sorted. --> 96 &sub-media-func-open; 97 &sub-media-func-close; 98 &sub-media-func-ioctl; 99 <!-- All ioctls go here. --> 100 &sub-media-ioc-device-info; 101 &sub-media-ioc-g-topology; 102 &sub-media-ioc-enum-entities; 103 &sub-media-ioc-enum-links; 104 &sub-media-ioc-setup-link; 105 </appendix>
