| <partinfo> |
| <authorgroup> |
| <author> |
| <firstname>Laurent</firstname> |
| <surname>Pinchart</surname> |
| <affiliation><address><email>laurent.pinchart@ideasonboard.com</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> |
| </partinfo> |
| |
| <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> |
| </chapter> |