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As video hardware pipelines become increasingly complex and configurable, the current hardware description through v4l2 subdevices reaches its limits. In addition to enumerating and configuring subdevices, video camera drivers need a way to discover and modify at runtime how those subdevices are connected. This is done through new elements called entities, pads and links. An entity is a basic media hardware 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. 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. 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. Links are stored in the source entity. To make backwards graph walk faster, a copy of all links is also stored in the sink entity. The copy is known as a backlink and is only used to help graph traversal. The entity API is made of three functions: - media_entity_init() initializes an entity. The caller must provide an array of pads as well as an estimated number of links. The links array is allocated dynamically and will be reallocated if it grows beyond the initial estimate. - media_entity_cleanup() frees resources allocated for an entity. It must be called during the cleanup phase after unregistering the entity and before freeing it. - media_entity_create_link() creates a link between two entities. An entry in the link array of each entity is allocated and stores pointers to source and sink pads. When a media device is unregistered, all its entities are unregistered automatically. The code is based on Hans Verkuil <hverkuil@xs4all.nl> initial work. Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com> Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi> Acked-by: Hans Verkuil <hverkuil@xs4all.nl> Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>
77 lines
3.5 KiB
XML
77 lines
3.5 KiB
XML
<partinfo>
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<authorgroup>
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<author>
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<firstname>Laurent</firstname>
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<surname>Pinchart</surname>
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<affiliation><address><email>laurent.pinchart@ideasonboard.com</email></address></affiliation>
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<contrib>Initial version.</contrib>
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</author>
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</authorgroup>
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<copyright>
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<year>2010</year>
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<holder>Laurent Pinchart</holder>
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</copyright>
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<revhistory>
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<!-- Put document revisions here, newest first. -->
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<revision>
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<revnumber>1.0.0</revnumber>
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<date>2010-11-10</date>
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<authorinitials>lp</authorinitials>
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<revremark>Initial revision</revremark>
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</revision>
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</revhistory>
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</partinfo>
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<title>Media Controller API</title>
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<chapter id="media_controller">
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<title>Media Controller</title>
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<section id="media-controller-intro">
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<title>Introduction</title>
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<para>Media devices increasingly handle multiple related functions. Many USB
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cameras include microphones, video capture hardware can also output video,
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or SoC camera interfaces also perform memory-to-memory operations similar to
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video codecs.</para>
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<para>Independent functions, even when implemented in the same hardware, can
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be modelled as separate devices. A USB camera with a microphone will be
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presented to userspace applications as V4L2 and ALSA capture devices. The
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devices' relationships (when using a webcam, end-users shouldn't have to
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manually select the associated USB microphone), while not made available
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directly to applications by the drivers, can usually be retrieved from
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sysfs.</para>
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<para>With more and more advanced SoC devices being introduced, the current
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approach will not scale. Device topologies are getting increasingly complex
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and can't always be represented by a tree structure. Hardware blocks are
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shared between different functions, creating dependencies between seemingly
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unrelated devices.</para>
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<para>Kernel abstraction APIs such as V4L2 and ALSA provide means for
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applications to access hardware parameters. As newer hardware expose an
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increasingly high number of those parameters, drivers need to guess what
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applications really require based on limited information, thereby
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implementing policies that belong to userspace.</para>
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<para>The media controller API aims at solving those problems.</para>
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</section>
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<section id="media-controller-model">
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<title>Media device model</title>
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<para>Discovering a device internal topology, and configuring it at runtime,
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is one of the goals of the media controller API. To achieve this, hardware
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devices are modelled as an oriented graph of building blocks called entities
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connected through pads.</para>
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<para>An entity is a basic media hardware or software building block. It can
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correspond to a large variety of logical blocks such as physical hardware
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devices (CMOS sensor for instance), logical hardware devices (a building
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block in a System-on-Chip image processing pipeline), DMA channels or
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physical connectors.</para>
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<para>A pad is a connection endpoint through which an entity can interact
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with other entities. Data (not restricted to video) produced by an entity
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flows from the entity's output to one or more entity inputs. Pads should not
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be confused with physical pins at chip boundaries.</para>
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<para>A link is a point-to-point oriented connection between two pads,
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either on the same entity or on different entities. Data flows from a source
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pad to a sink pad.</para>
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</section>
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</chapter>
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