[media] media: Entities, pads and links

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>

1 files changed, 20 insertions, 0 deletions

diff --git a/Documentation/DocBook/v4l/media-controller.xml b/Documentation/DocBook/v4l/media-controller.xml
index 253ddb4426c9..f89228d3ec2a 100644
--- a/Documentation/DocBook/v4l/media-controller.xml
+++ b/Documentation/DocBook/v4l/media-controller.xml
@@ -53,4 +53,24 @@
     implementing policies that belong to userspace.</para>
     <para>The media controller API aims at solving those problems.</para>
   </section>
+
+  <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 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>
+  </section>
 </chapter>