[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, 151 insertions, 0 deletions

diff --git a/Documentation/media-framework.txt b/Documentation/media-framework.txt
index 1844c3f1..0257bad 100644
--- a/Documentation/media-framework.txt
+++ b/Documentation/media-framework.txt
@@ -13,6 +13,30 @@ Documentation/DocBook/v4l/media-controller.xml. This document will focus on
 the kernel-side implementation of the media framework.
 
 
+Abstract media device model
+---------------------------
+
+Discovering a device internal topology, and configuring it at runtime, is one
+of the goals of the media framework. To achieve this, hardware devices are
+modeled as an oriented graph of building blocks called entities connected
+through pads.
+
+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.
+
+
 Media device
 ------------
 
@@ -65,3 +89,130 @@ Drivers unregister media device instances by calling
 	media_device_unregister(struct media_device *mdev);
 
 Unregistering a media device that hasn't been registered is *NOT* safe.
+
+
+Entities, pads and links
+------------------------
+
+- Entities
+
+Entities are represented by a struct media_entity instance, defined in
+include/media/media-entity.h. The structure is usually embedded into a
+higher-level structure, such as a v4l2_subdev or video_device instance,
+although drivers can allocate entities directly.
+
+Drivers initialize entities by calling
+
+	media_entity_init(struct media_entity *entity, u16 num_pads,
+			  struct media_pad *pads, u16 extra_links);
+
+The media_entity name, type, flags, revision and group_id fields can be
+initialized before or after calling media_entity_init. Entities embedded in
+higher-level standard structures can have some of those fields set by the
+higher-level framework.
+
+As the number of pads is known in advance, the pads array is not allocated
+dynamically but is managed by the entity driver. Most drivers will embed the
+pads array in a driver-specific structure, avoiding dynamic allocation.
+
+Drivers must set the direction of every pad in the pads array before calling
+media_entity_init. The function will initialize the other pads fields.
+
+Unlike the number of pads, the total number of links isn't always known in
+advance by the entity driver. As an initial estimate, media_entity_init
+pre-allocates a number of links equal to the number of pads plus an optional
+number of extra links. The links array will be reallocated if it grows beyond
+the initial estimate.
+
+Drivers register entities with a media device by calling
+
+	media_device_register_entity(struct media_device *mdev,
+				     struct media_entity *entity);
+
+Entities are identified by a unique positive integer ID. Drivers can provide an
+ID by filling the media_entity id field prior to registration, or request the
+media controller framework to assign an ID automatically. Drivers that provide
+IDs manually must ensure that all IDs are unique. IDs are not guaranteed to be
+contiguous even when they are all assigned automatically by the framework.
+
+Drivers unregister entities by calling
+
+	media_device_unregister_entity(struct media_entity *entity);
+
+Unregistering an entity will not change the IDs of the other entities, and the
+ID will never be reused for a newly registered entity.
+
+When a media device is unregistered, all its entities are unregistered
+automatically. No manual entities unregistration is then required.
+
+Drivers free resources associated with an entity by calling
+
+	media_entity_cleanup(struct media_entity *entity);
+
+This function must be called during the cleanup phase after unregistering the
+entity. Note that the media_entity instance itself must be freed explicitly by
+the driver if required.
+
+Entities have flags that describe the entity capabilities and state.
+
+	MEDIA_ENT_FL_DEFAULT indicates the default entity for a given type.
+	This can be used to report the default audio and video devices or the
+	default camera sensor.
+
+Logical entity groups can be defined by setting the group ID of all member
+entities to the same non-zero value. An entity group serves no purpose in the
+kernel, but is reported to userspace during entities enumeration. The group_id
+field belongs to the media device driver and must not by touched by entity
+drivers.
+
+Media device drivers should define groups if several entities are logically
+bound together. Example usages include reporting
+
+	- ALSA, VBI and video nodes that carry the same media stream
+	- lens and flash controllers associated with a sensor
+
+- Pads
+
+Pads are represented by a struct media_pad instance, defined in
+include/media/media-entity.h. Each entity stores its pads in a pads array
+managed by the entity driver. Drivers usually embed the array in a
+driver-specific structure.
+
+Pads are identified by their entity and their 0-based index in the pads array.
+Both information are stored in the media_pad structure, making the media_pad
+pointer the canonical way to store and pass link references.
+
+Pads have flags that describe the pad capabilities and state.
+
+	MEDIA_PAD_FL_SINK indicates that the pad supports sinking data.
+	MEDIA_PAD_FL_SOURCE indicates that the pad supports sourcing data.
+
+One and only one of MEDIA_PAD_FL_SINK and MEDIA_PAD_FL_SOURCE must be set for
+each pad.
+
+- Links
+
+Links are represented by a struct media_link instance, defined in
+include/media/media-entity.h. Each entity stores all links originating at or
+targetting any of its pads in a links array. A given link is thus stored
+twice, once in the source entity and once in the target entity. The array is
+pre-allocated and grows dynamically as needed.
+
+Drivers create links by calling
+
+	media_entity_create_link(struct media_entity *source, u16 source_pad,
+				 struct media_entity *sink,   u16 sink_pad,
+				 u32 flags);
+
+An entry in the link array of each entity is allocated and stores pointers
+to source and sink pads.
+
+Links have flags that describe the link capabilities and state.
+
+	MEDIA_LNK_FL_ENABLED indicates that the link is enabled and can be used
+	to transfer media data. When two or more links target a sink pad, only
+	one of them can be enabled at a time.
+	MEDIA_LNK_FL_IMMUTABLE indicates that the link enabled state can't be
+	modified at runtime. If MEDIA_LNK_FL_IMMUTABLE is set, then
+	MEDIA_LNK_FL_ENABLED must also be set since an immutable link is always
+	enabled.