[media] doc-rst: split media_drivers.rst into one file per API type

Just like the uAPI book is split into parts, let's split the
kAPI documentation. That should make easier to maintain, and
will split the final documentation into smaller html files.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

1 files changed, 247 insertions, 0 deletions

diff --git a/Documentation/media/kapi/mc-core.rst b/Documentation/media/kapi/mc-core.rst
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+Media Controller devices
+------------------------
+
+Media Controller
+~~~~~~~~~~~~~~~~
+
+
+The media controller userspace API is documented in DocBook format in
+Documentation/DocBook/media/v4l/media-controller.xml. This document 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
+modelled 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
+^^^^^^^^^^^^
+
+A media device is represented by a struct &media_device instance, defined in
+include/media/media-device.h. Allocation of the structure is handled by the
+media device driver, usually by embedding the &media_device instance in a
+larger driver-specific structure.
+
+Drivers register media device instances by calling
+__media_device_register() via the macro media_device_register()
+and unregistered by calling
+media_device_unregister().
+
+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 entity pads by calling
+media_entity_pads_init().
+
+Drivers register entities with a media device by calling
+media_device_register_entity()
+and unregistred by calling
+media_device_unregister_entity().
+
+Interfaces
+^^^^^^^^^^
+
+Interfaces are represented by a struct &media_interface instance, defined in
+include/media/media-entity.h. Currently, only one type of interface is
+defined: a device node. Such interfaces are represented by a struct
+&media_intf_devnode.
+
+Drivers initialize and create device node interfaces by calling
+media_devnode_create()
+and remove them by calling:
+media_devnode_remove().
+
+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.
+
+NOTE: 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. There are two types of links:
+
+1. pad to pad links:
+
+Associate two entities via their PADs. Each entity has a list that points
+to all links originating at or targeting any of its pads.
+A given link is thus stored twice, once in the source entity and once in
+the target entity.
+
+Drivers create pad to pad links by calling:
+media_create_pad_link() and remove with media_entity_remove_links().
+
+2. interface to entity links:
+
+Associate one interface to a Link.
+
+Drivers create interface to entity links by calling:
+media_create_intf_link() and remove with media_remove_intf_links().
+
+.. note::
+
+   Links can only be created after having both ends already created.
+
+Links have flags that describe the link capabilities and state. The
+valid values are described at media_create_pad_link() and
+media_create_intf_link().
+
+Graph traversal
+^^^^^^^^^^^^^^^
+
+The media framework provides APIs to iterate over entities in a graph.
+
+To iterate over all entities belonging to a media device, drivers can use
+the media_device_for_each_entity macro, defined in
+include/media/media-device.h.
+
+struct media_entity *entity;
+
+media_device_for_each_entity(entity, mdev) {
+// entity will point to each entity in turn
+...
+}
+
+Drivers might also need to iterate over all entities in a graph that can be
+reached only through enabled links starting at a given entity. The media
+framework provides a depth-first graph traversal API for that purpose.
+
+Note that graphs with cycles (whether directed or undirected) are *NOT*
+supported by the graph traversal API. To prevent infinite loops, the graph
+traversal code limits the maximum depth to MEDIA_ENTITY_ENUM_MAX_DEPTH,
+currently defined as 16.
+
+Drivers initiate a graph traversal by calling
+media_entity_graph_walk_start()
+
+The graph structure, provided by the caller, is initialized to start graph
+traversal at the given entity.
+
+Drivers can then retrieve the next entity by calling
+media_entity_graph_walk_next()
+
+When the graph traversal is complete the function will return NULL.
+
+Graph traversal can be interrupted at any moment. No cleanup function call
+is required and the graph structure can be freed normally.
+
+Helper functions can be used to find a link between two given pads, or a pad
+connected to another pad through an enabled link
+media_entity_find_link() and media_entity_remote_pad()
+
+Use count and power handling
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Due to the wide differences between drivers regarding power management
+needs, the media controller does not implement power management. However,
+the &media_entity structure includes a use_count field that media drivers
+can use to track the number of users of every entity for power management
+needs.
+
+The &media_entity.@use_count field is owned by media drivers and must not be
+touched by entity drivers. Access to the field must be protected by the
+&media_device.@graph_mutex lock.
+
+Links setup
+^^^^^^^^^^^
+
+Link properties can be modified at runtime by calling
+media_entity_setup_link()
+
+Pipelines and media streams
+^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+When starting streaming, drivers must notify all entities in the pipeline to
+prevent link states from being modified during streaming by calling
+media_entity_pipeline_start().
+
+The function will mark all entities connected to the given entity through
+enabled links, either directly or indirectly, as streaming.
+
+The &media_pipeline instance pointed to by the pipe argument will be stored
+in every entity in the pipeline. Drivers should embed the &media_pipeline
+structure in higher-level pipeline structures and can then access the
+pipeline through the &media_entity pipe field.
+
+Calls to media_entity_pipeline_start() can be nested. The pipeline pointer
+must be identical for all nested calls to the function.
+
+media_entity_pipeline_start() may return an error. In that case, it will
+clean up any of the changes it did by itself.
+
+When stopping the stream, drivers must notify the entities with
+media_entity_pipeline_stop().
+
+If multiple calls to media_entity_pipeline_start() have been made the same
+number of media_entity_pipeline_stop() calls are required to stop streaming.
+The &media_entity pipe field is reset to NULL on the last nested stop call.
+
+Link configuration will fail with -%EBUSY by default if either end of the
+link is a streaming entity. Links that can be modified while streaming must
+be marked with the %MEDIA_LNK_FL_DYNAMIC flag.
+
+If other operations need to be disallowed on streaming entities (such as
+changing entities configuration parameters) drivers can explicitly check the
+media_entity stream_count field to find out if an entity is streaming. This
+operation must be done with the media_device graph_mutex held.
+
+Link validation
+^^^^^^^^^^^^^^^
+
+Link validation is performed by media_entity_pipeline_start() for any
+entity which has sink pads in the pipeline. The
+&media_entity.@link_validate() callback is used for that purpose. In
+@link_validate() callback, entity driver should check that the properties of
+the source pad of the connected entity and its own sink pad match. It is up
+to the type of the entity (and in the end, the properties of the hardware)
+what matching actually means.
+
+Subsystems should facilitate link validation by providing subsystem specific
+helper functions to provide easy access for commonly needed information, and
+in the end provide a way to use driver-specific callbacks.
+
+.. kernel-doc:: include/media/media-device.h
+
+.. kernel-doc:: include/media/media-devnode.h
+
+.. kernel-doc:: include/media/media-entity.h
+