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-Linux kernel media framework
-============================
-
-This document describes the Linux kernel media framework, its data structures,
-functions and their usage.
-
-
-Introduction
-------------
-
-The media controller API is documented in DocBook format in
-Documentation/DocBook/media/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
-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(struct media_device *mdev);
-
-The caller is responsible for initializing the media_device structure before
-registration. The following fields must be set:
-
- - dev must point to the parent device (usually a pci_dev, usb_interface or
-   platform_device instance).
-
- - model must be filled with the device model name as a NUL-terminated UTF-8
-   string. The device/model revision must not be stored in this field.
-
-The following fields are optional:
-
- - serial is a unique serial number stored as a NUL-terminated ASCII string.
-   The field is big enough to store a GUID in text form. If the hardware
-   doesn't provide a unique serial number this field must be left empty.
-
- - bus_info represents the location of the device in the system as a
-   NUL-terminated ASCII string. For PCI/PCIe devices bus_info must be set to
-   "PCI:" (or "PCIe:") followed by the value of pci_name(). For USB devices,
-   the usb_make_path() function must be used. This field is used by
-   applications to distinguish between otherwise identical devices that don't
-   provide a serial number.
-
- - hw_revision is the hardware device revision in a driver-specific format.
-   When possible the revision should be formatted with the KERNEL_VERSION
-   macro.
-
- - driver_version is formatted with the KERNEL_VERSION macro. The version
-   minor must be incremented when new features are added to the userspace API
-   without breaking binary compatibility. The version major must be
-   incremented when binary compatibility is broken.
-
-Upon successful registration a character device named media[0-9]+ is created.
-The device major and minor numbers are dynamic. The model name is exported as
-a sysfs attribute.
-
-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
-targeting 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.
-
-
-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(struct media_entity_graph *graph,
-				      struct media_entity *entity);
-
-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(struct media_entity_graph *graph);
-
-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(struct media_pad *source,
-			       struct media_pad *sink);
-
-	media_entity_remote_pad(struct media_pad *pad);
-
-Refer to the kerneldoc documentation for more information.
-
-
-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 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(struct media_link *link, u32 flags);
-
-The flags argument contains the requested new link flags.
-
-The only configurable property is the ENABLED link flag to enable/disable a
-link. Links marked with the IMMUTABLE link flag can not be enabled or disabled.
-
-When a link is enabled or disabled, the media framework calls the
-link_setup operation for the two entities at the source and sink of the link,
-in that order. If the second link_setup call fails, another link_setup call is
-made on the first entity to restore the original link flags.
-
-Media device drivers can be notified of link setup operations by setting the
-media_device::link_notify pointer to a callback function. If provided, the
-notification callback will be called before enabling and after disabling
-links.
-
-Entity drivers must implement the link_setup operation if any of their links
-is non-immutable. The operation must either configure the hardware or store
-the configuration information to be applied later.
-
-Link configuration must not have any side effect on other links. If an enabled
-link at a sink pad prevents another link at the same pad from being enabled,
-the link_setup operation must return -EBUSY and can't implicitly disable the
-first enabled 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(struct media_entity *entity,
-				    struct media_pipeline *pipe);
-
-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(struct media_entity *entity);
-
-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.