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author | Mauro Carvalho Chehab <mchehab@osg.samsung.com> | 2015-12-11 10:21:39 (GMT) |
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committer | Mauro Carvalho Chehab <mchehab@osg.samsung.com> | 2016-01-11 14:19:04 (GMT) |
commit | cc2dd94a051c8a467c66a258391cb980c3fb3312 (patch) | |
tree | ae1c069061519529c3056d8ee9cdc1e6ce1c4c5d /include/media/media-device.h | |
parent | 8ed8c88c460fa6ce71deb9847f78a5bff4dfcb0e (diff) | |
download | linux-cc2dd94a051c8a467c66a258391cb980c3fb3312.tar.xz |
[media] DocBook: Move media-framework.txt contents to media-device.h
Instead of using a text file, let's put it together with the
struct documentation for the Media Controller.
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
Diffstat (limited to 'include/media/media-device.h')
-rw-r--r-- | include/media/media-device.h | 378 |
1 files changed, 378 insertions, 0 deletions
diff --git a/include/media/media-device.h b/include/media/media-device.h index 87ff299..6728528 100644 --- a/include/media/media-device.h +++ b/include/media/media-device.h @@ -30,6 +30,384 @@ #include <media/media-devnode.h> #include <media/media-entity.h> +/** + * DOC: Media Controller + * + * 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 entity pads by calling + * + * media_entity_pads_init(struct media_entity *entity, u16 num_pads, + * struct media_pad *pads); + * + * If no pads are needed, drivers could directly fill entity->num_pads + * with 0 and entity->pads with NULL or to call the above function that + * will do the same. + * + * The media_entity name, type and flags fields should be initialized before + * calling media_device_register_entity(). 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_pads_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_pads_init + * pre-allocates a number of links equal to the number of pads. 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. + * + * 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_create_pad_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. + */ + struct device; /** |