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Diffstat (limited to 'Documentation/media-framework.txt')
| -rw-r--r-- | Documentation/media-framework.txt | 151 |
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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. |
