4. Media Controller devices

4.1. Media Controller

The media controller userspace API is documented in the Media Controller uAPI book. This document focus on the kernel-side implementation of the media framework.

4.1.1. 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.

4.1.2. 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().

4.1.3. 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 v4l2_subdev or video_device instances, 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().

4.1.4. 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().

4.1.5. 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 struct media_pad, making the struct 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 or MEDIA_PAD_FL_SOURCE must be set for each pad.

4.1.7. 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.


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_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_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().

4.1.8. 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 struct media_entity 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.

4.1.10. 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_pipeline_start().

The function will mark all entities connected to the given entity through enabled links, either directly or indirectly, as streaming.

The struct media_pipeline instance pointed to by the pipe argument will be stored in every entity in the pipeline. Drivers should embed the struct media_pipeline in higher-level pipeline structures and can then access the pipeline through the struct media_entity pipe field.

Calls to media_pipeline_start() can be nested. The pipeline pointer must be identical for all nested calls to the function.

media_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_pipeline_stop().

If multiple calls to media_pipeline_start() have been made the same number of media_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.