1 Linux kernel media framework
2 ============================
4 This document describes the Linux kernel media framework, its data structures,
5 functions and their usage.
11 The media controller API is documented in DocBook format in
12 Documentation/DocBook/media/v4l/media-controller.xml. This document will focus
13 on the kernel-side implementation of the media framework.
16 Abstract media device model
17 ---------------------------
19 Discovering a device internal topology, and configuring it at runtime, is one
20 of the goals of the media framework. To achieve this, hardware devices are
21 modelled as an oriented graph of building blocks called entities connected
24 An entity is a basic media hardware building block. It can correspond to
25 a large variety of logical blocks such as physical hardware devices
26 (CMOS sensor for instance), logical hardware devices (a building block
27 in a System-on-Chip image processing pipeline), DMA channels or physical
30 A pad is a connection endpoint through which an entity can interact with
31 other entities. Data (not restricted to video) produced by an entity
32 flows from the entity's output to one or more entity inputs. Pads should
33 not be confused with physical pins at chip boundaries.
35 A link is a point-to-point oriented connection between two pads, either
36 on the same entity or on different entities. Data flows from a source
43 A media device is represented by a struct media_device instance, defined in
44 include/media/media-device.h. Allocation of the structure is handled by the
45 media device driver, usually by embedding the media_device instance in a
46 larger driver-specific structure.
48 Drivers register media device instances by calling
50 media_device_register(struct media_device *mdev);
52 The caller is responsible for initializing the media_device structure before
53 registration. The following fields must be set:
55 - dev must point to the parent device (usually a pci_dev, usb_interface or
56 platform_device instance).
58 - model must be filled with the device model name as a NUL-terminated UTF-8
59 string. The device/model revision must not be stored in this field.
61 The following fields are optional:
63 - serial is a unique serial number stored as a NUL-terminated ASCII string.
64 The field is big enough to store a GUID in text form. If the hardware
65 doesn't provide a unique serial number this field must be left empty.
67 - bus_info represents the location of the device in the system as a
68 NUL-terminated ASCII string. For PCI/PCIe devices bus_info must be set to
69 "PCI:" (or "PCIe:") followed by the value of pci_name(). For USB devices,
70 the usb_make_path() function must be used. This field is used by
71 applications to distinguish between otherwise identical devices that don't
72 provide a serial number.
74 - hw_revision is the hardware device revision in a driver-specific format.
75 When possible the revision should be formatted with the KERNEL_VERSION
78 - driver_version is formatted with the KERNEL_VERSION macro. The version
79 minor must be incremented when new features are added to the userspace API
80 without breaking binary compatibility. The version major must be
81 incremented when binary compatibility is broken.
83 Upon successful registration a character device named media[0-9]+ is created.
84 The device major and minor numbers are dynamic. The model name is exported as
87 Drivers unregister media device instances by calling
89 media_device_unregister(struct media_device *mdev);
91 Unregistering a media device that hasn't been registered is *NOT* safe.
94 Entities, pads and links
95 ------------------------
99 Entities are represented by a struct media_entity instance, defined in
100 include/media/media-entity.h. The structure is usually embedded into a
101 higher-level structure, such as a v4l2_subdev or video_device instance,
102 although drivers can allocate entities directly.
104 Drivers initialize entity pads by calling
106 media_entity_pads_init(struct media_entity *entity, u16 num_pads,
107 struct media_pad *pads);
109 If no pads are needed, drivers could directly fill entity->num_pads
110 with 0 and entity->pads with NULL or to call the above function that
113 The media_entity name, type, flags, revision and group_id fields should be
114 initialized before calling media_device_register_entity(). Entities embedded
115 in higher-level standard structures can have some of those fields set by the
116 higher-level framework.
118 As the number of pads is known in advance, the pads array is not allocated
119 dynamically but is managed by the entity driver. Most drivers will embed the
120 pads array in a driver-specific structure, avoiding dynamic allocation.
122 Drivers must set the direction of every pad in the pads array before calling
123 media_entity_pads_init. The function will initialize the other pads fields.
125 Unlike the number of pads, the total number of links isn't always known in
126 advance by the entity driver. As an initial estimate, media_entity_pads_init
127 pre-allocates a number of links equal to the number of pads. The links array
128 will be reallocated if it grows beyond the initial estimate.
130 Drivers register entities with a media device by calling
132 media_device_register_entity(struct media_device *mdev,
133 struct media_entity *entity);
135 Entities are identified by a unique positive integer ID. Drivers can provide an
136 ID by filling the media_entity id field prior to registration, or request the
137 media controller framework to assign an ID automatically. Drivers that provide
138 IDs manually must ensure that all IDs are unique. IDs are not guaranteed to be
139 contiguous even when they are all assigned automatically by the framework.
141 Drivers unregister entities by calling
143 media_device_unregister_entity(struct media_entity *entity);
145 Unregistering an entity will not change the IDs of the other entities, and the
146 ID will never be reused for a newly registered entity.
148 When a media device is unregistered, all its entities are unregistered
149 automatically. No manual entities unregistration is then required.
151 Drivers free resources associated with an entity by calling
153 media_entity_cleanup(struct media_entity *entity);
155 This function must be called during the cleanup phase after unregistering the
156 entity. Note that the media_entity instance itself must be freed explicitly by
157 the driver if required.
159 Entities have flags that describe the entity capabilities and state.
161 MEDIA_ENT_FL_DEFAULT indicates the default entity for a given type.
162 This can be used to report the default audio and video devices or the
163 default camera sensor.
165 Logical entity groups can be defined by setting the group ID of all member
166 entities to the same non-zero value. An entity group serves no purpose in the
167 kernel, but is reported to userspace during entities enumeration. The group_id
168 field belongs to the media device driver and must not by touched by entity
171 Media device drivers should define groups if several entities are logically
172 bound together. Example usages include reporting
174 - ALSA, VBI and video nodes that carry the same media stream
175 - lens and flash controllers associated with a sensor
179 Pads are represented by a struct media_pad instance, defined in
180 include/media/media-entity.h. Each entity stores its pads in a pads array
181 managed by the entity driver. Drivers usually embed the array in a
182 driver-specific structure.
184 Pads are identified by their entity and their 0-based index in the pads array.
185 Both information are stored in the media_pad structure, making the media_pad
186 pointer the canonical way to store and pass link references.
188 Pads have flags that describe the pad capabilities and state.
190 MEDIA_PAD_FL_SINK indicates that the pad supports sinking data.
191 MEDIA_PAD_FL_SOURCE indicates that the pad supports sourcing data.
193 One and only one of MEDIA_PAD_FL_SINK and MEDIA_PAD_FL_SOURCE must be set for
198 Links are represented by a struct media_link instance, defined in
199 include/media/media-entity.h. Each entity stores all links originating at or
200 targeting any of its pads in a links array. A given link is thus stored
201 twice, once in the source entity and once in the target entity. The array is
202 pre-allocated and grows dynamically as needed.
204 Drivers create links by calling
206 media_create_pad_link(struct media_entity *source, u16 source_pad,
207 struct media_entity *sink, u16 sink_pad,
210 An entry in the link array of each entity is allocated and stores pointers
211 to source and sink pads.
213 Links have flags that describe the link capabilities and state.
215 MEDIA_LNK_FL_ENABLED indicates that the link is enabled and can be used
216 to transfer media data. When two or more links target a sink pad, only
217 one of them can be enabled at a time.
218 MEDIA_LNK_FL_IMMUTABLE indicates that the link enabled state can't be
219 modified at runtime. If MEDIA_LNK_FL_IMMUTABLE is set, then
220 MEDIA_LNK_FL_ENABLED must also be set since an immutable link is always
227 The media framework provides APIs to iterate over entities in a graph.
229 To iterate over all entities belonging to a media device, drivers can use the
230 media_device_for_each_entity macro, defined in include/media/media-device.h.
232 struct media_entity *entity;
234 media_device_for_each_entity(entity, mdev) {
235 /* entity will point to each entity in turn */
239 Drivers might also need to iterate over all entities in a graph that can be
240 reached only through enabled links starting at a given entity. The media
241 framework provides a depth-first graph traversal API for that purpose.
243 Note that graphs with cycles (whether directed or undirected) are *NOT*
244 supported by the graph traversal API. To prevent infinite loops, the graph
245 traversal code limits the maximum depth to MEDIA_ENTITY_ENUM_MAX_DEPTH,
246 currently defined as 16.
248 Drivers initiate a graph traversal by calling
250 media_entity_graph_walk_start(struct media_entity_graph *graph,
251 struct media_entity *entity);
253 The graph structure, provided by the caller, is initialized to start graph
254 traversal at the given entity.
256 Drivers can then retrieve the next entity by calling
258 media_entity_graph_walk_next(struct media_entity_graph *graph);
260 When the graph traversal is complete the function will return NULL.
262 Graph traversal can be interrupted at any moment. No cleanup function call is
263 required and the graph structure can be freed normally.
265 Helper functions can be used to find a link between two given pads, or a pad
266 connected to another pad through an enabled link
268 media_entity_find_link(struct media_pad *source,
269 struct media_pad *sink);
271 media_entity_remote_pad(struct media_pad *pad);
273 Refer to the kerneldoc documentation for more information.
276 Use count and power handling
277 ----------------------------
279 Due to the wide differences between drivers regarding power management needs,
280 the media controller does not implement power management. However, the
281 media_entity structure includes a use_count field that media drivers can use to
282 track the number of users of every entity for power management needs.
284 The use_count field is owned by media drivers and must not be touched by entity
285 drivers. Access to the field must be protected by the media device graph_mutex
292 Link properties can be modified at runtime by calling
294 media_entity_setup_link(struct media_link *link, u32 flags);
296 The flags argument contains the requested new link flags.
298 The only configurable property is the ENABLED link flag to enable/disable a
299 link. Links marked with the IMMUTABLE link flag can not be enabled or disabled.
301 When a link is enabled or disabled, the media framework calls the
302 link_setup operation for the two entities at the source and sink of the link,
303 in that order. If the second link_setup call fails, another link_setup call is
304 made on the first entity to restore the original link flags.
306 Media device drivers can be notified of link setup operations by setting the
307 media_device::link_notify pointer to a callback function. If provided, the
308 notification callback will be called before enabling and after disabling
311 Entity drivers must implement the link_setup operation if any of their links
312 is non-immutable. The operation must either configure the hardware or store
313 the configuration information to be applied later.
315 Link configuration must not have any side effect on other links. If an enabled
316 link at a sink pad prevents another link at the same pad from being enabled,
317 the link_setup operation must return -EBUSY and can't implicitly disable the
321 Pipelines and media streams
322 ---------------------------
324 When starting streaming, drivers must notify all entities in the pipeline to
325 prevent link states from being modified during streaming by calling
327 media_entity_pipeline_start(struct media_entity *entity,
328 struct media_pipeline *pipe);
330 The function will mark all entities connected to the given entity through
331 enabled links, either directly or indirectly, as streaming.
333 The media_pipeline instance pointed to by the pipe argument will be stored in
334 every entity in the pipeline. Drivers should embed the media_pipeline structure
335 in higher-level pipeline structures and can then access the pipeline through
336 the media_entity pipe field.
338 Calls to media_entity_pipeline_start() can be nested. The pipeline pointer must
339 be identical for all nested calls to the function.
341 media_entity_pipeline_start() may return an error. In that case, it will
342 clean up any of the changes it did by itself.
344 When stopping the stream, drivers must notify the entities with
346 media_entity_pipeline_stop(struct media_entity *entity);
348 If multiple calls to media_entity_pipeline_start() have been made the same
349 number of media_entity_pipeline_stop() calls are required to stop streaming. The
350 media_entity pipe field is reset to NULL on the last nested stop call.
352 Link configuration will fail with -EBUSY by default if either end of the link is
353 a streaming entity. Links that can be modified while streaming must be marked
354 with the MEDIA_LNK_FL_DYNAMIC flag.
356 If other operations need to be disallowed on streaming entities (such as
357 changing entities configuration parameters) drivers can explicitly check the
358 media_entity stream_count field to find out if an entity is streaming. This
359 operation must be done with the media_device graph_mutex held.
365 Link validation is performed by media_entity_pipeline_start() for any
366 entity which has sink pads in the pipeline. The
367 media_entity::link_validate() callback is used for that purpose. In
368 link_validate() callback, entity driver should check that the properties of
369 the source pad of the connected entity and its own sink pad match. It is up
370 to the type of the entity (and in the end, the properties of the hardware)
371 what matching actually means.
373 Subsystems should facilitate link validation by providing subsystem specific
374 helper functions to provide easy access for commonly needed information, and
375 in the end provide a way to use driver-specific callbacks.