Root/Documentation/media-framework.txt

1Linux kernel media framework
2============================
3
4This document describes the Linux kernel media framework, its data structures,
5functions and their usage.
6
7
8Introduction
9------------
10
11The media controller API is documented in DocBook format in
12Documentation/DocBook/v4l/media-controller.xml. This document will focus on
13the kernel-side implementation of the media framework.
14
15
16Abstract media device model
17---------------------------
18
19Discovering a device internal topology, and configuring it at runtime, is one
20of the goals of the media framework. To achieve this, hardware devices are
21modeled as an oriented graph of building blocks called entities connected
22through pads.
23
24An entity is a basic media hardware building block. It can correspond to
25a large variety of logical blocks such as physical hardware devices
26(CMOS sensor for instance), logical hardware devices (a building block
27in a System-on-Chip image processing pipeline), DMA channels or physical
28connectors.
29
30A pad is a connection endpoint through which an entity can interact with
31other entities. Data (not restricted to video) produced by an entity
32flows from the entity's output to one or more entity inputs. Pads should
33not be confused with physical pins at chip boundaries.
34
35A link is a point-to-point oriented connection between two pads, either
36on the same entity or on different entities. Data flows from a source
37pad to a sink pad.
38
39
40Media device
41------------
42
43A media device is represented by a struct media_device instance, defined in
44include/media/media-device.h. Allocation of the structure is handled by the
45media device driver, usually by embedding the media_device instance in a
46larger driver-specific structure.
47
48Drivers register media device instances by calling
49
50    media_device_register(struct media_device *mdev);
51
52The caller is responsible for initializing the media_device structure before
53registration. The following fields must be set:
54
55 - dev must point to the parent device (usually a pci_dev, usb_interface or
56   platform_device instance).
57
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.
60
61The following fields are optional:
62
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.
66
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.
73
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
76   macro.
77
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.
82
83Upon successful registration a character device named media[0-9]+ is created.
84The device major and minor numbers are dynamic. The model name is exported as
85a sysfs attribute.
86
87Drivers unregister media device instances by calling
88
89    media_device_unregister(struct media_device *mdev);
90
91Unregistering a media device that hasn't been registered is *NOT* safe.
92
93
94Entities, pads and links
95------------------------
96
97- Entities
98
99Entities are represented by a struct media_entity instance, defined in
100include/media/media-entity.h. The structure is usually embedded into a
101higher-level structure, such as a v4l2_subdev or video_device instance,
102although drivers can allocate entities directly.
103
104Drivers initialize entities by calling
105
106    media_entity_init(struct media_entity *entity, u16 num_pads,
107              struct media_pad *pads, u16 extra_links);
108
109The media_entity name, type, flags, revision and group_id fields can be
110initialized before or after calling media_entity_init. Entities embedded in
111higher-level standard structures can have some of those fields set by the
112higher-level framework.
113
114As the number of pads is known in advance, the pads array is not allocated
115dynamically but is managed by the entity driver. Most drivers will embed the
116pads array in a driver-specific structure, avoiding dynamic allocation.
117
118Drivers must set the direction of every pad in the pads array before calling
119media_entity_init. The function will initialize the other pads fields.
120
121Unlike the number of pads, the total number of links isn't always known in
122advance by the entity driver. As an initial estimate, media_entity_init
123pre-allocates a number of links equal to the number of pads plus an optional
124number of extra links. The links array will be reallocated if it grows beyond
125the initial estimate.
126
127Drivers register entities with a media device by calling
128
129    media_device_register_entity(struct media_device *mdev,
130                     struct media_entity *entity);
131
132Entities are identified by a unique positive integer ID. Drivers can provide an
133ID by filling the media_entity id field prior to registration, or request the
134media controller framework to assign an ID automatically. Drivers that provide
135IDs manually must ensure that all IDs are unique. IDs are not guaranteed to be
136contiguous even when they are all assigned automatically by the framework.
137
138Drivers unregister entities by calling
139
140    media_device_unregister_entity(struct media_entity *entity);
141
142Unregistering an entity will not change the IDs of the other entities, and the
143ID will never be reused for a newly registered entity.
144
145When a media device is unregistered, all its entities are unregistered
146automatically. No manual entities unregistration is then required.
147
148Drivers free resources associated with an entity by calling
149
150    media_entity_cleanup(struct media_entity *entity);
151
152This function must be called during the cleanup phase after unregistering the
153entity. Note that the media_entity instance itself must be freed explicitly by
154the driver if required.
155
156Entities have flags that describe the entity capabilities and state.
157
158    MEDIA_ENT_FL_DEFAULT indicates the default entity for a given type.
159    This can be used to report the default audio and video devices or the
160    default camera sensor.
161
162Logical entity groups can be defined by setting the group ID of all member
163entities to the same non-zero value. An entity group serves no purpose in the
164kernel, but is reported to userspace during entities enumeration. The group_id
165field belongs to the media device driver and must not by touched by entity
166drivers.
167
168Media device drivers should define groups if several entities are logically
169bound together. Example usages include reporting
170
171    - ALSA, VBI and video nodes that carry the same media stream
172    - lens and flash controllers associated with a sensor
173
174- Pads
175
176Pads are represented by a struct media_pad instance, defined in
177include/media/media-entity.h. Each entity stores its pads in a pads array
178managed by the entity driver. Drivers usually embed the array in a
179driver-specific structure.
180
181Pads are identified by their entity and their 0-based index in the pads array.
182Both information are stored in the media_pad structure, making the media_pad
183pointer the canonical way to store and pass link references.
184
185Pads have flags that describe the pad capabilities and state.
186
187    MEDIA_PAD_FL_SINK indicates that the pad supports sinking data.
188    MEDIA_PAD_FL_SOURCE indicates that the pad supports sourcing data.
189
190One and only one of MEDIA_PAD_FL_SINK and MEDIA_PAD_FL_SOURCE must be set for
191each pad.
192
193- Links
194
195Links are represented by a struct media_link instance, defined in
196include/media/media-entity.h. Each entity stores all links originating at or
197targeting any of its pads in a links array. A given link is thus stored
198twice, once in the source entity and once in the target entity. The array is
199pre-allocated and grows dynamically as needed.
200
201Drivers create links by calling
202
203    media_entity_create_link(struct media_entity *source, u16 source_pad,
204                 struct media_entity *sink, u16 sink_pad,
205                 u32 flags);
206
207An entry in the link array of each entity is allocated and stores pointers
208to source and sink pads.
209
210Links have flags that describe the link capabilities and state.
211
212    MEDIA_LNK_FL_ENABLED indicates that the link is enabled and can be used
213    to transfer media data. When two or more links target a sink pad, only
214    one of them can be enabled at a time.
215    MEDIA_LNK_FL_IMMUTABLE indicates that the link enabled state can't be
216    modified at runtime. If MEDIA_LNK_FL_IMMUTABLE is set, then
217    MEDIA_LNK_FL_ENABLED must also be set since an immutable link is always
218    enabled.
219
220
221Graph traversal
222---------------
223
224The media framework provides APIs to iterate over entities in a graph.
225
226To iterate over all entities belonging to a media device, drivers can use the
227media_device_for_each_entity macro, defined in include/media/media-device.h.
228
229    struct media_entity *entity;
230
231    media_device_for_each_entity(entity, mdev) {
232        /* entity will point to each entity in turn */
233        ...
234    }
235
236Drivers might also need to iterate over all entities in a graph that can be
237reached only through enabled links starting at a given entity. The media
238framework provides a depth-first graph traversal API for that purpose.
239
240Note that graphs with cycles (whether directed or undirected) are *NOT*
241supported by the graph traversal API. To prevent infinite loops, the graph
242traversal code limits the maximum depth to MEDIA_ENTITY_ENUM_MAX_DEPTH,
243currently defined as 16.
244
245Drivers initiate a graph traversal by calling
246
247    media_entity_graph_walk_start(struct media_entity_graph *graph,
248                      struct media_entity *entity);
249
250The graph structure, provided by the caller, is initialized to start graph
251traversal at the given entity.
252
253Drivers can then retrieve the next entity by calling
254
255    media_entity_graph_walk_next(struct media_entity_graph *graph);
256
257When the graph traversal is complete the function will return NULL.
258
259Graph traversal can be interrupted at any moment. No cleanup function call is
260required and the graph structure can be freed normally.
261
262Helper functions can be used to find a link between two given pads, or a pad
263connected to another pad through an enabled link
264
265    media_entity_find_link(struct media_pad *source,
266                   struct media_pad *sink);
267
268    media_entity_remote_source(struct media_pad *pad);
269
270Refer to the kerneldoc documentation for more information.
271
272
273Use count and power handling
274----------------------------
275
276Due to the wide differences between drivers regarding power management needs,
277the media controller does not implement power management. However, the
278media_entity structure includes a use_count field that media drivers can use to
279track the number of users of every entity for power management needs.
280
281The use_count field is owned by media drivers and must not be touched by entity
282drivers. Access to the field must be protected by the media device graph_mutex
283lock.
284
285
286Links setup
287-----------
288
289Link properties can be modified at runtime by calling
290
291    media_entity_setup_link(struct media_link *link, u32 flags);
292
293The flags argument contains the requested new link flags.
294
295The only configurable property is the ENABLED link flag to enable/disable a
296link. Links marked with the IMMUTABLE link flag can not be enabled or disabled.
297
298When a link is enabled or disabled, the media framework calls the
299link_setup operation for the two entities at the source and sink of the link,
300in that order. If the second link_setup call fails, another link_setup call is
301made on the first entity to restore the original link flags.
302
303Media device drivers can be notified of link setup operations by setting the
304media_device::link_notify pointer to a callback function. If provided, the
305notification callback will be called before enabling and after disabling
306links.
307
308Entity drivers must implement the link_setup operation if any of their links
309is non-immutable. The operation must either configure the hardware or store
310the configuration information to be applied later.
311
312Link configuration must not have any side effect on other links. If an enabled
313link at a sink pad prevents another link at the same pad from being enabled,
314the link_setup operation must return -EBUSY and can't implicitly disable the
315first enabled link.
316
317
318Pipelines and media streams
319---------------------------
320
321When starting streaming, drivers must notify all entities in the pipeline to
322prevent link states from being modified during streaming by calling
323
324    media_entity_pipeline_start(struct media_entity *entity,
325                    struct media_pipeline *pipe);
326
327The function will mark all entities connected to the given entity through
328enabled links, either directly or indirectly, as streaming.
329
330The media_pipeline instance pointed to by the pipe argument will be stored in
331every entity in the pipeline. Drivers should embed the media_pipeline structure
332in higher-level pipeline structures and can then access the pipeline through
333the media_entity pipe field.
334
335Calls to media_entity_pipeline_start() can be nested. The pipeline pointer must
336be identical for all nested calls to the function.
337
338When stopping the stream, drivers must notify the entities with
339
340    media_entity_pipeline_stop(struct media_entity *entity);
341
342If multiple calls to media_entity_pipeline_start() have been made the same
343number of media_entity_pipeline_stop() calls are required to stop streaming. The
344media_entity pipe field is reset to NULL on the last nested stop call.
345
346Link configuration will fail with -EBUSY by default if either end of the link is
347a streaming entity. Links that can be modified while streaming must be marked
348with the MEDIA_LNK_FL_DYNAMIC flag.
349
350If other operations need to be disallowed on streaming entities (such as
351changing entities configuration parameters) drivers can explicitly check the
352media_entity stream_count field to find out if an entity is streaming. This
353operation must be done with the media_device graph_mutex held.
354

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