1 .. SPDX-License-Identifier: GPL-2.0
3 Media Controller devices
4 ------------------------
9 The media controller userspace API is documented in
10 :ref:`the Media Controller uAPI book <media_controller>`. This document focus
11 on the kernel-side implementation of the media framework.
13 Abstract media device model
14 ^^^^^^^^^^^^^^^^^^^^^^^^^^^
16 Discovering a device internal topology, and configuring it at runtime, is one
17 of the goals of the media framework. To achieve this, hardware devices are
18 modelled as an oriented graph of building blocks called entities connected
21 An entity is a basic media hardware building block. It can correspond to
22 a large variety of logical blocks such as physical hardware devices
23 (CMOS sensor for instance), logical hardware devices (a building block
24 in a System-on-Chip image processing pipeline), DMA channels or physical
27 A pad is a connection endpoint through which an entity can interact with
28 other entities. Data (not restricted to video) produced by an entity
29 flows from the entity's output to one or more entity inputs. Pads should
30 not be confused with physical pins at chip boundaries.
32 A link is a point-to-point oriented connection between two pads, either
33 on the same entity or on different entities. Data flows from a source
39 A media device is represented by a struct media_device
40 instance, defined in ``include/media/media-device.h``.
41 Allocation of the structure is handled by the media device driver, usually by
42 embedding the :c:type:`media_device` instance in a larger driver-specific
45 Drivers initialise media device instances by calling
46 :c:func:`media_device_init()`. After initialising a media device instance, it is
47 registered by calling :c:func:`__media_device_register()` via the macro
48 ``media_device_register()`` and unregistered by calling
49 :c:func:`media_device_unregister()`. An initialised media device must be
50 eventually cleaned up by calling :c:func:`media_device_cleanup()`.
52 Note that it is not allowed to unregister a media device instance that was not
53 previously registered, or clean up a media device instance that was not
54 previously initialised.
59 Entities are represented by a struct media_entity
60 instance, defined in ``include/media/media-entity.h``. The structure is usually
61 embedded into a higher-level structure, such as
62 :c:type:`v4l2_subdev` or :c:type:`video_device`
63 instances, although drivers can allocate entities directly.
65 Drivers initialize entity pads by calling
66 :c:func:`media_entity_pads_init()`.
68 Drivers register entities with a media device by calling
69 :c:func:`media_device_register_entity()`
70 and unregistered by calling
71 :c:func:`media_device_unregister_entity()`.
76 Interfaces are represented by a
77 struct media_interface instance, defined in
78 ``include/media/media-entity.h``. Currently, only one type of interface is
79 defined: a device node. Such interfaces are represented by a
80 struct media_intf_devnode.
82 Drivers initialize and create device node interfaces by calling
83 :c:func:`media_devnode_create()`
84 and remove them by calling:
85 :c:func:`media_devnode_remove()`.
89 Pads are represented by a struct media_pad instance,
90 defined in ``include/media/media-entity.h``. Each entity stores its pads in
91 a pads array managed by the entity driver. Drivers usually embed the array in
92 a driver-specific structure.
94 Pads are identified by their entity and their 0-based index in the pads
97 Both information are stored in the struct media_pad,
98 making the struct media_pad pointer the canonical way
99 to store and pass link references.
101 Pads have flags that describe the pad capabilities and state.
103 ``MEDIA_PAD_FL_SINK`` indicates that the pad supports sinking data.
104 ``MEDIA_PAD_FL_SOURCE`` indicates that the pad supports sourcing data.
108 One and only one of ``MEDIA_PAD_FL_SINK`` or ``MEDIA_PAD_FL_SOURCE`` must
114 Links are represented by a struct media_link instance,
115 defined in ``include/media/media-entity.h``. There are two types of links:
117 **1. pad to pad links**:
119 Associate two entities via their PADs. Each entity has a list that points
120 to all links originating at or targeting any of its pads.
121 A given link is thus stored twice, once in the source entity and once in
124 Drivers create pad to pad links by calling:
125 :c:func:`media_create_pad_link()` and remove with
126 :c:func:`media_entity_remove_links()`.
128 **2. interface to entity links**:
130 Associate one interface to a Link.
132 Drivers create interface to entity links by calling:
133 :c:func:`media_create_intf_link()` and remove with
134 :c:func:`media_remove_intf_links()`.
138 Links can only be created after having both ends already created.
140 Links have flags that describe the link capabilities and state. The
141 valid values are described at :c:func:`media_create_pad_link()` and
142 :c:func:`media_create_intf_link()`.
147 The media framework provides APIs to iterate over entities in a graph.
149 To iterate over all entities belonging to a media device, drivers can use
150 the media_device_for_each_entity macro, defined in
151 ``include/media/media-device.h``.
155 struct media_entity *entity;
157 media_device_for_each_entity(entity, mdev) {
158 // entity will point to each entity in turn
162 Drivers might also need to iterate over all entities in a graph that can be
163 reached only through enabled links starting at a given entity. The media
164 framework provides a depth-first graph traversal API for that purpose.
168 Graphs with cycles (whether directed or undirected) are **NOT**
169 supported by the graph traversal API. To prevent infinite loops, the graph
170 traversal code limits the maximum depth to ``MEDIA_ENTITY_ENUM_MAX_DEPTH``,
171 currently defined as 16.
173 Drivers initiate a graph traversal by calling
174 :c:func:`media_graph_walk_start()`
176 The graph structure, provided by the caller, is initialized to start graph
177 traversal at the given entity.
179 Drivers can then retrieve the next entity by calling
180 :c:func:`media_graph_walk_next()`
182 When the graph traversal is complete the function will return ``NULL``.
184 Graph traversal can be interrupted at any moment. No cleanup function call
185 is required and the graph structure can be freed normally.
187 Helper functions can be used to find a link between two given pads, or a pad
188 connected to another pad through an enabled link
189 :c:func:`media_entity_find_link()` and
190 :c:func:`media_entity_remote_pad()`.
192 Use count and power handling
193 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
195 Due to the wide differences between drivers regarding power management
196 needs, the media controller does not implement power management. However,
197 the struct media_entity includes a ``use_count``
198 field that media drivers
199 can use to track the number of users of every entity for power management
202 The :c:type:`media_entity<media_entity>`.\ ``use_count`` field is owned by
203 media drivers and must not be
204 touched by entity drivers. Access to the field must be protected by the
205 :c:type:`media_device`.\ ``graph_mutex`` lock.
210 Link properties can be modified at runtime by calling
211 :c:func:`media_entity_setup_link()`.
213 Pipelines and media streams
214 ^^^^^^^^^^^^^^^^^^^^^^^^^^^
216 When starting streaming, drivers must notify all entities in the pipeline to
217 prevent link states from being modified during streaming by calling
218 :c:func:`media_pipeline_start()`.
220 The function will mark all entities connected to the given entity through
221 enabled links, either directly or indirectly, as streaming.
223 The struct media_pipeline instance pointed to by
224 the pipe argument will be stored in every entity in the pipeline.
225 Drivers should embed the struct media_pipeline
226 in higher-level pipeline structures and can then access the
227 pipeline through the struct media_entity
230 Calls to :c:func:`media_pipeline_start()` can be nested.
231 The pipeline pointer must be identical for all nested calls to the function.
233 :c:func:`media_pipeline_start()` may return an error. In that case,
234 it will clean up any of the changes it did by itself.
236 When stopping the stream, drivers must notify the entities with
237 :c:func:`media_pipeline_stop()`.
239 If multiple calls to :c:func:`media_pipeline_start()` have been
240 made the same number of :c:func:`media_pipeline_stop()` calls
241 are required to stop streaming.
242 The :c:type:`media_entity`.\ ``pipe`` field is reset to ``NULL`` on the last
245 Link configuration will fail with ``-EBUSY`` by default if either end of the
246 link is a streaming entity. Links that can be modified while streaming must
247 be marked with the ``MEDIA_LNK_FL_DYNAMIC`` flag.
249 If other operations need to be disallowed on streaming entities (such as
250 changing entities configuration parameters) drivers can explicitly check the
251 media_entity stream_count field to find out if an entity is streaming. This
252 operation must be done with the media_device graph_mutex held.
257 Link validation is performed by :c:func:`media_pipeline_start()`
258 for any entity which has sink pads in the pipeline. The
259 :c:type:`media_entity`.\ ``link_validate()`` callback is used for that
260 purpose. In ``link_validate()`` callback, entity driver should check
261 that the properties of the source pad of the connected entity and its own
262 sink pad match. It is up to the type of the entity (and in the end, the
263 properties of the hardware) what matching actually means.
265 Subsystems should facilitate link validation by providing subsystem specific
266 helper functions to provide easy access for commonly needed information, and
267 in the end provide a way to use driver-specific callbacks.
269 Media Controller Device Allocator API
270 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
272 When the media device belongs to more than one driver, the shared media
273 device is allocated with the shared struct device as the key for look ups.
275 The shared media device should stay in registered state until the last
276 driver unregisters it. In addition, the media device should be released when
277 all the references are released. Each driver gets a reference to the media
278 device during probe, when it allocates the media device. If media device is
279 already allocated, the allocate API bumps up the refcount and returns the
280 existing media device. The driver puts the reference back in its disconnect
281 routine when it calls :c:func:`media_device_delete()`.
283 The media device is unregistered and cleaned up from the kref put handler to
284 ensure that the media device stays in registered state until the last driver
285 unregisters the media device.
289 Drivers should use the appropriate media-core routines to manage the shared
290 media device life-time handling the two states:
291 1. allocate -> register -> delete
292 2. get reference to already registered device -> delete
294 call :c:func:`media_device_delete()` routine to make sure the shared media
295 device delete is handled correctly.
298 Call :c:func:`media_device_usb_allocate()` to allocate or get a reference
299 Call :c:func:`media_device_register()`, if media devnode isn't registered
301 **driver disconnect:**
302 Call :c:func:`media_device_delete()` to free the media_device. Freeing is
303 handled by the kref put handler.
308 .. kernel-doc:: include/media/media-device.h
310 .. kernel-doc:: include/media/media-devnode.h
312 .. kernel-doc:: include/media/media-entity.h
314 .. kernel-doc:: include/media/media-request.h
316 .. kernel-doc:: include/media/media-dev-allocator.h