1.. SPDX-License-Identifier: GPL-2.0
2
3V4L2 sub-devices
4----------------
5
6Many drivers need to communicate with sub-devices. These devices can do all
7sort of tasks, but most commonly they handle audio and/or video muxing,
8encoding or decoding. For webcams common sub-devices are sensors and camera
9controllers.
10
11Usually these are I2C devices, but not necessarily. In order to provide the
12driver with a consistent interface to these sub-devices the
13:c:type:`v4l2_subdev` struct (v4l2-subdev.h) was created.
14
15Each sub-device driver must have a :c:type:`v4l2_subdev` struct. This struct
16can be stand-alone for simple sub-devices or it might be embedded in a larger
17struct if more state information needs to be stored. Usually there is a
18low-level device struct (e.g. ``i2c_client``) that contains the device data as
19setup by the kernel. It is recommended to store that pointer in the private
20data of :c:type:`v4l2_subdev` using :c:func:`v4l2_set_subdevdata`. That makes
21it easy to go from a :c:type:`v4l2_subdev` to the actual low-level bus-specific
22device data.
23
24You also need a way to go from the low-level struct to :c:type:`v4l2_subdev`.
25For the common i2c_client struct the i2c_set_clientdata() call is used to store
26a :c:type:`v4l2_subdev` pointer, for other buses you may have to use other
27methods.
28
29Bridges might also need to store per-subdev private data, such as a pointer to
30bridge-specific per-subdev private data. The :c:type:`v4l2_subdev` structure
31provides host private data for that purpose that can be accessed with
32:c:func:`v4l2_get_subdev_hostdata` and :c:func:`v4l2_set_subdev_hostdata`.
33
34From the bridge driver perspective, you load the sub-device module and somehow
35obtain the :c:type:`v4l2_subdev` pointer. For i2c devices this is easy: you call
36``i2c_get_clientdata()``. For other buses something similar needs to be done.
37Helper functions exist for sub-devices on an I2C bus that do most of this
38tricky work for you.
39
40Each :c:type:`v4l2_subdev` contains function pointers that sub-device drivers
41can implement (or leave ``NULL`` if it is not applicable). Since sub-devices can
42do so many different things and you do not want to end up with a huge ops struct
43of which only a handful of ops are commonly implemented, the function pointers
44are sorted according to category and each category has its own ops struct.
45
46The top-level ops struct contains pointers to the category ops structs, which
47may be NULL if the subdev driver does not support anything from that category.
48
49It looks like this:
50
51.. code-block:: c
52
53	struct v4l2_subdev_core_ops {
54		int (*log_status)(struct v4l2_subdev *sd);
55		int (*init)(struct v4l2_subdev *sd, u32 val);
56		...
57	};
58
59	struct v4l2_subdev_tuner_ops {
60		...
61	};
62
63	struct v4l2_subdev_audio_ops {
64		...
65	};
66
67	struct v4l2_subdev_video_ops {
68		...
69	};
70
71	struct v4l2_subdev_pad_ops {
72		...
73	};
74
75	struct v4l2_subdev_ops {
76		const struct v4l2_subdev_core_ops  *core;
77		const struct v4l2_subdev_tuner_ops *tuner;
78		const struct v4l2_subdev_audio_ops *audio;
79		const struct v4l2_subdev_video_ops *video;
80		const struct v4l2_subdev_pad_ops *video;
81	};
82
83The core ops are common to all subdevs, the other categories are implemented
84depending on the sub-device. E.g. a video device is unlikely to support the
85audio ops and vice versa.
86
87This setup limits the number of function pointers while still making it easy
88to add new ops and categories.
89
90A sub-device driver initializes the :c:type:`v4l2_subdev` struct using:
91
92	:c:func:`v4l2_subdev_init <v4l2_subdev_init>`
93	(:c:type:`sd <v4l2_subdev>`, &\ :c:type:`ops <v4l2_subdev_ops>`).
94
95
96Afterwards you need to initialize :c:type:`sd <v4l2_subdev>`->name with a
97unique name and set the module owner. This is done for you if you use the
98i2c helper functions.
99
100If integration with the media framework is needed, you must initialize the
101:c:type:`media_entity` struct embedded in the :c:type:`v4l2_subdev` struct
102(entity field) by calling :c:func:`media_entity_pads_init`, if the entity has
103pads:
104
105.. code-block:: c
106
107	struct media_pad *pads = &my_sd->pads;
108	int err;
109
110	err = media_entity_pads_init(&sd->entity, npads, pads);
111
112The pads array must have been previously initialized. There is no need to
113manually set the struct media_entity function and name fields, but the
114revision field must be initialized if needed.
115
116A reference to the entity will be automatically acquired/released when the
117subdev device node (if any) is opened/closed.
118
119Don't forget to cleanup the media entity before the sub-device is destroyed:
120
121.. code-block:: c
122
123	media_entity_cleanup(&sd->entity);
124
125If the subdev driver intends to process video and integrate with the media
126framework, it must implement format related functionality using
127:c:type:`v4l2_subdev_pad_ops` instead of :c:type:`v4l2_subdev_video_ops`.
128
129In that case, the subdev driver may set the link_validate field to provide
130its own link validation function. The link validation function is called for
131every link in the pipeline where both of the ends of the links are V4L2
132sub-devices. The driver is still responsible for validating the correctness
133of the format configuration between sub-devices and video nodes.
134
135If link_validate op is not set, the default function
136:c:func:`v4l2_subdev_link_validate_default` is used instead. This function
137ensures that width, height and the media bus pixel code are equal on both source
138and sink of the link. Subdev drivers are also free to use this function to
139perform the checks mentioned above in addition to their own checks.
140
141Subdev registration
142~~~~~~~~~~~~~~~~~~~
143
144There are currently two ways to register subdevices with the V4L2 core. The
145first (traditional) possibility is to have subdevices registered by bridge
146drivers. This can be done when the bridge driver has the complete information
147about subdevices connected to it and knows exactly when to register them. This
148is typically the case for internal subdevices, like video data processing units
149within SoCs or complex PCI(e) boards, camera sensors in USB cameras or connected
150to SoCs, which pass information about them to bridge drivers, usually in their
151platform data.
152
153There are however also situations where subdevices have to be registered
154asynchronously to bridge devices. An example of such a configuration is a Device
155Tree based system where information about subdevices is made available to the
156system independently from the bridge devices, e.g. when subdevices are defined
157in DT as I2C device nodes. The API used in this second case is described further
158below.
159
160Using one or the other registration method only affects the probing process, the
161run-time bridge-subdevice interaction is in both cases the same.
162
163In the **synchronous** case a device (bridge) driver needs to register the
164:c:type:`v4l2_subdev` with the v4l2_device:
165
166	:c:func:`v4l2_device_register_subdev <v4l2_device_register_subdev>`
167	(:c:type:`v4l2_dev <v4l2_device>`, :c:type:`sd <v4l2_subdev>`).
168
169This can fail if the subdev module disappeared before it could be registered.
170After this function was called successfully the subdev->dev field points to
171the :c:type:`v4l2_device`.
172
173If the v4l2_device parent device has a non-NULL mdev field, the sub-device
174entity will be automatically registered with the media device.
175
176You can unregister a sub-device using:
177
178	:c:func:`v4l2_device_unregister_subdev <v4l2_device_unregister_subdev>`
179	(:c:type:`sd <v4l2_subdev>`).
180
181
182Afterwards the subdev module can be unloaded and
183:c:type:`sd <v4l2_subdev>`->dev == ``NULL``.
184
185In the **asynchronous** case subdevice probing can be invoked independently of
186the bridge driver availability. The subdevice driver then has to verify whether
187all the requirements for a successful probing are satisfied. This can include a
188check for a master clock availability. If any of the conditions aren't satisfied
189the driver might decide to return ``-EPROBE_DEFER`` to request further reprobing
190attempts. Once all conditions are met the subdevice shall be registered using
191the :c:func:`v4l2_async_register_subdev` function. Unregistration is
192performed using the :c:func:`v4l2_async_unregister_subdev` call. Subdevices
193registered this way are stored in a global list of subdevices, ready to be
194picked up by bridge drivers.
195
196Bridge drivers in turn have to register a notifier object. This is
197performed using the :c:func:`v4l2_async_notifier_register` call. To
198unregister the notifier the driver has to call
199:c:func:`v4l2_async_notifier_unregister`. The former of the two functions
200takes two arguments: a pointer to struct :c:type:`v4l2_device` and a
201pointer to struct :c:type:`v4l2_async_notifier`.
202
203Before registering the notifier, bridge drivers must do two things:
204first, the notifier must be initialized using the
205:c:func:`v4l2_async_notifier_init`. Second, bridge drivers can then
206begin to form a list of subdevice descriptors that the bridge device
207needs for its operation. Subdevice descriptors are added to the notifier
208using the :c:func:`v4l2_async_notifier_add_subdev` call. This function
209takes two arguments: a pointer to struct :c:type:`v4l2_async_notifier`,
210and a pointer to the subdevice descripter, which is of type struct
211:c:type:`v4l2_async_subdev`.
212
213The V4L2 core will then use these descriptors to match asynchronously
214registered subdevices to them. If a match is detected the ``.bound()``
215notifier callback is called. After all subdevices have been located the
216.complete() callback is called. When a subdevice is removed from the
217system the .unbind() method is called. All three callbacks are optional.
218
219Calling subdev operations
220~~~~~~~~~~~~~~~~~~~~~~~~~
221
222The advantage of using :c:type:`v4l2_subdev` is that it is a generic struct and
223does not contain any knowledge about the underlying hardware. So a driver might
224contain several subdevs that use an I2C bus, but also a subdev that is
225controlled through GPIO pins. This distinction is only relevant when setting
226up the device, but once the subdev is registered it is completely transparent.
227
228Once te subdev has been registered you can call an ops function either
229directly:
230
231.. code-block:: c
232
233	err = sd->ops->core->g_std(sd, &norm);
234
235but it is better and easier to use this macro:
236
237.. code-block:: c
238
239	err = v4l2_subdev_call(sd, core, g_std, &norm);
240
241The macro will do the right ``NULL`` pointer checks and returns ``-ENODEV``
242if :c:type:`sd <v4l2_subdev>` is ``NULL``, ``-ENOIOCTLCMD`` if either
243:c:type:`sd <v4l2_subdev>`->core or :c:type:`sd <v4l2_subdev>`->core->g_std is ``NULL``, or the actual result of the
244:c:type:`sd <v4l2_subdev>`->ops->core->g_std ops.
245
246It is also possible to call all or a subset of the sub-devices:
247
248.. code-block:: c
249
250	v4l2_device_call_all(v4l2_dev, 0, core, g_std, &norm);
251
252Any subdev that does not support this ops is skipped and error results are
253ignored. If you want to check for errors use this:
254
255.. code-block:: c
256
257	err = v4l2_device_call_until_err(v4l2_dev, 0, core, g_std, &norm);
258
259Any error except ``-ENOIOCTLCMD`` will exit the loop with that error. If no
260errors (except ``-ENOIOCTLCMD``) occurred, then 0 is returned.
261
262The second argument to both calls is a group ID. If 0, then all subdevs are
263called. If non-zero, then only those whose group ID match that value will
264be called. Before a bridge driver registers a subdev it can set
265:c:type:`sd <v4l2_subdev>`->grp_id to whatever value it wants (it's 0 by
266default). This value is owned by the bridge driver and the sub-device driver
267will never modify or use it.
268
269The group ID gives the bridge driver more control how callbacks are called.
270For example, there may be multiple audio chips on a board, each capable of
271changing the volume. But usually only one will actually be used when the
272user want to change the volume. You can set the group ID for that subdev to
273e.g. AUDIO_CONTROLLER and specify that as the group ID value when calling
274``v4l2_device_call_all()``. That ensures that it will only go to the subdev
275that needs it.
276
277If the sub-device needs to notify its v4l2_device parent of an event, then
278it can call ``v4l2_subdev_notify(sd, notification, arg)``. This macro checks
279whether there is a ``notify()`` callback defined and returns ``-ENODEV`` if not.
280Otherwise the result of the ``notify()`` call is returned.
281
282V4L2 sub-device userspace API
283-----------------------------
284
285Bridge drivers traditionally expose one or multiple video nodes to userspace,
286and control subdevices through the :c:type:`v4l2_subdev_ops` operations in
287response to video node operations. This hides the complexity of the underlying
288hardware from applications. For complex devices, finer-grained control of the
289device than what the video nodes offer may be required. In those cases, bridge
290drivers that implement :ref:`the media controller API <media_controller>` may
291opt for making the subdevice operations directly accessible from userpace.
292
293Device nodes named ``v4l-subdev``\ *X* can be created in ``/dev`` to access
294sub-devices directly. If a sub-device supports direct userspace configuration
295it must set the ``V4L2_SUBDEV_FL_HAS_DEVNODE`` flag before being registered.
296
297After registering sub-devices, the :c:type:`v4l2_device` driver can create
298device nodes for all registered sub-devices marked with
299``V4L2_SUBDEV_FL_HAS_DEVNODE`` by calling
300:c:func:`v4l2_device_register_subdev_nodes`. Those device nodes will be
301automatically removed when sub-devices are unregistered.
302
303The device node handles a subset of the V4L2 API.
304
305``VIDIOC_QUERYCTRL``,
306``VIDIOC_QUERYMENU``,
307``VIDIOC_G_CTRL``,
308``VIDIOC_S_CTRL``,
309``VIDIOC_G_EXT_CTRLS``,
310``VIDIOC_S_EXT_CTRLS`` and
311``VIDIOC_TRY_EXT_CTRLS``:
312
313	The controls ioctls are identical to the ones defined in V4L2. They
314	behave identically, with the only exception that they deal only with
315	controls implemented in the sub-device. Depending on the driver, those
316	controls can be also be accessed through one (or several) V4L2 device
317	nodes.
318
319``VIDIOC_DQEVENT``,
320``VIDIOC_SUBSCRIBE_EVENT`` and
321``VIDIOC_UNSUBSCRIBE_EVENT``
322
323	The events ioctls are identical to the ones defined in V4L2. They
324	behave identically, with the only exception that they deal only with
325	events generated by the sub-device. Depending on the driver, those
326	events can also be reported by one (or several) V4L2 device nodes.
327
328	Sub-device drivers that want to use events need to set the
329	``V4L2_SUBDEV_FL_HAS_EVENTS`` :c:type:`v4l2_subdev`.flags before registering
330	the sub-device. After registration events can be queued as usual on the
331	:c:type:`v4l2_subdev`.devnode device node.
332
333	To properly support events, the ``poll()`` file operation is also
334	implemented.
335
336Private ioctls
337
338	All ioctls not in the above list are passed directly to the sub-device
339	driver through the core::ioctl operation.
340
341Read-only sub-device userspace API
342----------------------------------
343
344Bridge drivers that control their connected subdevices through direct calls to
345the kernel API realized by :c:type:`v4l2_subdev_ops` structure do not usually
346want userspace to be able to change the same parameters through the subdevice
347device node and thus do not usually register any.
348
349It is sometimes useful to report to userspace the current subdevice
350configuration through a read-only API, that does not permit applications to
351change to the device parameters but allows interfacing to the subdevice device
352node to inspect them.
353
354For instance, to implement cameras based on computational photography, userspace
355needs to know the detailed camera sensor configuration (in terms of skipping,
356binning, cropping and scaling) for each supported output resolution. To support
357such use cases, bridge drivers may expose the subdevice operations to userspace
358through a read-only API.
359
360To create a read-only device node for all the subdevices registered with the
361``V4L2_SUBDEV_FL_HAS_DEVNODE`` set, the :c:type:`v4l2_device` driver should call
362:c:func:`v4l2_device_register_ro_subdev_nodes`.
363
364Access to the following ioctls for userspace applications is restricted on
365sub-device device nodes registered with
366:c:func:`v4l2_device_register_ro_subdev_nodes`.
367
368``VIDIOC_SUBDEV_S_FMT``,
369``VIDIOC_SUBDEV_S_CROP``,
370``VIDIOC_SUBDEV_S_SELECTION``:
371
372	These ioctls are only allowed on a read-only subdevice device node
373	for the :ref:`V4L2_SUBDEV_FORMAT_TRY <v4l2-subdev-format-whence>`
374	formats and selection rectangles.
375
376``VIDIOC_SUBDEV_S_FRAME_INTERVAL``,
377``VIDIOC_SUBDEV_S_DV_TIMINGS``,
378``VIDIOC_SUBDEV_S_STD``:
379
380	These ioctls are not allowed on a read-only subdevice node.
381
382In case the ioctl is not allowed, or the format to modify is set to
383``V4L2_SUBDEV_FORMAT_ACTIVE``, the core returns a negative error code and
384the errno variable is set to ``-EPERM``.
385
386I2C sub-device drivers
387----------------------
388
389Since these drivers are so common, special helper functions are available to
390ease the use of these drivers (``v4l2-common.h``).
391
392The recommended method of adding :c:type:`v4l2_subdev` support to an I2C driver
393is to embed the :c:type:`v4l2_subdev` struct into the state struct that is
394created for each I2C device instance. Very simple devices have no state
395struct and in that case you can just create a :c:type:`v4l2_subdev` directly.
396
397A typical state struct would look like this (where 'chipname' is replaced by
398the name of the chip):
399
400.. code-block:: c
401
402	struct chipname_state {
403		struct v4l2_subdev sd;
404		...  /* additional state fields */
405	};
406
407Initialize the :c:type:`v4l2_subdev` struct as follows:
408
409.. code-block:: c
410
411	v4l2_i2c_subdev_init(&state->sd, client, subdev_ops);
412
413This function will fill in all the fields of :c:type:`v4l2_subdev` ensure that
414the :c:type:`v4l2_subdev` and i2c_client both point to one another.
415
416You should also add a helper inline function to go from a :c:type:`v4l2_subdev`
417pointer to a chipname_state struct:
418
419.. code-block:: c
420
421	static inline struct chipname_state *to_state(struct v4l2_subdev *sd)
422	{
423		return container_of(sd, struct chipname_state, sd);
424	}
425
426Use this to go from the :c:type:`v4l2_subdev` struct to the ``i2c_client``
427struct:
428
429.. code-block:: c
430
431	struct i2c_client *client = v4l2_get_subdevdata(sd);
432
433And this to go from an ``i2c_client`` to a :c:type:`v4l2_subdev` struct:
434
435.. code-block:: c
436
437	struct v4l2_subdev *sd = i2c_get_clientdata(client);
438
439Make sure to call
440:c:func:`v4l2_device_unregister_subdev`\ (:c:type:`sd <v4l2_subdev>`)
441when the ``remove()`` callback is called. This will unregister the sub-device
442from the bridge driver. It is safe to call this even if the sub-device was
443never registered.
444
445You need to do this because when the bridge driver destroys the i2c adapter
446the ``remove()`` callbacks are called of the i2c devices on that adapter.
447After that the corresponding v4l2_subdev structures are invalid, so they
448have to be unregistered first. Calling
449:c:func:`v4l2_device_unregister_subdev`\ (:c:type:`sd <v4l2_subdev>`)
450from the ``remove()`` callback ensures that this is always done correctly.
451
452
453The bridge driver also has some helper functions it can use:
454
455.. code-block:: c
456
457	struct v4l2_subdev *sd = v4l2_i2c_new_subdev(v4l2_dev, adapter,
458					"module_foo", "chipid", 0x36, NULL);
459
460This loads the given module (can be ``NULL`` if no module needs to be loaded)
461and calls :c:func:`i2c_new_client_device` with the given ``i2c_adapter`` and
462chip/address arguments. If all goes well, then it registers the subdev with
463the v4l2_device.
464
465You can also use the last argument of :c:func:`v4l2_i2c_new_subdev` to pass
466an array of possible I2C addresses that it should probe. These probe addresses
467are only used if the previous argument is 0. A non-zero argument means that you
468know the exact i2c address so in that case no probing will take place.
469
470Both functions return ``NULL`` if something went wrong.
471
472Note that the chipid you pass to :c:func:`v4l2_i2c_new_subdev` is usually
473the same as the module name. It allows you to specify a chip variant, e.g.
474"saa7114" or "saa7115". In general though the i2c driver autodetects this.
475The use of chipid is something that needs to be looked at more closely at a
476later date. It differs between i2c drivers and as such can be confusing.
477To see which chip variants are supported you can look in the i2c driver code
478for the i2c_device_id table. This lists all the possibilities.
479
480There are one more helper function:
481
482:c:func:`v4l2_i2c_new_subdev_board` uses an :c:type:`i2c_board_info` struct
483which is passed to the i2c driver and replaces the irq, platform_data and addr
484arguments.
485
486If the subdev supports the s_config core ops, then that op is called with
487the irq and platform_data arguments after the subdev was setup.
488
489The :c:func:`v4l2_i2c_new_subdev` function will call
490:c:func:`v4l2_i2c_new_subdev_board`, internally filling a
491:c:type:`i2c_board_info` structure using the ``client_type`` and the
492``addr`` to fill it.
493
494V4L2 sub-device functions and data structures
495---------------------------------------------
496
497.. kernel-doc:: include/media/v4l2-subdev.h
498