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1 .. SPDX-License-Identifier: GPL-2.0
3 V4L2 sub-devices
4 ----------------
6 Many drivers need to communicate with sub-devices. These devices can do all
8 encoding or decoding. For webcams common sub-devices are sensors and camera
12 driver with a consistent interface to these sub-devices the
13 :c:type:`v4l2_subdev` struct (v4l2-subdev.h) was created.
15 Each sub-device driver must have a :c:type:`v4l2_subdev` struct. This struct
16 can be stand-alone for simple sub-devices or it might be embedded in a larger
17 struct if more state information needs to be stored. Usually there is a
18 low-level device struct (e.g. ``i2c_client``) that contains the device data as
20 data of :c:type:`v4l2_subdev` using :c:func:`v4l2_set_subdevdata`. That makes
21 it easy to go from a :c:type:`v4l2_subdev` to the actual low-level bus-specific
24 You also need a way to go from the low-level struct to :c:type:`v4l2_subdev`.
26 a :c:type:`v4l2_subdev` pointer, for other buses you may have to use other
29 Bridges might also need to store per-subdev private data, such as a pointer to
30 bridge-specific per-subdev private data. The :c:type:`v4l2_subdev` structure
32 :c:func:`v4l2_get_subdev_hostdata` and :c:func:`v4l2_set_subdev_hostdata`.
34 From the bridge driver perspective, you load the sub-device module and somehow
35 obtain the :c:type:`v4l2_subdev` pointer. For i2c devices this is easy: you call
37 Helper functions exist for sub-devices on an I2C bus that do most of this
40 Each :c:type:`v4l2_subdev` contains function pointers that sub-device drivers
41 can implement (or leave ``NULL`` if it is not applicable). Since sub-devices can
42 do so many different things and you do not want to end up with a huge ops struct
43 of which only a handful of ops are commonly implemented, the function pointers
46 The top-level ops struct contains pointers to the category ops structs, which
51 .. code-block:: c
84 depending on the sub-device. E.g. a video device is unlikely to support the
90 A sub-device driver initializes the :c:type:`v4l2_subdev` struct using:
92 :c:func:`v4l2_subdev_init <v4l2_subdev_init>`
93 (:c:type:`sd <v4l2_subdev>`, &\ :c:type:`ops <v4l2_subdev_ops>`).
96 Afterwards you need to initialize :c:type:`sd <v4l2_subdev>`->name with a
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
105 .. code-block:: c
107 struct media_pad *pads = &my_sd->pads;
110 err = media_entity_pads_init(&sd->entity, npads, pads);
116 A reference to the entity will be automatically acquired/released when the
119 Don't forget to cleanup the media entity before the sub-device is destroyed:
121 .. code-block:: c
123 media_entity_cleanup(&sd->entity);
127 :c:type:`v4l2_subdev_pad_ops` instead of :c:type:`v4l2_subdev_video_ops`.
132 sub-devices. The driver is still responsible for validating the correctness
133 of the format configuration between sub-devices and video nodes.
136 :c:func:`v4l2_subdev_link_validate_default` is used instead. This function
154 asynchronously to bridge devices. An example of such a configuration is a Device
161 run-time bridge-subdevice interaction is in both cases the same.
163 In the **synchronous** case a device (bridge) driver needs to register the
164 :c:type:`v4l2_subdev` with the v4l2_device:
166 :c:func:`v4l2_device_register_subdev <v4l2_device_register_subdev>`
167 (:c:type:`v4l2_dev <v4l2_device>`, :c:type:`sd <v4l2_subdev>`).
170 After this function was called successfully the subdev->dev field points to
171 the :c:type:`v4l2_device`.
173 If the v4l2_device parent device has a non-NULL mdev field, the sub-device
176 You can unregister a sub-device using:
178 :c:func:`v4l2_device_unregister_subdev <v4l2_device_unregister_subdev>`
179 (:c:type:`sd <v4l2_subdev>`).
183 :c:type:`sd <v4l2_subdev>`->dev == ``NULL``.
187 all the requirements for a successful probing are satisfied. This can include a
188 check for a master clock availability. If any of the conditions aren't satisfied
189 the driver might decide to return ``-EPROBE_DEFER`` to request further reprobing
191 the :c:func:`v4l2_async_register_subdev` function. Unregistration is
192 performed using the :c:func:`v4l2_async_unregister_subdev` call. Subdevices
193 registered this way are stored in a global list of subdevices, ready to be
196 Bridge drivers in turn have to register a notifier object. This is
197 performed using the :c:func:`v4l2_async_notifier_register` call. To
199 :c:func:`v4l2_async_notifier_unregister`. The former of the two functions
200 takes two arguments: a pointer to struct :c:type:`v4l2_device` and a
201 pointer to struct :c:type:`v4l2_async_notifier`.
205 :c:func:`v4l2_async_notifier_init`. Second, bridge drivers can then
206 begin to form a list of subdevice descriptors that the bridge device
208 using the :c:func:`v4l2_async_notifier_add_subdev` call. This function
209 takes two arguments: a pointer to struct :c:type:`v4l2_async_notifier`,
210 and a pointer to the subdevice descripter, which is of type struct
211 :c:type:`v4l2_async_subdev`.
214 registered subdevices to them. If a match is detected the ``.bound()``
216 .complete() callback is called. When a subdevice is removed from the
222 The advantage of using :c:type:`v4l2_subdev` is that it is a generic struct and
223 does not contain any knowledge about the underlying hardware. So a driver might
224 contain several subdevs that use an I2C bus, but also a subdev that is
231 .. code-block:: c
233 err = sd->ops->core->g_std(sd, &norm);
237 .. code-block:: c
241 The macro will do the right ``NULL`` pointer checks and returns ``-ENODEV``
242 if :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 act…
244 :c:type:`sd <v4l2_subdev>`->ops->core->g_std ops.
246 It is also possible to call all or a subset of the sub-devices:
248 .. code-block:: c
255 .. code-block:: c
259 Any error except ``-ENOIOCTLCMD`` will exit the loop with that error. If no
260 errors (except ``-ENOIOCTLCMD``) occurred, then 0 is returned.
262 The second argument to both calls is a group ID. If 0, then all subdevs are
263 called. If non-zero, then only those whose group ID match that value will
264 be 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
266 default). This value is owned by the bridge driver and the sub-device driver
270 For example, there may be multiple audio chips on a board, each capable of
277 If the sub-device needs to notify its v4l2_device parent of an event, then
279 whether there is a ``notify()`` callback defined and returns ``-ENODEV`` if not.
282 V4L2 sub-device userspace API
283 -----------------------------
286 and control subdevices through the :c:type:`v4l2_subdev_ops` operations in
288 hardware from applications. For complex devices, finer-grained control of the
293 Device nodes named ``v4l-subdev``\ *X* can be created in ``/dev`` to access
294 sub-devices directly. If a sub-device supports direct userspace configuration
297 After registering sub-devices, the :c:type:`v4l2_device` driver can create
298 device nodes for all registered sub-devices marked with
300 :c:func:`v4l2_device_register_subdev_nodes`. Those device nodes will be
301 automatically removed when sub-devices are unregistered.
303 The device node handles a subset of the V4L2 API.
315 controls implemented in the sub-device. Depending on the driver, those
325 events generated by the sub-device. Depending on the driver, those
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.
338 All ioctls not in the above list are passed directly to the sub-device
341 Read-only sub-device userspace API
342 ----------------------------------
345 the kernel API realized by :c:type:`v4l2_subdev_ops` structure do not usually
350 configuration through a read-only API, that does not permit applications to
358 through a read-only API.
360 To 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`.
365 sub-device device nodes registered with
366 :c:func:`v4l2_device_register_ro_subdev_nodes`.
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>`
380 These ioctls are not allowed on a read-only subdevice node.
383 ``V4L2_SUBDEV_FORMAT_ACTIVE``, the core returns a negative error code and
384 the errno variable is set to ``-EPERM``.
386 I2C sub-device drivers
387 ----------------------
390 ease the use of these drivers (``v4l2-common.h``).
392 The recommended method of adding :c:type:`v4l2_subdev` support to an I2C driver
393 is to embed the :c:type:`v4l2_subdev` struct into the state struct that is
395 struct and in that case you can just create a :c:type:`v4l2_subdev` directly.
397 A typical state struct would look like this (where 'chipname' is replaced by
400 .. code-block:: c
407 Initialize the :c:type:`v4l2_subdev` struct as follows:
409 .. code-block:: c
411 v4l2_i2c_subdev_init(&state->sd, client, subdev_ops);
413 This function will fill in all the fields of :c:type:`v4l2_subdev` ensure that
414 the :c:type:`v4l2_subdev` and i2c_client both point to one another.
416 You should also add a helper inline function to go from a :c:type:`v4l2_subdev`
417 pointer to a chipname_state struct:
419 .. code-block:: c
426 Use this to go from the :c:type:`v4l2_subdev` struct to the ``i2c_client``
429 .. code-block:: c
433 And this to go from an ``i2c_client`` to a :c:type:`v4l2_subdev` struct:
435 .. code-block:: c
440 :c:func:`v4l2_device_unregister_subdev`\ (:c:type:`sd <v4l2_subdev>`)
441 when the ``remove()`` callback is called. This will unregister the sub-device
442 from the bridge driver. It is safe to call this even if the sub-device was
449 :c:func:`v4l2_device_unregister_subdev`\ (:c:type:`sd <v4l2_subdev>`)
455 .. code-block:: c
461 and calls :c:func:`i2c_new_client_device` with the given ``i2c_adapter`` and
465 You can also use the last argument of :c:func:`v4l2_i2c_new_subdev` to pass
467 are only used if the previous argument is 0. A non-zero argument means that you
472 Note that the chipid you pass to :c:func:`v4l2_i2c_new_subdev` is usually
473 the same as the module name. It allows you to specify a chip variant, e.g.
475 The use of chipid is something that needs to be looked at more closely at a
482 :c:func:`v4l2_i2c_new_subdev_board` uses an :c:type:`i2c_board_info` struct
489 The :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
494 V4L2 sub-device functions and data structures
495 ---------------------------------------------
497 .. kernel-doc:: include/media/v4l2-subdev.h