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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);
125 If a sub-device driver implements sink pads, the subdev driver may set the
126 link_validate field in :c:type:`v4l2_subdev_pad_ops` to provide its own link
130 between sub-devices and video nodes.
133 :c:func:`v4l2_subdev_link_validate_default` is used instead. This function
151 asynchronously to bridge devices. An example of such a configuration is a Device
158 run-time bridge-subdevice interaction is in both cases the same.
160 In the **synchronous** case a device (bridge) driver needs to register the
161 :c:type:`v4l2_subdev` with the v4l2_device:
163 	:c:func:`v4l2_device_register_subdev <v4l2_device_register_subdev>`
164 	(:c:type:`v4l2_dev <v4l2_device>`, :c:type:`sd <v4l2_subdev>`).
167 After this function was called successfully the subdev->dev field points to
168 the :c:type:`v4l2_device`.
170 If the v4l2_device parent device has a non-NULL mdev field, the sub-device
173 You can unregister a sub-device using:
175 	:c:func:`v4l2_device_unregister_subdev <v4l2_device_unregister_subdev>`
176 	(:c:type:`sd <v4l2_subdev>`).
180 :c:type:`sd <v4l2_subdev>`->dev == ``NULL``.
184 all the requirements for a successful probing are satisfied. This can include a
185 check for a master clock availability. If any of the conditions aren't satisfied
186 the driver might decide to return ``-EPROBE_DEFER`` to request further reprobing
188 the :c:func:`v4l2_async_register_subdev` function. Unregistration is
189 performed using the :c:func:`v4l2_async_unregister_subdev` call. Subdevices
190 registered this way are stored in a global list of subdevices, ready to be
193 Bridge drivers in turn have to register a notifier object. This is
194 performed using the :c:func:`v4l2_async_nf_register` call. To
196 :c:func:`v4l2_async_nf_unregister`. The former of the two functions
197 takes two arguments: a pointer to struct :c:type:`v4l2_device` and a
198 pointer to struct :c:type:`v4l2_async_notifier`.
201 notifier must be initialized using the :c:func:`v4l2_async_nf_init`.
202 Second, bridge drivers can then begin to form a list of subdevice descriptors
204 to add subdevice descriptors to a notifier, depending on the type of device and
207 :c:func:`v4l2_async_nf_add_fwnode_remote` and
208 :c:func:`v4l2_async_nf_add_i2c` are for bridge and ISP drivers for
209 registering their async sub-devices with the notifier.
211 :c:func:`v4l2_async_register_subdev_sensor` is a helper function for
212 sensor drivers registering their own async sub-device, but it also registers a
213 notifier and further registers async sub-devices for lens and flash devices
214 found in firmware. The notifier for the sub-device is unregistered with the
215 async sub-device.
217 These functions allocate an async sub-device descriptor which is of type struct
218 :c:type:`v4l2_async_subdev` embedded in a driver-specific struct. The &struct
219 :c:type:`v4l2_async_subdev` shall be the first member of this struct:
221 .. code-block:: c
241 registered subdevices to them. If a match is detected the ``.bound()``
243 .complete() callback is called. When a subdevice is removed from the
246 Drivers can store any type of custom data in their driver-specific
247 :c:type:`v4l2_async_subdev` wrapper. If any of that data requires special
250 :c:type:`v4l2_async_subdev`.
255 The advantage of using :c:type:`v4l2_subdev` is that it is a generic struct and
256 does not contain any knowledge about the underlying hardware. So a driver might
257 contain several subdevs that use an I2C bus, but also a subdev that is
264 .. code-block:: c
266 	err = sd->ops->core->g_std(sd, &norm);
270 .. code-block:: c
274 The macro will do the right ``NULL`` pointer checks and returns ``-ENODEV``
275 if :c:type:`sd <v4l2_subdev>` is ``NULL``, ``-ENOIOCTLCMD`` if either
276 :c:type:`sd <v4l2_subdev>`->core or :c:type:`sd <v4l2_subdev>`->core->g_std is ``NULL``, or the act…
277 :c:type:`sd <v4l2_subdev>`->ops->core->g_std ops.
279 It is also possible to call all or a subset of the sub-devices:
281 .. code-block:: c
288 .. code-block:: c
292 Any error except ``-ENOIOCTLCMD`` will exit the loop with that error. If no
293 errors (except ``-ENOIOCTLCMD``) occurred, then 0 is returned.
295 The second argument to both calls is a group ID. If 0, then all subdevs are
296 called. If non-zero, then only those whose group ID match that value will
297 be called. Before a bridge driver registers a subdev it can set
298 :c:type:`sd <v4l2_subdev>`->grp_id to whatever value it wants (it's 0 by
299 default). This value is owned by the bridge driver and the sub-device driver
303 For example, there may be multiple audio chips on a board, each capable of
310 If the sub-device needs to notify its v4l2_device parent of an event, then
312 whether there is a ``notify()`` callback defined and returns ``-ENODEV`` if not.
315 V4L2 sub-device userspace API
316 -----------------------------
319 and control subdevices through the :c:type:`v4l2_subdev_ops` operations in
321 hardware from applications. For complex devices, finer-grained control of the
326 Device nodes named ``v4l-subdev``\ *X* can be created in ``/dev`` to access
327 sub-devices directly. If a sub-device supports direct userspace configuration
330 After registering sub-devices, the :c:type:`v4l2_device` driver can create
331 device nodes for all registered sub-devices marked with
333 :c:func:`v4l2_device_register_subdev_nodes`. Those device nodes will be
334 automatically removed when sub-devices are unregistered.
336 The device node handles a subset of the V4L2 API.
348 	controls implemented in the sub-device. Depending on the driver, those
358 	events generated by the sub-device. Depending on the driver, those
361 	Sub-device drivers that want to use events need to set the
362 	``V4L2_SUBDEV_FL_HAS_EVENTS`` :c:type:`v4l2_subdev`.flags before registering
363 	the sub-device. After registration events can be queued as usual on the
364 	:c:type:`v4l2_subdev`.devnode device node.
371 	All ioctls not in the above list are passed directly to the sub-device
374 Read-only sub-device userspace API
375 ----------------------------------
378 the kernel API realized by :c:type:`v4l2_subdev_ops` structure do not usually
383 configuration through a read-only API, that does not permit applications to
391 through a read-only API.
393 To create a read-only device node for all the subdevices registered with the
394 ``V4L2_SUBDEV_FL_HAS_DEVNODE`` set, the :c:type:`v4l2_device` driver should call
395 :c:func:`v4l2_device_register_ro_subdev_nodes`.
398 sub-device device nodes registered with
399 :c:func:`v4l2_device_register_ro_subdev_nodes`.
405 	These ioctls are only allowed on a read-only subdevice device node
406 	for the :ref:`V4L2_SUBDEV_FORMAT_TRY <v4l2-subdev-format-whence>`
413 	These ioctls are not allowed on a read-only subdevice node.
416 ``V4L2_SUBDEV_FORMAT_ACTIVE``, the core returns a negative error code and
417 the errno variable is set to ``-EPERM``.
419 I2C sub-device drivers
420 ----------------------
423 ease the use of these drivers (``v4l2-common.h``).
425 The recommended method of adding :c:type:`v4l2_subdev` support to an I2C driver
426 is to embed the :c:type:`v4l2_subdev` struct into the state struct that is
428 struct and in that case you can just create a :c:type:`v4l2_subdev` directly.
430 A typical state struct would look like this (where 'chipname' is replaced by
433 .. code-block:: c
440 Initialize the :c:type:`v4l2_subdev` struct as follows:
442 .. code-block:: c
444 	v4l2_i2c_subdev_init(&state->sd, client, subdev_ops);
446 This function will fill in all the fields of :c:type:`v4l2_subdev` ensure that
447 the :c:type:`v4l2_subdev` and i2c_client both point to one another.
449 You should also add a helper inline function to go from a :c:type:`v4l2_subdev`
450 pointer to a chipname_state struct:
452 .. code-block:: c
459 Use this to go from the :c:type:`v4l2_subdev` struct to the ``i2c_client``
462 .. code-block:: c
466 And this to go from an ``i2c_client`` to a :c:type:`v4l2_subdev` struct:
468 .. code-block:: c
473 :c:func:`v4l2_device_unregister_subdev`\ (:c:type:`sd <v4l2_subdev>`)
474 when the ``remove()`` callback is called. This will unregister the sub-device
475 from the bridge driver. It is safe to call this even if the sub-device was
482 :c:func:`v4l2_device_unregister_subdev`\ (:c:type:`sd <v4l2_subdev>`)
488 .. code-block:: c
494 and calls :c:func:`i2c_new_client_device` with the given ``i2c_adapter`` and
498 You can also use the last argument of :c:func:`v4l2_i2c_new_subdev` to pass
500 are only used if the previous argument is 0. A non-zero argument means that you
505 Note that the chipid you pass to :c:func:`v4l2_i2c_new_subdev` is usually
506 the same as the module name. It allows you to specify a chip variant, e.g.
508 The use of chipid is something that needs to be looked at more closely at a
515 :c:func:`v4l2_i2c_new_subdev_board` uses an :c:type:`i2c_board_info` struct
522 The :c:func:`v4l2_i2c_new_subdev` function will call
523 :c:func:`v4l2_i2c_new_subdev_board`, internally filling a
524 :c:type:`i2c_board_info` structure using the ``client_type`` and the
528 -------------------------------------
539 To simplify the subdev drivers the V4L2 subdev API now optionally supports a
541 :c:type:`v4l2_subdev_state`. One instance of state, which contains the active
542 device configuration, is stored in the sub-device itself as part of
543 the :c:type:`v4l2_subdev` structure, while the core associates a try state to
547 Sub-device drivers can opt-in and use state to manage their active configuration
548 by initializing the subdevice state with a call to v4l2_subdev_init_finalize()
549 before registering the sub-device. They must also call v4l2_subdev_cleanup()
550 to release all the allocated resources before unregistering the sub-device.
551 The core automatically allocates and initializes a state for each open file
555 V4L2 sub-device operations that use both the :ref:`ACTIVE and TRY formats
556 <v4l2-subdev-format-whence>` receive the correct state to operate on through
558 caller by calling :c:func:`v4l2_subdev_lock_state()` and
559 :c:func:`v4l2_subdev_unlock_state()`. The caller can do so by calling the subdev
560 operation through the :c:func:`v4l2_subdev_call_state_active()` macro.
562 Operations that do not receive a state parameter implicitly operate on the
564 calling :c:func:`v4l2_subdev_lock_and_get_active_state()`. The sub-device active
565 state must equally be released by calling :c:func:`v4l2_subdev_unlock_state()`.
567 Drivers must never manually access the state stored in the :c:type:`v4l2_subdev`
571 operations, many existing device drivers pass a NULL state when calling
572 operations with :c:func:`v4l2_subdev_call()`. This legacy construct causes
574 as they expect to receive the appropriate state as a parameter. To help the
575 conversion of subdevice drivers to a managed active state without having to
578 the callee's active state with :c:func:`v4l2_subdev_lock_and_get_active_state()`,
583 future these parts should be combined into a single state. For the time being
584 we need a way to handle the locking for these parts. This can be accomplished
585 by sharing a lock. The v4l2_ctrl_handler already supports this via its 'lock'
589 .. code-block:: c
591 	sd->ctrl_handler->lock = &priv->mutex;
592 	sd->state_lock = &priv->mutex;
596 V4L2 sub-device functions and data structures
597 ---------------------------------------------
599 .. kernel-doc:: include/media/v4l2-subdev.h