Adding upstream version 6.1.76.upstream/6.1.76upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 599 insertions, 0 deletions

diff --git a/Documentation/driver-api/media/v4l2-subdev.rst b/Documentation/driver-api/media/v4l2-subdev.rst
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+.. SPDX-License-Identifier: GPL-2.0
+
+V4L2 sub-devices
+----------------
+
+Many drivers need to communicate with sub-devices. These devices can do all
+sort of tasks, but most commonly they handle audio and/or video muxing,
+encoding or decoding. For webcams common sub-devices are sensors and camera
+controllers.
+
+Usually these are I2C devices, but not necessarily. In order to provide the
+driver with a consistent interface to these sub-devices the
+:c:type:`v4l2_subdev` struct (v4l2-subdev.h) was created.
+
+Each sub-device driver must have a :c:type:`v4l2_subdev` struct. This struct
+can be stand-alone for simple sub-devices or it might be embedded in a larger
+struct if more state information needs to be stored. Usually there is a
+low-level device struct (e.g. ``i2c_client``) that contains the device data as
+setup by the kernel. It is recommended to store that pointer in the private
+data of :c:type:`v4l2_subdev` using :c:func:`v4l2_set_subdevdata`. That makes
+it easy to go from a :c:type:`v4l2_subdev` to the actual low-level bus-specific
+device data.
+
+You also need a way to go from the low-level struct to :c:type:`v4l2_subdev`.
+For the common i2c_client struct the i2c_set_clientdata() call is used to store
+a :c:type:`v4l2_subdev` pointer, for other buses you may have to use other
+methods.
+
+Bridges might also need to store per-subdev private data, such as a pointer to
+bridge-specific per-subdev private data. The :c:type:`v4l2_subdev` structure
+provides host private data for that purpose that can be accessed with
+:c:func:`v4l2_get_subdev_hostdata` and :c:func:`v4l2_set_subdev_hostdata`.
+
+From the bridge driver perspective, you load the sub-device module and somehow
+obtain the :c:type:`v4l2_subdev` pointer. For i2c devices this is easy: you call
+``i2c_get_clientdata()``. For other buses something similar needs to be done.
+Helper functions exist for sub-devices on an I2C bus that do most of this
+tricky work for you.
+
+Each :c:type:`v4l2_subdev` contains function pointers that sub-device drivers
+can implement (or leave ``NULL`` if it is not applicable). Since sub-devices can
+do so many different things and you do not want to end up with a huge ops struct
+of which only a handful of ops are commonly implemented, the function pointers
+are sorted according to category and each category has its own ops struct.
+
+The top-level ops struct contains pointers to the category ops structs, which
+may be NULL if the subdev driver does not support anything from that category.
+
+It looks like this:
+
+.. code-block:: c
+
+	struct v4l2_subdev_core_ops {
+		int (*log_status)(struct v4l2_subdev *sd);
+		int (*init)(struct v4l2_subdev *sd, u32 val);
+		...
+	};
+
+	struct v4l2_subdev_tuner_ops {
+		...
+	};
+
+	struct v4l2_subdev_audio_ops {
+		...
+	};
+
+	struct v4l2_subdev_video_ops {
+		...
+	};
+
+	struct v4l2_subdev_pad_ops {
+		...
+	};
+
+	struct v4l2_subdev_ops {
+		const struct v4l2_subdev_core_ops  *core;
+		const struct v4l2_subdev_tuner_ops *tuner;
+		const struct v4l2_subdev_audio_ops *audio;
+		const struct v4l2_subdev_video_ops *video;
+		const struct v4l2_subdev_pad_ops *video;
+	};
+
+The core ops are common to all subdevs, the other categories are implemented
+depending on the sub-device. E.g. a video device is unlikely to support the
+audio ops and vice versa.
+
+This setup limits the number of function pointers while still making it easy
+to add new ops and categories.
+
+A sub-device driver initializes the :c:type:`v4l2_subdev` struct using:
+
+	:c:func:`v4l2_subdev_init <v4l2_subdev_init>`
+	(:c:type:`sd <v4l2_subdev>`, &\ :c:type:`ops <v4l2_subdev_ops>`).
+
+
+Afterwards you need to initialize :c:type:`sd <v4l2_subdev>`->name with a
+unique name and set the module owner. This is done for you if you use the
+i2c helper functions.
+
+If integration with the media framework is needed, you must initialize the
+:c:type:`media_entity` struct embedded in the :c:type:`v4l2_subdev` struct
+(entity field) by calling :c:func:`media_entity_pads_init`, if the entity has
+pads:
+
+.. code-block:: c
+
+	struct media_pad *pads = &my_sd->pads;
+	int err;
+
+	err = media_entity_pads_init(&sd->entity, npads, pads);
+
+The pads array must have been previously initialized. There is no need to
+manually set the struct media_entity function and name fields, but the
+revision field must be initialized if needed.
+
+A reference to the entity will be automatically acquired/released when the
+subdev device node (if any) is opened/closed.
+
+Don't forget to cleanup the media entity before the sub-device is destroyed:
+
+.. code-block:: c
+
+	media_entity_cleanup(&sd->entity);
+
+If a sub-device driver implements sink pads, the subdev driver may set the
+link_validate field in :c:type:`v4l2_subdev_pad_ops` to provide its own link
+validation function. For every link in the pipeline, the link_validate pad
+operation of the sink end of the link is called. In both cases the driver is
+still responsible for validating the correctness of the format configuration
+between sub-devices and video nodes.
+
+If link_validate op is not set, the default function
+:c:func:`v4l2_subdev_link_validate_default` is used instead. This function
+ensures that width, height and the media bus pixel code are equal on both source
+and sink of the link. Subdev drivers are also free to use this function to
+perform the checks mentioned above in addition to their own checks.
+
+Subdev registration
+~~~~~~~~~~~~~~~~~~~
+
+There are currently two ways to register subdevices with the V4L2 core. The
+first (traditional) possibility is to have subdevices registered by bridge
+drivers. This can be done when the bridge driver has the complete information
+about subdevices connected to it and knows exactly when to register them. This
+is typically the case for internal subdevices, like video data processing units
+within SoCs or complex PCI(e) boards, camera sensors in USB cameras or connected
+to SoCs, which pass information about them to bridge drivers, usually in their
+platform data.
+
+There are however also situations where subdevices have to be registered
+asynchronously to bridge devices. An example of such a configuration is a Device
+Tree based system where information about subdevices is made available to the
+system independently from the bridge devices, e.g. when subdevices are defined
+in DT as I2C device nodes. The API used in this second case is described further
+below.
+
+Using one or the other registration method only affects the probing process, the
+run-time bridge-subdevice interaction is in both cases the same.
+
+In the **synchronous** case a device (bridge) driver needs to register the
+:c:type:`v4l2_subdev` with the v4l2_device:
+
+	:c:func:`v4l2_device_register_subdev <v4l2_device_register_subdev>`
+	(:c:type:`v4l2_dev <v4l2_device>`, :c:type:`sd <v4l2_subdev>`).
+
+This can fail if the subdev module disappeared before it could be registered.
+After this function was called successfully the subdev->dev field points to
+the :c:type:`v4l2_device`.
+
+If the v4l2_device parent device has a non-NULL mdev field, the sub-device
+entity will be automatically registered with the media device.
+
+You can unregister a sub-device using:
+
+	:c:func:`v4l2_device_unregister_subdev <v4l2_device_unregister_subdev>`
+	(:c:type:`sd <v4l2_subdev>`).
+
+
+Afterwards the subdev module can be unloaded and
+:c:type:`sd <v4l2_subdev>`->dev == ``NULL``.
+
+In the **asynchronous** case subdevice probing can be invoked independently of
+the bridge driver availability. The subdevice driver then has to verify whether
+all the requirements for a successful probing are satisfied. This can include a
+check for a master clock availability. If any of the conditions aren't satisfied
+the driver might decide to return ``-EPROBE_DEFER`` to request further reprobing
+attempts. Once all conditions are met the subdevice shall be registered using
+the :c:func:`v4l2_async_register_subdev` function. Unregistration is
+performed using the :c:func:`v4l2_async_unregister_subdev` call. Subdevices
+registered this way are stored in a global list of subdevices, ready to be
+picked up by bridge drivers.
+
+Bridge drivers in turn have to register a notifier object. This is
+performed using the :c:func:`v4l2_async_nf_register` call. To
+unregister the notifier the driver has to call
+:c:func:`v4l2_async_nf_unregister`. The former of the two functions
+takes two arguments: a pointer to struct :c:type:`v4l2_device` and a
+pointer to struct :c:type:`v4l2_async_notifier`.
+
+Before registering the notifier, bridge drivers must do two things: first, the
+notifier must be initialized using the :c:func:`v4l2_async_nf_init`.
+Second, bridge drivers can then begin to form a list of subdevice descriptors
+that the bridge device needs for its operation. Several functions are available
+to add subdevice descriptors to a notifier, depending on the type of device and
+the needs of the driver.
+
+:c:func:`v4l2_async_nf_add_fwnode_remote` and
+:c:func:`v4l2_async_nf_add_i2c` are for bridge and ISP drivers for
+registering their async sub-devices with the notifier.
+
+:c:func:`v4l2_async_register_subdev_sensor` is a helper function for
+sensor drivers registering their own async sub-device, but it also registers a
+notifier and further registers async sub-devices for lens and flash devices
+found in firmware. The notifier for the sub-device is unregistered with the
+async sub-device.
+
+These functions allocate an async sub-device descriptor which is of type struct
+:c:type:`v4l2_async_subdev` embedded in a driver-specific struct. The &struct
+:c:type:`v4l2_async_subdev` shall be the first member of this struct:
+
+.. code-block:: c
+
+	struct my_async_subdev {
+		struct v4l2_async_subdev asd;
+		...
+	};
+
+	struct my_async_subdev *my_asd;
+	struct fwnode_handle *ep;
+
+	...
+
+	my_asd = v4l2_async_nf_add_fwnode_remote(&notifier, ep,
+						 struct my_async_subdev);
+	fwnode_handle_put(ep);
+
+	if (IS_ERR(asd))
+		return PTR_ERR(asd);
+
+The V4L2 core will then use these descriptors to match asynchronously
+registered subdevices to them. If a match is detected the ``.bound()``
+notifier callback is called. After all subdevices have been located the
+.complete() callback is called. When a subdevice is removed from the
+system the .unbind() method is called. All three callbacks are optional.
+
+Drivers can store any type of custom data in their driver-specific
+:c:type:`v4l2_async_subdev` wrapper. If any of that data requires special
+handling when the structure is freed, drivers must implement the ``.destroy()``
+notifier callback. The framework will call it right before freeing the
+:c:type:`v4l2_async_subdev`.
+
+Calling subdev operations
+~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The advantage of using :c:type:`v4l2_subdev` is that it is a generic struct and
+does not contain any knowledge about the underlying hardware. So a driver might
+contain several subdevs that use an I2C bus, but also a subdev that is
+controlled through GPIO pins. This distinction is only relevant when setting
+up the device, but once the subdev is registered it is completely transparent.
+
+Once the subdev has been registered you can call an ops function either
+directly:
+
+.. code-block:: c
+
+	err = sd->ops->core->g_std(sd, &norm);
+
+but it is better and easier to use this macro:
+
+.. code-block:: c
+
+	err = v4l2_subdev_call(sd, core, g_std, &norm);
+
+The macro will do the right ``NULL`` pointer checks and returns ``-ENODEV``
+if :c:type:`sd <v4l2_subdev>` is ``NULL``, ``-ENOIOCTLCMD`` if either
+:c:type:`sd <v4l2_subdev>`->core or :c:type:`sd <v4l2_subdev>`->core->g_std is ``NULL``, or the actual result of the
+:c:type:`sd <v4l2_subdev>`->ops->core->g_std ops.
+
+It is also possible to call all or a subset of the sub-devices:
+
+.. code-block:: c
+
+	v4l2_device_call_all(v4l2_dev, 0, core, g_std, &norm);
+
+Any subdev that does not support this ops is skipped and error results are
+ignored. If you want to check for errors use this:
+
+.. code-block:: c
+
+	err = v4l2_device_call_until_err(v4l2_dev, 0, core, g_std, &norm);
+
+Any error except ``-ENOIOCTLCMD`` will exit the loop with that error. If no
+errors (except ``-ENOIOCTLCMD``) occurred, then 0 is returned.
+
+The second argument to both calls is a group ID. If 0, then all subdevs are
+called. If non-zero, then only those whose group ID match that value will
+be called. Before a bridge driver registers a subdev it can set
+:c:type:`sd <v4l2_subdev>`->grp_id to whatever value it wants (it's 0 by
+default). This value is owned by the bridge driver and the sub-device driver
+will never modify or use it.
+
+The group ID gives the bridge driver more control how callbacks are called.
+For example, there may be multiple audio chips on a board, each capable of
+changing the volume. But usually only one will actually be used when the
+user want to change the volume. You can set the group ID for that subdev to
+e.g. AUDIO_CONTROLLER and specify that as the group ID value when calling
+``v4l2_device_call_all()``. That ensures that it will only go to the subdev
+that needs it.
+
+If the sub-device needs to notify its v4l2_device parent of an event, then
+it can call ``v4l2_subdev_notify(sd, notification, arg)``. This macro checks
+whether there is a ``notify()`` callback defined and returns ``-ENODEV`` if not.
+Otherwise the result of the ``notify()`` call is returned.
+
+V4L2 sub-device userspace API
+-----------------------------
+
+Bridge drivers traditionally expose one or multiple video nodes to userspace,
+and control subdevices through the :c:type:`v4l2_subdev_ops` operations in
+response to video node operations. This hides the complexity of the underlying
+hardware from applications. For complex devices, finer-grained control of the
+device than what the video nodes offer may be required. In those cases, bridge
+drivers that implement :ref:`the media controller API <media_controller>` may
+opt for making the subdevice operations directly accessible from userpace.
+
+Device nodes named ``v4l-subdev``\ *X* can be created in ``/dev`` to access
+sub-devices directly. If a sub-device supports direct userspace configuration
+it must set the ``V4L2_SUBDEV_FL_HAS_DEVNODE`` flag before being registered.
+
+After registering sub-devices, the :c:type:`v4l2_device` driver can create
+device nodes for all registered sub-devices marked with
+``V4L2_SUBDEV_FL_HAS_DEVNODE`` by calling
+:c:func:`v4l2_device_register_subdev_nodes`. Those device nodes will be
+automatically removed when sub-devices are unregistered.
+
+The device node handles a subset of the V4L2 API.
+
+``VIDIOC_QUERYCTRL``,
+``VIDIOC_QUERYMENU``,
+``VIDIOC_G_CTRL``,
+``VIDIOC_S_CTRL``,
+``VIDIOC_G_EXT_CTRLS``,
+``VIDIOC_S_EXT_CTRLS`` and
+``VIDIOC_TRY_EXT_CTRLS``:
+
+	The controls ioctls are identical to the ones defined in V4L2. They
+	behave identically, with the only exception that they deal only with
+	controls implemented in the sub-device. Depending on the driver, those
+	controls can be also be accessed through one (or several) V4L2 device
+	nodes.
+
+``VIDIOC_DQEVENT``,
+``VIDIOC_SUBSCRIBE_EVENT`` and
+``VIDIOC_UNSUBSCRIBE_EVENT``
+
+	The events ioctls are identical to the ones defined in V4L2. They
+	behave identically, with the only exception that they deal only with
+	events generated by the sub-device. Depending on the driver, those
+	events can also be reported by one (or several) V4L2 device nodes.
+
+	Sub-device drivers that want to use events need to set the
+	``V4L2_SUBDEV_FL_HAS_EVENTS`` :c:type:`v4l2_subdev`.flags before registering
+	the sub-device. After registration events can be queued as usual on the
+	:c:type:`v4l2_subdev`.devnode device node.
+
+	To properly support events, the ``poll()`` file operation is also
+	implemented.
+
+Private ioctls
+
+	All ioctls not in the above list are passed directly to the sub-device
+	driver through the core::ioctl operation.
+
+Read-only sub-device userspace API
+----------------------------------
+
+Bridge drivers that control their connected subdevices through direct calls to
+the kernel API realized by :c:type:`v4l2_subdev_ops` structure do not usually
+want userspace to be able to change the same parameters through the subdevice
+device node and thus do not usually register any.
+
+It is sometimes useful to report to userspace the current subdevice
+configuration through a read-only API, that does not permit applications to
+change to the device parameters but allows interfacing to the subdevice device
+node to inspect them.
+
+For instance, to implement cameras based on computational photography, userspace
+needs to know the detailed camera sensor configuration (in terms of skipping,
+binning, cropping and scaling) for each supported output resolution. To support
+such use cases, bridge drivers may expose the subdevice operations to userspace
+through a read-only API.
+
+To create a read-only device node for all the subdevices registered with the
+``V4L2_SUBDEV_FL_HAS_DEVNODE`` set, the :c:type:`v4l2_device` driver should call
+:c:func:`v4l2_device_register_ro_subdev_nodes`.
+
+Access to the following ioctls for userspace applications is restricted on
+sub-device device nodes registered with
+:c:func:`v4l2_device_register_ro_subdev_nodes`.
+
+``VIDIOC_SUBDEV_S_FMT``,
+``VIDIOC_SUBDEV_S_CROP``,
+``VIDIOC_SUBDEV_S_SELECTION``:
+
+	These ioctls are only allowed on a read-only subdevice device node
+	for the :ref:`V4L2_SUBDEV_FORMAT_TRY <v4l2-subdev-format-whence>`
+	formats and selection rectangles.
+
+``VIDIOC_SUBDEV_S_FRAME_INTERVAL``,
+``VIDIOC_SUBDEV_S_DV_TIMINGS``,
+``VIDIOC_SUBDEV_S_STD``:
+
+	These ioctls are not allowed on a read-only subdevice node.
+
+In case the ioctl is not allowed, or the format to modify is set to
+``V4L2_SUBDEV_FORMAT_ACTIVE``, the core returns a negative error code and
+the errno variable is set to ``-EPERM``.
+
+I2C sub-device drivers
+----------------------
+
+Since these drivers are so common, special helper functions are available to
+ease the use of these drivers (``v4l2-common.h``).
+
+The recommended method of adding :c:type:`v4l2_subdev` support to an I2C driver
+is to embed the :c:type:`v4l2_subdev` struct into the state struct that is
+created for each I2C device instance. Very simple devices have no state
+struct and in that case you can just create a :c:type:`v4l2_subdev` directly.
+
+A typical state struct would look like this (where 'chipname' is replaced by
+the name of the chip):
+
+.. code-block:: c
+
+	struct chipname_state {
+		struct v4l2_subdev sd;
+		...  /* additional state fields */
+	};
+
+Initialize the :c:type:`v4l2_subdev` struct as follows:
+
+.. code-block:: c
+
+	v4l2_i2c_subdev_init(&state->sd, client, subdev_ops);
+
+This function will fill in all the fields of :c:type:`v4l2_subdev` ensure that
+the :c:type:`v4l2_subdev` and i2c_client both point to one another.
+
+You should also add a helper inline function to go from a :c:type:`v4l2_subdev`
+pointer to a chipname_state struct:
+
+.. code-block:: c
+
+	static inline struct chipname_state *to_state(struct v4l2_subdev *sd)
+	{
+		return container_of(sd, struct chipname_state, sd);
+	}
+
+Use this to go from the :c:type:`v4l2_subdev` struct to the ``i2c_client``
+struct:
+
+.. code-block:: c
+
+	struct i2c_client *client = v4l2_get_subdevdata(sd);
+
+And this to go from an ``i2c_client`` to a :c:type:`v4l2_subdev` struct:
+
+.. code-block:: c
+
+	struct v4l2_subdev *sd = i2c_get_clientdata(client);
+
+Make sure to call
+:c:func:`v4l2_device_unregister_subdev`\ (:c:type:`sd <v4l2_subdev>`)
+when the ``remove()`` callback is called. This will unregister the sub-device
+from the bridge driver. It is safe to call this even if the sub-device was
+never registered.
+
+You need to do this because when the bridge driver destroys the i2c adapter
+the ``remove()`` callbacks are called of the i2c devices on that adapter.
+After that the corresponding v4l2_subdev structures are invalid, so they
+have to be unregistered first. Calling
+:c:func:`v4l2_device_unregister_subdev`\ (:c:type:`sd <v4l2_subdev>`)
+from the ``remove()`` callback ensures that this is always done correctly.
+
+
+The bridge driver also has some helper functions it can use:
+
+.. code-block:: c
+
+	struct v4l2_subdev *sd = v4l2_i2c_new_subdev(v4l2_dev, adapter,
+					"module_foo", "chipid", 0x36, NULL);
+
+This loads the given module (can be ``NULL`` if no module needs to be loaded)
+and calls :c:func:`i2c_new_client_device` with the given ``i2c_adapter`` and
+chip/address arguments. If all goes well, then it registers the subdev with
+the v4l2_device.
+
+You can also use the last argument of :c:func:`v4l2_i2c_new_subdev` to pass
+an array of possible I2C addresses that it should probe. These probe addresses
+are only used if the previous argument is 0. A non-zero argument means that you
+know the exact i2c address so in that case no probing will take place.
+
+Both functions return ``NULL`` if something went wrong.
+
+Note that the chipid you pass to :c:func:`v4l2_i2c_new_subdev` is usually
+the same as the module name. It allows you to specify a chip variant, e.g.
+"saa7114" or "saa7115". In general though the i2c driver autodetects this.
+The use of chipid is something that needs to be looked at more closely at a
+later date. It differs between i2c drivers and as such can be confusing.
+To see which chip variants are supported you can look in the i2c driver code
+for the i2c_device_id table. This lists all the possibilities.
+
+There are one more helper function:
+
+:c:func:`v4l2_i2c_new_subdev_board` uses an :c:type:`i2c_board_info` struct
+which is passed to the i2c driver and replaces the irq, platform_data and addr
+arguments.
+
+If the subdev supports the s_config core ops, then that op is called with
+the irq and platform_data arguments after the subdev was setup.
+
+The :c:func:`v4l2_i2c_new_subdev` function will call
+:c:func:`v4l2_i2c_new_subdev_board`, internally filling a
+:c:type:`i2c_board_info` structure using the ``client_type`` and the
+``addr`` to fill it.
+
+Centrally managed subdev active state
+-------------------------------------
+
+Traditionally V4L2 subdev drivers maintained internal state for the active
+device configuration. This is often implemented as e.g. an array of struct
+v4l2_mbus_framefmt, one entry for each pad, and similarly for crop and compose
+rectangles.
+
+In addition to the active configuration, each subdev file handle has an array of
+struct v4l2_subdev_pad_config, managed by the V4L2 core, which contains the try
+configuration.
+
+To simplify the subdev drivers the V4L2 subdev API now optionally supports a
+centrally managed active configuration represented by
+:c:type:`v4l2_subdev_state`. One instance of state, which contains the active
+device configuration, is stored in the sub-device itself as part of
+the :c:type:`v4l2_subdev` structure, while the core associates a try state to
+each open file handle, to store the try configuration related to that file
+handle.
+
+Sub-device drivers can opt-in and use state to manage their active configuration
+by initializing the subdevice state with a call to v4l2_subdev_init_finalize()
+before registering the sub-device. They must also call v4l2_subdev_cleanup()
+to release all the allocated resources before unregistering the sub-device.
+The core automatically allocates and initializes a state for each open file
+handle to store the try configurations and frees it when closing the file
+handle.
+
+V4L2 sub-device operations that use both the :ref:`ACTIVE and TRY formats
+<v4l2-subdev-format-whence>` receive the correct state to operate on through
+the 'state' parameter. The state must be locked and unlocked by the
+caller by calling :c:func:`v4l2_subdev_lock_state()` and
+:c:func:`v4l2_subdev_unlock_state()`. The caller can do so by calling the subdev
+operation through the :c:func:`v4l2_subdev_call_state_active()` macro.
+
+Operations that do not receive a state parameter implicitly operate on the
+subdevice active state, which drivers can exclusively access by
+calling :c:func:`v4l2_subdev_lock_and_get_active_state()`. The sub-device active
+state must equally be released by calling :c:func:`v4l2_subdev_unlock_state()`.
+
+Drivers must never manually access the state stored in the :c:type:`v4l2_subdev`
+or in the file handle without going through the designated helpers.
+
+While the V4L2 core passes the correct try or active state to the subdevice
+operations, many existing device drivers pass a NULL state when calling
+operations with :c:func:`v4l2_subdev_call()`. This legacy construct causes
+issues with subdevice drivers that let the V4L2 core manage the active state,
+as they expect to receive the appropriate state as a parameter. To help the
+conversion of subdevice drivers to a managed active state without having to
+convert all callers at the same time, an additional wrapper layer has been
+added to v4l2_subdev_call(), which handles the NULL case by geting and locking
+the callee's active state with :c:func:`v4l2_subdev_lock_and_get_active_state()`,
+and unlocking the state after the call.
+
+The whole subdev state is in reality split into three parts: the
+v4l2_subdev_state, subdev controls and subdev driver's internal state. In the
+future these parts should be combined into a single state. For the time being
+we need a way to handle the locking for these parts. This can be accomplished
+by sharing a lock. The v4l2_ctrl_handler already supports this via its 'lock'
+pointer and the same model is used with states. The driver can do the following
+before calling v4l2_subdev_init_finalize():
+
+.. code-block:: c
+
+	sd->ctrl_handler->lock = &priv->mutex;
+	sd->state_lock = &priv->mutex;
+
+This shares the driver's private mutex between the controls and the states.
+
+V4L2 sub-device functions and data structures
+---------------------------------------------
+
+.. kernel-doc:: include/media/v4l2-subdev.h