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 :cpp: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 busses 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
:cpp:func:`v4l2_get_subdev_hostdata` and :cpp: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 busses something similar needs to be done.
Helper functions exists 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:

	:cpp: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 :cpp: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 :c:type:`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 the subdev driver intends to process video and integrate with the media
framework, it must implement format related functionality using
:c:type:`v4l2_subdev_pad_ops` instead of :c:type:`v4l2_subdev_video_ops`.

In that case, the subdev driver may set the link_validate field to provide
its own link validation function. The link validation function is called for
every link in the pipeline where both of the ends of the links are V4L2
sub-devices. 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
:cpp: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.

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:

	:cpp: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:

	:cpp: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``.

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 to 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.

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.

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 :cpp:func:`v4l2_async_register_subdev` function. Unregistration is
performed using the :cpp: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 with an array of
subdevice descriptors that the bridge device needs for its operation. This is
performed using the :cpp:func:`v4l2_async_notifier_register` call. To
unregister the notifier the driver has to call
:cpp:func:`v4l2_async_notifier_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`. The latter contains a pointer to an array
of pointers to subdevice descriptors of type struct :c:type:`v4l2_async_subdev`
type. 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.

V4L2 subdev kAPI
^^^^^^^^^^^^^^^^

.. kernel-doc:: include/media/v4l2-subdev.h

V4L2 subdev async kAPI
^^^^^^^^^^^^^^^^^^^^^^

.. kernel-doc:: include/media/v4l2-async.h