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 v4l2_subdev struct
(v4l2-subdev.h) was created.

Each sub-device driver must have a 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 v4l2_subdev using v4l2_set_subdevdata(). That makes it easy to go
from a v4l2_subdev to the actual low-level bus-specific device data.

You also need a way to go from the low-level struct to v4l2_subdev. For the
common i2c_client struct the i2c_set_clientdata() call is used to store a
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 v4l2_subdev structure provides
host private data for that purpose that can be accessed with
v4l2_get_subdev_hostdata() and v4l2_set_subdev_hostdata().

From the bridge driver perspective you load the sub-device module and somehow
obtain the 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 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:: none

	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 v4l2_subdev struct using:

.. code-block:: none

	v4l2_subdev_init(sd, &ops);

Afterwards you need to initialize 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
media_entity struct embedded in the v4l2_subdev struct (entity field) by
calling media_entity_pads_init(), if the entity has pads:

.. code-block:: none

	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:: none

	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
v4l2_subdev_pad_ops instead of 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
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
v4l2_subdev with the v4l2_device:

.. code-block:: none

	int err = v4l2_device_register_subdev(v4l2_dev, sd);

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 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:

.. code-block:: none

	v4l2_device_unregister_subdev(sd);

Afterwards the subdev module can be unloaded and sd->dev == NULL.

You can call an ops function either directly:

.. code-block:: none

	err = sd->ops->core->g_std(sd, &norm);

but it is better and easier to use this macro:

.. code-block:: none

	err = v4l2_subdev_call(sd, core, g_std, &norm);

The macro will to the right NULL pointer checks and returns -ENODEV if subdev
is NULL, -ENOIOCTLCMD if either subdev->core or subdev->core->g_std is
NULL, or the actual result of the subdev->ops->core->g_std ops.

It is also possible to call all or a subset of the sub-devices:

.. code-block:: none

	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:: none

	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 sd->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 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 v4l2_async_register_subdev() function. Unregistration is performed using
the 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 v4l2_async_notifier_register() call. To unregister the
notifier the driver has to call v4l2_async_notifier_unregister(). The former of
the two functions takes two arguments: a pointer to struct v4l2_device and a
pointer to struct v4l2_async_notifier. The latter contains a pointer to an array
of pointers to subdevice descriptors of type struct 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