![Mauro Carvalho Chehab](/assets/img/avatar_default.png)
Use codespell to fix lots of typos over frontends. Manually verified to avoid false-positives. Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org> Acked-by: Philipp Zabel <p.zabel@pengutronix.de> Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
445 lines
18 KiB
ReStructuredText
445 lines
18 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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V4L2 sub-devices
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----------------
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Many drivers need to communicate with sub-devices. These devices can do all
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sort of tasks, but most commonly they handle audio and/or video muxing,
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encoding or decoding. For webcams common sub-devices are sensors and camera
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controllers.
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Usually these are I2C devices, but not necessarily. In order to provide the
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driver with a consistent interface to these sub-devices the
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:c:type:`v4l2_subdev` struct (v4l2-subdev.h) was created.
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Each sub-device driver must have a :c:type:`v4l2_subdev` struct. This struct
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can be stand-alone for simple sub-devices or it might be embedded in a larger
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struct if more state information needs to be stored. Usually there is a
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low-level device struct (e.g. ``i2c_client``) that contains the device data as
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setup by the kernel. It is recommended to store that pointer in the private
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data of :c:type:`v4l2_subdev` using :c:func:`v4l2_set_subdevdata`. That makes
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it easy to go from a :c:type:`v4l2_subdev` to the actual low-level bus-specific
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device data.
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You also need a way to go from the low-level struct to :c:type:`v4l2_subdev`.
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For the common i2c_client struct the i2c_set_clientdata() call is used to store
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a :c:type:`v4l2_subdev` pointer, for other buses you may have to use other
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methods.
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Bridges might also need to store per-subdev private data, such as a pointer to
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bridge-specific per-subdev private data. The :c:type:`v4l2_subdev` structure
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provides host private data for that purpose that can be accessed with
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:c:func:`v4l2_get_subdev_hostdata` and :c:func:`v4l2_set_subdev_hostdata`.
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From the bridge driver perspective, you load the sub-device module and somehow
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obtain the :c:type:`v4l2_subdev` pointer. For i2c devices this is easy: you call
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``i2c_get_clientdata()``. For other buses something similar needs to be done.
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Helper functions exists for sub-devices on an I2C bus that do most of this
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tricky work for you.
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Each :c:type:`v4l2_subdev` contains function pointers that sub-device drivers
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can implement (or leave ``NULL`` if it is not applicable). Since sub-devices can
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do so many different things and you do not want to end up with a huge ops struct
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of which only a handful of ops are commonly implemented, the function pointers
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are sorted according to category and each category has its own ops struct.
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The top-level ops struct contains pointers to the category ops structs, which
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may be NULL if the subdev driver does not support anything from that category.
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It looks like this:
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.. code-block:: c
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struct v4l2_subdev_core_ops {
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int (*log_status)(struct v4l2_subdev *sd);
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int (*init)(struct v4l2_subdev *sd, u32 val);
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...
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};
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struct v4l2_subdev_tuner_ops {
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...
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};
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struct v4l2_subdev_audio_ops {
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...
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};
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struct v4l2_subdev_video_ops {
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...
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};
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struct v4l2_subdev_pad_ops {
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...
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};
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struct v4l2_subdev_ops {
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const struct v4l2_subdev_core_ops *core;
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const struct v4l2_subdev_tuner_ops *tuner;
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const struct v4l2_subdev_audio_ops *audio;
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const struct v4l2_subdev_video_ops *video;
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const struct v4l2_subdev_pad_ops *video;
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};
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The core ops are common to all subdevs, the other categories are implemented
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depending on the sub-device. E.g. a video device is unlikely to support the
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audio ops and vice versa.
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This setup limits the number of function pointers while still making it easy
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to add new ops and categories.
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A sub-device driver initializes the :c:type:`v4l2_subdev` struct using:
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:c:func:`v4l2_subdev_init <v4l2_subdev_init>`
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(:c:type:`sd <v4l2_subdev>`, &\ :c:type:`ops <v4l2_subdev_ops>`).
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Afterwards you need to initialize :c:type:`sd <v4l2_subdev>`->name with a
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unique name and set the module owner. This is done for you if you use the
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i2c helper functions.
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If integration with the media framework is needed, you must initialize the
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:c:type:`media_entity` struct embedded in the :c:type:`v4l2_subdev` struct
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(entity field) by calling :c:func:`media_entity_pads_init`, if the entity has
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pads:
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.. code-block:: c
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struct media_pad *pads = &my_sd->pads;
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int err;
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err = media_entity_pads_init(&sd->entity, npads, pads);
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The pads array must have been previously initialized. There is no need to
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manually set the struct :c:type:`media_entity` function and name fields, but the
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revision field must be initialized if needed.
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A reference to the entity will be automatically acquired/released when the
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subdev device node (if any) is opened/closed.
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Don't forget to cleanup the media entity before the sub-device is destroyed:
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.. code-block:: c
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media_entity_cleanup(&sd->entity);
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If the subdev driver intends to process video and integrate with the media
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framework, it must implement format related functionality using
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:c:type:`v4l2_subdev_pad_ops` instead of :c:type:`v4l2_subdev_video_ops`.
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In that case, the subdev driver may set the link_validate field to provide
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its own link validation function. The link validation function is called for
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every link in the pipeline where both of the ends of the links are V4L2
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sub-devices. The driver is still responsible for validating the correctness
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of the format configuration between sub-devices and video nodes.
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If link_validate op is not set, the default function
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:c:func:`v4l2_subdev_link_validate_default` is used instead. This function
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ensures that width, height and the media bus pixel code are equal on both source
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and sink of the link. Subdev drivers are also free to use this function to
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perform the checks mentioned above in addition to their own checks.
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There are currently two ways to register subdevices with the V4L2 core. The
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first (traditional) possibility is to have subdevices registered by bridge
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drivers. This can be done when the bridge driver has the complete information
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about subdevices connected to it and knows exactly when to register them. This
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is typically the case for internal subdevices, like video data processing units
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within SoCs or complex PCI(e) boards, camera sensors in USB cameras or connected
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to SoCs, which pass information about them to bridge drivers, usually in their
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platform data.
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There are however also situations where subdevices have to be registered
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asynchronously to bridge devices. An example of such a configuration is a Device
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Tree based system where information about subdevices is made available to the
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system independently from the bridge devices, e.g. when subdevices are defined
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in DT as I2C device nodes. The API used in this second case is described further
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below.
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Using one or the other registration method only affects the probing process, the
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run-time bridge-subdevice interaction is in both cases the same.
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In the synchronous case a device (bridge) driver needs to register the
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:c:type:`v4l2_subdev` with the v4l2_device:
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:c:func:`v4l2_device_register_subdev <v4l2_device_register_subdev>`
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(:c:type:`v4l2_dev <v4l2_device>`, :c:type:`sd <v4l2_subdev>`).
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This can fail if the subdev module disappeared before it could be registered.
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After this function was called successfully the subdev->dev field points to
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the :c:type:`v4l2_device`.
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If the v4l2_device parent device has a non-NULL mdev field, the sub-device
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entity will be automatically registered with the media device.
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You can unregister a sub-device using:
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:c:func:`v4l2_device_unregister_subdev <v4l2_device_unregister_subdev>`
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(:c:type:`sd <v4l2_subdev>`).
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Afterwards the subdev module can be unloaded and
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:c:type:`sd <v4l2_subdev>`->dev == ``NULL``.
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You can call an ops function either directly:
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.. code-block:: c
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err = sd->ops->core->g_std(sd, &norm);
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but it is better and easier to use this macro:
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.. code-block:: c
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err = v4l2_subdev_call(sd, core, g_std, &norm);
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The macro will to the right ``NULL`` pointer checks and returns ``-ENODEV``
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if :c:type:`sd <v4l2_subdev>` is ``NULL``, ``-ENOIOCTLCMD`` if either
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:c:type:`sd <v4l2_subdev>`->core or :c:type:`sd <v4l2_subdev>`->core->g_std is ``NULL``, or the actual result of the
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:c:type:`sd <v4l2_subdev>`->ops->core->g_std ops.
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It is also possible to call all or a subset of the sub-devices:
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.. code-block:: c
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v4l2_device_call_all(v4l2_dev, 0, core, g_std, &norm);
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Any subdev that does not support this ops is skipped and error results are
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ignored. If you want to check for errors use this:
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.. code-block:: c
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err = v4l2_device_call_until_err(v4l2_dev, 0, core, g_std, &norm);
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Any error except ``-ENOIOCTLCMD`` will exit the loop with that error. If no
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errors (except ``-ENOIOCTLCMD``) occurred, then 0 is returned.
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The second argument to both calls is a group ID. If 0, then all subdevs are
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called. If non-zero, then only those whose group ID match that value will
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be called. Before a bridge driver registers a subdev it can set
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:c:type:`sd <v4l2_subdev>`->grp_id to whatever value it wants (it's 0 by
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default). This value is owned by the bridge driver and the sub-device driver
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will never modify or use it.
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The group ID gives the bridge driver more control how callbacks are called.
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For example, there may be multiple audio chips on a board, each capable of
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changing the volume. But usually only one will actually be used when the
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user want to change the volume. You can set the group ID for that subdev to
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e.g. AUDIO_CONTROLLER and specify that as the group ID value when calling
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``v4l2_device_call_all()``. That ensures that it will only go to the subdev
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that needs it.
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If the sub-device needs to notify its v4l2_device parent of an event, then
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it can call ``v4l2_subdev_notify(sd, notification, arg)``. This macro checks
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whether there is a ``notify()`` callback defined and returns ``-ENODEV`` if not.
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Otherwise the result of the ``notify()`` call is returned.
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The advantage of using :c:type:`v4l2_subdev` is that it is a generic struct and
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does not contain any knowledge about the underlying hardware. So a driver might
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contain several subdevs that use an I2C bus, but also a subdev that is
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controlled through GPIO pins. This distinction is only relevant when setting
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up the device, but once the subdev is registered it is completely transparent.
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In the asynchronous case subdevice probing can be invoked independently of the
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bridge driver availability. The subdevice driver then has to verify whether all
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the requirements for a successful probing are satisfied. This can include a
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check for a master clock availability. If any of the conditions aren't satisfied
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the driver might decide to return ``-EPROBE_DEFER`` to request further reprobing
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attempts. Once all conditions are met the subdevice shall be registered using
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the :c:func:`v4l2_async_register_subdev` function. Unregistration is
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performed using the :c:func:`v4l2_async_unregister_subdev` call. Subdevices
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registered this way are stored in a global list of subdevices, ready to be
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picked up by bridge drivers.
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Bridge drivers in turn have to register a notifier object. This is
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performed using the :c:func:`v4l2_async_notifier_register` call. To
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unregister the notifier the driver has to call
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:c:func:`v4l2_async_notifier_unregister`. The former of the two functions
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takes two arguments: a pointer to struct :c:type:`v4l2_device` and a
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pointer to struct :c:type:`v4l2_async_notifier`.
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Before registering the notifier, bridge drivers must do two things:
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first, the notifier must be initialized using the
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:c:func:`v4l2_async_notifier_init`. Second, bridge drivers can then
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begin to form a list of subdevice descriptors that the bridge device
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needs for its operation. Subdevice descriptors are added to the notifier
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using the :c:func:`v4l2_async_notifier_add_subdev` call. This function
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takes two arguments: a pointer to struct :c:type:`v4l2_async_notifier`,
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and a pointer to the subdevice descripter, which is of type struct
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:c:type:`v4l2_async_subdev`.
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The V4L2 core will then use these descriptors to match asynchronously
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registered subdevices to them. If a match is detected the ``.bound()``
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notifier callback is called. After all subdevices have been located the
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.complete() callback is called. When a subdevice is removed from the
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system the .unbind() method is called. All three callbacks are optional.
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V4L2 sub-device userspace API
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-----------------------------
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Beside exposing a kernel API through the :c:type:`v4l2_subdev_ops` structure,
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V4L2 sub-devices can also be controlled directly by userspace applications.
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Device nodes named ``v4l-subdev``\ *X* can be created in ``/dev`` to access
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sub-devices directly. If a sub-device supports direct userspace configuration
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it must set the ``V4L2_SUBDEV_FL_HAS_DEVNODE`` flag before being registered.
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After registering sub-devices, the :c:type:`v4l2_device` driver can create
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device nodes for all registered sub-devices marked with
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``V4L2_SUBDEV_FL_HAS_DEVNODE`` by calling
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:c:func:`v4l2_device_register_subdev_nodes`. Those device nodes will be
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automatically removed when sub-devices are unregistered.
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The device node handles a subset of the V4L2 API.
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``VIDIOC_QUERYCTRL``,
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``VIDIOC_QUERYMENU``,
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``VIDIOC_G_CTRL``,
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``VIDIOC_S_CTRL``,
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``VIDIOC_G_EXT_CTRLS``,
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``VIDIOC_S_EXT_CTRLS`` and
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``VIDIOC_TRY_EXT_CTRLS``:
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The controls ioctls are identical to the ones defined in V4L2. They
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behave identically, with the only exception that they deal only with
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controls implemented in the sub-device. Depending on the driver, those
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controls can be also be accessed through one (or several) V4L2 device
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nodes.
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``VIDIOC_DQEVENT``,
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``VIDIOC_SUBSCRIBE_EVENT`` and
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``VIDIOC_UNSUBSCRIBE_EVENT``
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The events ioctls are identical to the ones defined in V4L2. They
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behave identically, with the only exception that they deal only with
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events generated by the sub-device. Depending on the driver, those
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events can also be reported by one (or several) V4L2 device nodes.
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Sub-device drivers that want to use events need to set the
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``V4L2_SUBDEV_USES_EVENTS`` :c:type:`v4l2_subdev`.flags and initialize
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:c:type:`v4l2_subdev`.nevents to events queue depth before registering
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the sub-device. After registration events can be queued as usual on the
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:c:type:`v4l2_subdev`.devnode device node.
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To properly support events, the ``poll()`` file operation is also
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implemented.
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Private ioctls
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All ioctls not in the above list are passed directly to the sub-device
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driver through the core::ioctl operation.
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I2C sub-device drivers
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----------------------
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Since these drivers are so common, special helper functions are available to
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ease the use of these drivers (``v4l2-common.h``).
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The recommended method of adding :c:type:`v4l2_subdev` support to an I2C driver
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is to embed the :c:type:`v4l2_subdev` struct into the state struct that is
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created for each I2C device instance. Very simple devices have no state
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struct and in that case you can just create a :c:type:`v4l2_subdev` directly.
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A typical state struct would look like this (where 'chipname' is replaced by
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the name of the chip):
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.. code-block:: c
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struct chipname_state {
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struct v4l2_subdev sd;
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... /* additional state fields */
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};
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Initialize the :c:type:`v4l2_subdev` struct as follows:
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.. code-block:: c
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v4l2_i2c_subdev_init(&state->sd, client, subdev_ops);
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This function will fill in all the fields of :c:type:`v4l2_subdev` ensure that
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the :c:type:`v4l2_subdev` and i2c_client both point to one another.
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You should also add a helper inline function to go from a :c:type:`v4l2_subdev`
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pointer to a chipname_state struct:
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.. code-block:: c
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static inline struct chipname_state *to_state(struct v4l2_subdev *sd)
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{
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return container_of(sd, struct chipname_state, sd);
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}
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Use this to go from the :c:type:`v4l2_subdev` struct to the ``i2c_client``
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struct:
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.. code-block:: c
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struct i2c_client *client = v4l2_get_subdevdata(sd);
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And this to go from an ``i2c_client`` to a :c:type:`v4l2_subdev` struct:
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.. code-block:: c
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struct v4l2_subdev *sd = i2c_get_clientdata(client);
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Make sure to call
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:c:func:`v4l2_device_unregister_subdev`\ (:c:type:`sd <v4l2_subdev>`)
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when the ``remove()`` callback is called. This will unregister the sub-device
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from the bridge driver. It is safe to call this even if the sub-device was
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never registered.
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You need to do this because when the bridge driver destroys the i2c adapter
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the ``remove()`` callbacks are called of the i2c devices on that adapter.
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After that the corresponding v4l2_subdev structures are invalid, so they
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have to be unregistered first. Calling
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:c:func:`v4l2_device_unregister_subdev`\ (:c:type:`sd <v4l2_subdev>`)
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from the ``remove()`` callback ensures that this is always done correctly.
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The bridge driver also has some helper functions it can use:
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.. code-block:: c
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struct v4l2_subdev *sd = v4l2_i2c_new_subdev(v4l2_dev, adapter,
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"module_foo", "chipid", 0x36, NULL);
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This loads the given module (can be ``NULL`` if no module needs to be loaded)
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and calls :c:func:`i2c_new_device` with the given ``i2c_adapter`` and
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chip/address arguments. If all goes well, then it registers the subdev with
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the v4l2_device.
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You can also use the last argument of :c:func:`v4l2_i2c_new_subdev` to pass
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an array of possible I2C addresses that it should probe. These probe addresses
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are only used if the previous argument is 0. A non-zero argument means that you
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know the exact i2c address so in that case no probing will take place.
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Both functions return ``NULL`` if something went wrong.
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Note that the chipid you pass to :c:func:`v4l2_i2c_new_subdev` is usually
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the same as the module name. It allows you to specify a chip variant, e.g.
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"saa7114" or "saa7115". In general though the i2c driver autodetects this.
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The use of chipid is something that needs to be looked at more closely at a
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later date. It differs between i2c drivers and as such can be confusing.
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To see which chip variants are supported you can look in the i2c driver code
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for the i2c_device_id table. This lists all the possibilities.
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There are one more helper function:
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:c:func:`v4l2_i2c_new_subdev_board` uses an :c:type:`i2c_board_info` struct
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which is passed to the i2c driver and replaces the irq, platform_data and addr
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arguments.
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If the subdev supports the s_config core ops, then that op is called with
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the irq and platform_data arguments after the subdev was setup.
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The :c:func:`v4l2_i2c_new_subdev` function will call
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:c:func:`v4l2_i2c_new_subdev_board`, internally filling a
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:c:type:`i2c_board_info` structure using the ``client_type`` and the
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``addr`` to fill it.
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V4L2 sub-device functions and data structures
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|
---------------------------------------------
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.. kernel-doc:: include/media/v4l2-subdev.h
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.. kernel-doc:: include/media/v4l2-async.h
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