diff options
Diffstat (limited to 'Documentation/media/kapi/v4l2-subdev.rst')
| -rw-r--r-- | Documentation/media/kapi/v4l2-subdev.rst | 445 |
1 files changed, 445 insertions, 0 deletions
diff --git a/Documentation/media/kapi/v4l2-subdev.rst b/Documentation/media/kapi/v4l2-subdev.rst new file mode 100644 index 000000000000..d767b61e9842 --- /dev/null +++ b/Documentation/media/kapi/v4l2-subdev.rst @@ -0,0 +1,445 @@ +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 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 +:c: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: + + :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 :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 +: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. + +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``. + +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 :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 with an array of +subdevice descriptors that the bridge device needs for its operation. This is +performed using the :c:func:`v4l2_async_notifier_register` call. To +unregister the notifier the driver has to call +:c: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 sub-device userspace API +----------------------------- + +Beside exposing a kernel API through the :c:type:`v4l2_subdev_ops` structure, +V4L2 sub-devices can also be controlled directly by userspace applications. + +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_USES_EVENTS`` :c:type:`v4l2_subdev`.flags and initialize + :c:type:`v4l2_subdev`.nevents to events queue depth 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. + + +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_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 two more helper functions: + +:c:func:`v4l2_i2c_new_subdev_cfg`: this function adds new irq and +platform_data arguments and has both 'addr' and 'probed_addrs' arguments: +if addr is not 0 then that will be used (non-probing variant), otherwise the +probed_addrs are probed. + +For example: this will probe for address 0x10: + +.. code-block:: c + + struct v4l2_subdev *sd = v4l2_i2c_new_subdev_cfg(v4l2_dev, adapter, + "module_foo", "chipid", 0, NULL, 0, I2C_ADDRS(0x10)); + +: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 older :c:func:`v4l2_i2c_new_subdev` and +:c:func:`v4l2_i2c_new_probed_subdev` functions will call ``s_config`` as +well, but with irq set to 0 and platform_data set to ``NULL``. + +V4L2 sub-device functions and data structures +--------------------------------------------- + +.. kernel-doc:: include/media/v4l2-subdev.h + +.. kernel-doc:: include/media/v4l2-async.h |