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1.. SPDX-License-Identifier: GPL-2.02 3V4L2 sub-devices4----------------5 6Many drivers need to communicate with sub-devices. These devices can do all7sort of tasks, but most commonly they handle audio and/or video muxing,8encoding or decoding. For webcams common sub-devices are sensors and camera9controllers.10 11Usually these are I2C devices, but not necessarily. In order to provide the12driver with a consistent interface to these sub-devices the13:c:type:`v4l2_subdev` struct (v4l2-subdev.h) was created.14 15Each sub-device driver must have a :c:type:`v4l2_subdev` struct. This struct16can be stand-alone for simple sub-devices or it might be embedded in a larger17struct if more state information needs to be stored. Usually there is a18low-level device struct (e.g. ``i2c_client``) that contains the device data as19setup by the kernel. It is recommended to store that pointer in the private20data of :c:type:`v4l2_subdev` using :c:func:`v4l2_set_subdevdata`. That makes21it easy to go from a :c:type:`v4l2_subdev` to the actual low-level bus-specific22device data.23 24You also need a way to go from the low-level struct to :c:type:`v4l2_subdev`.25For the common i2c_client struct the i2c_set_clientdata() call is used to store26a :c:type:`v4l2_subdev` pointer, for other buses you may have to use other27methods.28 29Bridges might also need to store per-subdev private data, such as a pointer to30bridge-specific per-subdev private data. The :c:type:`v4l2_subdev` structure31provides host private data for that purpose that can be accessed with32:c:func:`v4l2_get_subdev_hostdata` and :c:func:`v4l2_set_subdev_hostdata`.33 34From the bridge driver perspective, you load the sub-device module and somehow35obtain the :c:type:`v4l2_subdev` pointer. For i2c devices this is easy: you call36``i2c_get_clientdata()``. For other buses something similar needs to be done.37Helper functions exist for sub-devices on an I2C bus that do most of this38tricky work for you.39 40Each :c:type:`v4l2_subdev` contains function pointers that sub-device drivers41can implement (or leave ``NULL`` if it is not applicable). Since sub-devices can42do so many different things and you do not want to end up with a huge ops struct43of which only a handful of ops are commonly implemented, the function pointers44are sorted according to category and each category has its own ops struct.45 46The top-level ops struct contains pointers to the category ops structs, which47may be NULL if the subdev driver does not support anything from that category.48 49It looks like this:50 51.. code-block:: c52 53 struct v4l2_subdev_core_ops {54 int (*log_status)(struct v4l2_subdev *sd);55 int (*init)(struct v4l2_subdev *sd, u32 val);56 ...57 };58 59 struct v4l2_subdev_tuner_ops {60 ...61 };62 63 struct v4l2_subdev_audio_ops {64 ...65 };66 67 struct v4l2_subdev_video_ops {68 ...69 };70 71 struct v4l2_subdev_pad_ops {72 ...73 };74 75 struct v4l2_subdev_ops {76 const struct v4l2_subdev_core_ops *core;77 const struct v4l2_subdev_tuner_ops *tuner;78 const struct v4l2_subdev_audio_ops *audio;79 const struct v4l2_subdev_video_ops *video;80 const struct v4l2_subdev_pad_ops *video;81 };82 83The core ops are common to all subdevs, the other categories are implemented84depending on the sub-device. E.g. a video device is unlikely to support the85audio ops and vice versa.86 87This setup limits the number of function pointers while still making it easy88to add new ops and categories.89 90A sub-device driver initializes the :c:type:`v4l2_subdev` struct using:91 92 :c:func:`v4l2_subdev_init <v4l2_subdev_init>`93 (:c:type:`sd <v4l2_subdev>`, &\ :c:type:`ops <v4l2_subdev_ops>`).94 95 96Afterwards you need to initialize :c:type:`sd <v4l2_subdev>`->name with a97unique name and set the module owner. This is done for you if you use the98i2c helper functions.99 100If integration with the media framework is needed, you must initialize the101:c:type:`media_entity` struct embedded in the :c:type:`v4l2_subdev` struct102(entity field) by calling :c:func:`media_entity_pads_init`, if the entity has103pads:104 105.. code-block:: c106 107 struct media_pad *pads = &my_sd->pads;108 int err;109 110 err = media_entity_pads_init(&sd->entity, npads, pads);111 112The pads array must have been previously initialized. There is no need to113manually set the struct media_entity function and name fields, but the114revision field must be initialized if needed.115 116A reference to the entity will be automatically acquired/released when the117subdev device node (if any) is opened/closed.118 119Don't forget to cleanup the media entity before the sub-device is destroyed:120 121.. code-block:: c122 123 media_entity_cleanup(&sd->entity);124 125If a sub-device driver implements sink pads, the subdev driver may set the126link_validate field in :c:type:`v4l2_subdev_pad_ops` to provide its own link127validation function. For every link in the pipeline, the link_validate pad128operation of the sink end of the link is called. In both cases the driver is129still responsible for validating the correctness of the format configuration130between sub-devices and video nodes.131 132If link_validate op is not set, the default function133:c:func:`v4l2_subdev_link_validate_default` is used instead. This function134ensures that width, height and the media bus pixel code are equal on both source135and sink of the link. Subdev drivers are also free to use this function to136perform the checks mentioned above in addition to their own checks.137 138Subdev registration139~~~~~~~~~~~~~~~~~~~140 141There are currently two ways to register subdevices with the V4L2 core. The142first (traditional) possibility is to have subdevices registered by bridge143drivers. This can be done when the bridge driver has the complete information144about subdevices connected to it and knows exactly when to register them. This145is typically the case for internal subdevices, like video data processing units146within SoCs or complex PCI(e) boards, camera sensors in USB cameras or connected147to SoCs, which pass information about them to bridge drivers, usually in their148platform data.149 150There are however also situations where subdevices have to be registered151asynchronously to bridge devices. An example of such a configuration is a Device152Tree based system where information about subdevices is made available to the153system independently from the bridge devices, e.g. when subdevices are defined154in DT as I2C device nodes. The API used in this second case is described further155below.156 157Using one or the other registration method only affects the probing process, the158run-time bridge-subdevice interaction is in both cases the same.159 160Registering synchronous sub-devices161^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^162 163In the **synchronous** case a device (bridge) driver needs to register the164:c:type:`v4l2_subdev` with the v4l2_device:165 166 :c:func:`v4l2_device_register_subdev <v4l2_device_register_subdev>`167 (:c:type:`v4l2_dev <v4l2_device>`, :c:type:`sd <v4l2_subdev>`).168 169This can fail if the subdev module disappeared before it could be registered.170After this function was called successfully the subdev->dev field points to171the :c:type:`v4l2_device`.172 173If the v4l2_device parent device has a non-NULL mdev field, the sub-device174entity will be automatically registered with the media device.175 176You can unregister a sub-device using:177 178 :c:func:`v4l2_device_unregister_subdev <v4l2_device_unregister_subdev>`179 (:c:type:`sd <v4l2_subdev>`).180 181Afterwards the subdev module can be unloaded and182:c:type:`sd <v4l2_subdev>`->dev == ``NULL``.183 184.. _media-registering-async-subdevs:185 186Registering asynchronous sub-devices187^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^188 189In the **asynchronous** case subdevice probing can be invoked independently of190the bridge driver availability. The subdevice driver then has to verify whether191all the requirements for a successful probing are satisfied. This can include a192check for a master clock availability. If any of the conditions aren't satisfied193the driver might decide to return ``-EPROBE_DEFER`` to request further reprobing194attempts. Once all conditions are met the subdevice shall be registered using195the :c:func:`v4l2_async_register_subdev` function. Unregistration is196performed using the :c:func:`v4l2_async_unregister_subdev` call. Subdevices197registered this way are stored in a global list of subdevices, ready to be198picked up by bridge drivers.199 200Drivers must complete all initialization of the sub-device before201registering it using :c:func:`v4l2_async_register_subdev`, including202enabling runtime PM. This is because the sub-device becomes accessible203as soon as it gets registered.204 205Asynchronous sub-device notifiers206^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^207 208Bridge drivers in turn have to register a notifier object. This is performed209using the :c:func:`v4l2_async_nf_register` call. To unregister the notifier the210driver has to call :c:func:`v4l2_async_nf_unregister`. Before releasing memory211of an unregister notifier, it must be cleaned up by calling212:c:func:`v4l2_async_nf_cleanup`.213 214Before registering the notifier, bridge drivers must do two things: first, the215notifier must be initialized using the :c:func:`v4l2_async_nf_init`. Second,216bridge drivers can then begin to form a list of async connection descriptors217that the bridge device needs for its218operation. :c:func:`v4l2_async_nf_add_fwnode`,219:c:func:`v4l2_async_nf_add_fwnode_remote` and :c:func:`v4l2_async_nf_add_i2c`220 221Async connection descriptors describe connections to external sub-devices the222drivers for which are not yet probed. Based on an async connection, a media data223or ancillary link may be created when the related sub-device becomes224available. There may be one or more async connections to a given sub-device but225this is not known at the time of adding the connections to the notifier. Async226connections are bound as matching async sub-devices are found, one by one.227 228Asynchronous sub-device notifier for sub-devices229^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^230 231A driver that registers an asynchronous sub-device may also register an232asynchronous notifier. This is called an asynchronous sub-device notifier and the233process is similar to that of a bridge driver apart from that the notifier is234initialised using :c:func:`v4l2_async_subdev_nf_init` instead. A sub-device235notifier may complete only after the V4L2 device becomes available, i.e. there's236a path via async sub-devices and notifiers to a notifier that is not an237asynchronous sub-device notifier.238 239Asynchronous sub-device registration helper for camera sensor drivers240^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^241 242:c:func:`v4l2_async_register_subdev_sensor` is a helper function for sensor243drivers registering their own async connection, but it also registers a notifier244and further registers async connections for lens and flash devices found in245firmware. The notifier for the sub-device is unregistered and cleaned up with246the async sub-device, using :c:func:`v4l2_async_unregister_subdev`.247 248Asynchronous sub-device notifier example249^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^250 251These functions allocate an async connection descriptor which is of type struct252:c:type:`v4l2_async_connection` embedded in a driver-specific struct. The &struct253:c:type:`v4l2_async_connection` shall be the first member of this struct:254 255.. code-block:: c256 257 struct my_async_connection {258 struct v4l2_async_connection asc;259 ...260 };261 262 struct my_async_connection *my_asc;263 struct fwnode_handle *ep;264 265 ...266 267 my_asc = v4l2_async_nf_add_fwnode_remote(¬ifier, ep,268 struct my_async_connection);269 fwnode_handle_put(ep);270 271 if (IS_ERR(my_asc))272 return PTR_ERR(my_asc);273 274Asynchronous sub-device notifier callbacks275^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^276 277The V4L2 core will then use these connection descriptors to match asynchronously278registered subdevices to them. If a match is detected the ``.bound()`` notifier279callback is called. After all connections have been bound the .complete()280callback is called. When a connection is removed from the system the281``.unbind()`` method is called. All three callbacks are optional.282 283Drivers can store any type of custom data in their driver-specific284:c:type:`v4l2_async_connection` wrapper. If any of that data requires special285handling when the structure is freed, drivers must implement the ``.destroy()``286notifier callback. The framework will call it right before freeing the287:c:type:`v4l2_async_connection`.288 289Calling subdev operations290~~~~~~~~~~~~~~~~~~~~~~~~~291 292The advantage of using :c:type:`v4l2_subdev` is that it is a generic struct and293does not contain any knowledge about the underlying hardware. So a driver might294contain several subdevs that use an I2C bus, but also a subdev that is295controlled through GPIO pins. This distinction is only relevant when setting296up the device, but once the subdev is registered it is completely transparent.297 298Once the subdev has been registered you can call an ops function either299directly:300 301.. code-block:: c302 303 err = sd->ops->core->g_std(sd, &norm);304 305but it is better and easier to use this macro:306 307.. code-block:: c308 309 err = v4l2_subdev_call(sd, core, g_std, &norm);310 311The macro will do the right ``NULL`` pointer checks and returns ``-ENODEV``312if :c:type:`sd <v4l2_subdev>` is ``NULL``, ``-ENOIOCTLCMD`` if either313:c:type:`sd <v4l2_subdev>`->core or :c:type:`sd <v4l2_subdev>`->core->g_std is ``NULL``, or the actual result of the314:c:type:`sd <v4l2_subdev>`->ops->core->g_std ops.315 316It is also possible to call all or a subset of the sub-devices:317 318.. code-block:: c319 320 v4l2_device_call_all(v4l2_dev, 0, core, g_std, &norm);321 322Any subdev that does not support this ops is skipped and error results are323ignored. If you want to check for errors use this:324 325.. code-block:: c326 327 err = v4l2_device_call_until_err(v4l2_dev, 0, core, g_std, &norm);328 329Any error except ``-ENOIOCTLCMD`` will exit the loop with that error. If no330errors (except ``-ENOIOCTLCMD``) occurred, then 0 is returned.331 332The second argument to both calls is a group ID. If 0, then all subdevs are333called. If non-zero, then only those whose group ID match that value will334be called. Before a bridge driver registers a subdev it can set335:c:type:`sd <v4l2_subdev>`->grp_id to whatever value it wants (it's 0 by336default). This value is owned by the bridge driver and the sub-device driver337will never modify or use it.338 339The group ID gives the bridge driver more control how callbacks are called.340For example, there may be multiple audio chips on a board, each capable of341changing the volume. But usually only one will actually be used when the342user want to change the volume. You can set the group ID for that subdev to343e.g. AUDIO_CONTROLLER and specify that as the group ID value when calling344``v4l2_device_call_all()``. That ensures that it will only go to the subdev345that needs it.346 347If the sub-device needs to notify its v4l2_device parent of an event, then348it can call ``v4l2_subdev_notify(sd, notification, arg)``. This macro checks349whether there is a ``notify()`` callback defined and returns ``-ENODEV`` if not.350Otherwise the result of the ``notify()`` call is returned.351 352V4L2 sub-device userspace API353-----------------------------354 355Bridge drivers traditionally expose one or multiple video nodes to userspace,356and control subdevices through the :c:type:`v4l2_subdev_ops` operations in357response to video node operations. This hides the complexity of the underlying358hardware from applications. For complex devices, finer-grained control of the359device than what the video nodes offer may be required. In those cases, bridge360drivers that implement :ref:`the media controller API <media_controller>` may361opt for making the subdevice operations directly accessible from userspace.362 363Device nodes named ``v4l-subdev``\ *X* can be created in ``/dev`` to access364sub-devices directly. If a sub-device supports direct userspace configuration365it must set the ``V4L2_SUBDEV_FL_HAS_DEVNODE`` flag before being registered.366 367After registering sub-devices, the :c:type:`v4l2_device` driver can create368device nodes for all registered sub-devices marked with369``V4L2_SUBDEV_FL_HAS_DEVNODE`` by calling370:c:func:`v4l2_device_register_subdev_nodes`. Those device nodes will be371automatically removed when sub-devices are unregistered.372 373The device node handles a subset of the V4L2 API.374 375``VIDIOC_QUERYCTRL``,376``VIDIOC_QUERYMENU``,377``VIDIOC_G_CTRL``,378``VIDIOC_S_CTRL``,379``VIDIOC_G_EXT_CTRLS``,380``VIDIOC_S_EXT_CTRLS`` and381``VIDIOC_TRY_EXT_CTRLS``:382 383 The controls ioctls are identical to the ones defined in V4L2. They384 behave identically, with the only exception that they deal only with385 controls implemented in the sub-device. Depending on the driver, those386 controls can be also be accessed through one (or several) V4L2 device387 nodes.388 389``VIDIOC_DQEVENT``,390``VIDIOC_SUBSCRIBE_EVENT`` and391``VIDIOC_UNSUBSCRIBE_EVENT``392 393 The events ioctls are identical to the ones defined in V4L2. They394 behave identically, with the only exception that they deal only with395 events generated by the sub-device. Depending on the driver, those396 events can also be reported by one (or several) V4L2 device nodes.397 398 Sub-device drivers that want to use events need to set the399 ``V4L2_SUBDEV_FL_HAS_EVENTS`` :c:type:`v4l2_subdev`.flags before registering400 the sub-device. After registration events can be queued as usual on the401 :c:type:`v4l2_subdev`.devnode device node.402 403 To properly support events, the ``poll()`` file operation is also404 implemented.405 406Private ioctls407 408 All ioctls not in the above list are passed directly to the sub-device409 driver through the core::ioctl operation.410 411Read-only sub-device userspace API412----------------------------------413 414Bridge drivers that control their connected subdevices through direct calls to415the kernel API realized by :c:type:`v4l2_subdev_ops` structure do not usually416want userspace to be able to change the same parameters through the subdevice417device node and thus do not usually register any.418 419It is sometimes useful to report to userspace the current subdevice420configuration through a read-only API, that does not permit applications to421change to the device parameters but allows interfacing to the subdevice device422node to inspect them.423 424For instance, to implement cameras based on computational photography, userspace425needs to know the detailed camera sensor configuration (in terms of skipping,426binning, cropping and scaling) for each supported output resolution. To support427such use cases, bridge drivers may expose the subdevice operations to userspace428through a read-only API.429 430To create a read-only device node for all the subdevices registered with the431``V4L2_SUBDEV_FL_HAS_DEVNODE`` set, the :c:type:`v4l2_device` driver should call432:c:func:`v4l2_device_register_ro_subdev_nodes`.433 434Access to the following ioctls for userspace applications is restricted on435sub-device device nodes registered with436:c:func:`v4l2_device_register_ro_subdev_nodes`.437 438``VIDIOC_SUBDEV_S_FMT``,439``VIDIOC_SUBDEV_S_CROP``,440``VIDIOC_SUBDEV_S_SELECTION``:441 442 These ioctls are only allowed on a read-only subdevice device node443 for the :ref:`V4L2_SUBDEV_FORMAT_TRY <v4l2-subdev-format-whence>`444 formats and selection rectangles.445 446``VIDIOC_SUBDEV_S_FRAME_INTERVAL``,447``VIDIOC_SUBDEV_S_DV_TIMINGS``,448``VIDIOC_SUBDEV_S_STD``:449 450 These ioctls are not allowed on a read-only subdevice node.451 452In case the ioctl is not allowed, or the format to modify is set to453``V4L2_SUBDEV_FORMAT_ACTIVE``, the core returns a negative error code and454the errno variable is set to ``-EPERM``.455 456I2C sub-device drivers457----------------------458 459Since these drivers are so common, special helper functions are available to460ease the use of these drivers (``v4l2-common.h``).461 462The recommended method of adding :c:type:`v4l2_subdev` support to an I2C driver463is to embed the :c:type:`v4l2_subdev` struct into the state struct that is464created for each I2C device instance. Very simple devices have no state465struct and in that case you can just create a :c:type:`v4l2_subdev` directly.466 467A typical state struct would look like this (where 'chipname' is replaced by468the name of the chip):469 470.. code-block:: c471 472 struct chipname_state {473 struct v4l2_subdev sd;474 ... /* additional state fields */475 };476 477Initialize the :c:type:`v4l2_subdev` struct as follows:478 479.. code-block:: c480 481 v4l2_i2c_subdev_init(&state->sd, client, subdev_ops);482 483This function will fill in all the fields of :c:type:`v4l2_subdev` ensure that484the :c:type:`v4l2_subdev` and i2c_client both point to one another.485 486You should also add a helper inline function to go from a :c:type:`v4l2_subdev`487pointer to a chipname_state struct:488 489.. code-block:: c490 491 static inline struct chipname_state *to_state(struct v4l2_subdev *sd)492 {493 return container_of(sd, struct chipname_state, sd);494 }495 496Use this to go from the :c:type:`v4l2_subdev` struct to the ``i2c_client``497struct:498 499.. code-block:: c500 501 struct i2c_client *client = v4l2_get_subdevdata(sd);502 503And this to go from an ``i2c_client`` to a :c:type:`v4l2_subdev` struct:504 505.. code-block:: c506 507 struct v4l2_subdev *sd = i2c_get_clientdata(client);508 509Make sure to call510:c:func:`v4l2_device_unregister_subdev`\ (:c:type:`sd <v4l2_subdev>`)511when the ``remove()`` callback is called. This will unregister the sub-device512from the bridge driver. It is safe to call this even if the sub-device was513never registered.514 515You need to do this because when the bridge driver destroys the i2c adapter516the ``remove()`` callbacks are called of the i2c devices on that adapter.517After that the corresponding v4l2_subdev structures are invalid, so they518have to be unregistered first. Calling519:c:func:`v4l2_device_unregister_subdev`\ (:c:type:`sd <v4l2_subdev>`)520from the ``remove()`` callback ensures that this is always done correctly.521 522 523The bridge driver also has some helper functions it can use:524 525.. code-block:: c526 527 struct v4l2_subdev *sd = v4l2_i2c_new_subdev(v4l2_dev, adapter,528 "module_foo", "chipid", 0x36, NULL);529 530This loads the given module (can be ``NULL`` if no module needs to be loaded)531and calls :c:func:`i2c_new_client_device` with the given ``i2c_adapter`` and532chip/address arguments. If all goes well, then it registers the subdev with533the v4l2_device.534 535You can also use the last argument of :c:func:`v4l2_i2c_new_subdev` to pass536an array of possible I2C addresses that it should probe. These probe addresses537are only used if the previous argument is 0. A non-zero argument means that you538know the exact i2c address so in that case no probing will take place.539 540Both functions return ``NULL`` if something went wrong.541 542Note that the chipid you pass to :c:func:`v4l2_i2c_new_subdev` is usually543the same as the module name. It allows you to specify a chip variant, e.g.544"saa7114" or "saa7115". In general though the i2c driver autodetects this.545The use of chipid is something that needs to be looked at more closely at a546later date. It differs between i2c drivers and as such can be confusing.547To see which chip variants are supported you can look in the i2c driver code548for the i2c_device_id table. This lists all the possibilities.549 550There are one more helper function:551 552:c:func:`v4l2_i2c_new_subdev_board` uses an :c:type:`i2c_board_info` struct553which is passed to the i2c driver and replaces the irq, platform_data and addr554arguments.555 556If the subdev supports the s_config core ops, then that op is called with557the irq and platform_data arguments after the subdev was setup.558 559The :c:func:`v4l2_i2c_new_subdev` function will call560:c:func:`v4l2_i2c_new_subdev_board`, internally filling a561:c:type:`i2c_board_info` structure using the ``client_type`` and the562``addr`` to fill it.563 564Centrally managed subdev active state565-------------------------------------566 567Traditionally V4L2 subdev drivers maintained internal state for the active568device configuration. This is often implemented as e.g. an array of struct569v4l2_mbus_framefmt, one entry for each pad, and similarly for crop and compose570rectangles.571 572In addition to the active configuration, each subdev file handle has a struct573v4l2_subdev_state, managed by the V4L2 core, which contains the try574configuration.575 576To simplify the subdev drivers the V4L2 subdev API now optionally supports a577centrally managed active configuration represented by578:c:type:`v4l2_subdev_state`. One instance of state, which contains the active579device configuration, is stored in the sub-device itself as part of580the :c:type:`v4l2_subdev` structure, while the core associates a try state to581each open file handle, to store the try configuration related to that file582handle.583 584Sub-device drivers can opt-in and use state to manage their active configuration585by initializing the subdevice state with a call to v4l2_subdev_init_finalize()586before registering the sub-device. They must also call v4l2_subdev_cleanup()587to release all the allocated resources before unregistering the sub-device.588The core automatically allocates and initializes a state for each open file589handle to store the try configurations and frees it when closing the file590handle.591 592V4L2 sub-device operations that use both the :ref:`ACTIVE and TRY formats593<v4l2-subdev-format-whence>` receive the correct state to operate on through594the 'state' parameter. The state must be locked and unlocked by the595caller by calling :c:func:`v4l2_subdev_lock_state()` and596:c:func:`v4l2_subdev_unlock_state()`. The caller can do so by calling the subdev597operation through the :c:func:`v4l2_subdev_call_state_active()` macro.598 599Operations that do not receive a state parameter implicitly operate on the600subdevice active state, which drivers can exclusively access by601calling :c:func:`v4l2_subdev_lock_and_get_active_state()`. The sub-device active602state must equally be released by calling :c:func:`v4l2_subdev_unlock_state()`.603 604Drivers must never manually access the state stored in the :c:type:`v4l2_subdev`605or in the file handle without going through the designated helpers.606 607While the V4L2 core passes the correct try or active state to the subdevice608operations, many existing device drivers pass a NULL state when calling609operations with :c:func:`v4l2_subdev_call()`. This legacy construct causes610issues with subdevice drivers that let the V4L2 core manage the active state,611as they expect to receive the appropriate state as a parameter. To help the612conversion of subdevice drivers to a managed active state without having to613convert all callers at the same time, an additional wrapper layer has been614added to v4l2_subdev_call(), which handles the NULL case by getting and locking615the callee's active state with :c:func:`v4l2_subdev_lock_and_get_active_state()`,616and unlocking the state after the call.617 618The whole subdev state is in reality split into three parts: the619v4l2_subdev_state, subdev controls and subdev driver's internal state. In the620future these parts should be combined into a single state. For the time being621we need a way to handle the locking for these parts. This can be accomplished622by sharing a lock. The v4l2_ctrl_handler already supports this via its 'lock'623pointer and the same model is used with states. The driver can do the following624before calling v4l2_subdev_init_finalize():625 626.. code-block:: c627 628 sd->ctrl_handler->lock = &priv->mutex;629 sd->state_lock = &priv->mutex;630 631This shares the driver's private mutex between the controls and the states.632 633Streams, multiplexed media pads and internal routing634----------------------------------------------------635 636A subdevice driver can implement support for multiplexed streams by setting637the V4L2_SUBDEV_FL_STREAMS subdev flag and implementing support for638centrally managed subdev active state, routing and stream based639configuration.640 641V4L2 sub-device functions and data structures642---------------------------------------------643 644.. kernel-doc:: include/media/v4l2-subdev.h645