[media] v4l2-subdev.rst: add documentation from v4l2-framework.rst
There are lots of documentation about V4L2 subdevices at v4l2-framework.rst. Move them to its specific chapter at v4l2-subdev.rst. Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
This commit is contained in:
Mauro Carvalho Chehab
parent
02ca08b8ae
commit
840b14d983
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@ -80,263 +80,6 @@ The V4L2 framework also optionally integrates with the media framework. If a
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driver sets the struct v4l2_device mdev field, sub-devices and video nodes
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will automatically appear in the media framework as entities.
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struct v4l2_subdev
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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 v4l2_subdev struct
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(v4l2-subdev.h) was created.
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Each sub-device driver must have a v4l2_subdev struct. This struct can be
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stand-alone for simple sub-devices or it might be embedded in a larger struct
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if more state information needs to be stored. Usually there is a low-level
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device struct (e.g. i2c_client) that contains the device data as setup
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by the kernel. It is recommended to store that pointer in the private
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data of v4l2_subdev using v4l2_set_subdevdata(). That makes it easy to go
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from a v4l2_subdev to the actual low-level bus-specific device data.
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You also need a way to go from the low-level struct to v4l2_subdev. For the
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common i2c_client struct the i2c_set_clientdata() call is used to store a
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v4l2_subdev pointer, for other busses you may have to use other 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 v4l2_subdev structure provides
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host private data for that purpose that can be accessed with
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v4l2_get_subdev_hostdata() and 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 v4l2_subdev pointer. For i2c devices this is easy: you call
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i2c_get_clientdata(). For other busses 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 v4l2_subdev contains function pointers that sub-device drivers can
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implement (or leave NULL if it is not applicable). Since sub-devices can do
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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:: none
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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 v4l2_subdev struct using:
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.. code-block:: none
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v4l2_subdev_init(sd, &ops);
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Afterwards you need to initialize subdev->name with a unique name and set the
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module owner. This is done for you if you use the i2c helper functions.
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If integration with the media framework is needed, you must initialize the
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media_entity struct embedded in the v4l2_subdev struct (entity field) by
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calling media_entity_pads_init(), if the entity has pads:
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.. code-block:: none
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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 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:: none
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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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v4l2_subdev_pad_ops instead of 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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v4l2_subdev_link_validate_default() is used instead. This function ensures
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that width, height and the media bus pixel code are equal on both source and
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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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v4l2_subdev with the v4l2_device:
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.. code-block:: none
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int err = v4l2_device_register_subdev(v4l2_dev, sd);
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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 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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.. code-block:: none
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v4l2_device_unregister_subdev(sd);
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Afterwards the subdev module can be unloaded and sd->dev == NULL.
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You can call an ops function either directly:
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.. code-block:: none
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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:: none
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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 if subdev
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is NULL, -ENOIOCTLCMD if either subdev->core or subdev->core->g_std is
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NULL, or the actual result of the 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:: none
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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:: none
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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 sd->grp_id
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to whatever value it wants (it's 0 by default). This value is owned by the
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bridge driver and the sub-device driver 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 v4l2_subdev is that it is a generic struct and does
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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 v4l2_async_register_subdev() function. Unregistration is performed using
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the v4l2_async_unregister_subdev() call. Subdevices registered this way are
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stored in a global list of subdevices, ready to be picked up by bridge drivers.
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Bridge drivers in turn have to register a notifier object with an array of
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subdevice descriptors that the bridge device needs for its operation. This is
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performed using the v4l2_async_notifier_register() call. To unregister the
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notifier the driver has to call v4l2_async_notifier_unregister(). The former of
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the two functions takes two arguments: a pointer to struct v4l2_device and a
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pointer to struct v4l2_async_notifier. The latter contains a pointer to an array
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of pointers to subdevice descriptors of type struct v4l2_async_subdev type. The
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V4L2 core will then use these descriptors to match asynchronously registered
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subdevices to them. If a match is detected the .bound() notifier callback is
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called. After all subdevices have been located the .complete() callback is
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called. When a subdevice is removed from the system the .unbind() method is
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called. All three callbacks are optional.
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V4L2 sub-device userspace API
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-----------------------------
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@ -1,3 +1,259 @@
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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 v4l2_subdev struct
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(v4l2-subdev.h) was created.
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Each sub-device driver must have a v4l2_subdev struct. This struct can be
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stand-alone for simple sub-devices or it might be embedded in a larger struct
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if more state information needs to be stored. Usually there is a low-level
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device struct (e.g. i2c_client) that contains the device data as setup
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by the kernel. It is recommended to store that pointer in the private
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data of v4l2_subdev using v4l2_set_subdevdata(). That makes it easy to go
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from a v4l2_subdev to the actual low-level bus-specific device data.
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You also need a way to go from the low-level struct to v4l2_subdev. For the
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common i2c_client struct the i2c_set_clientdata() call is used to store a
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v4l2_subdev pointer, for other busses you may have to use other 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 v4l2_subdev structure provides
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host private data for that purpose that can be accessed with
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v4l2_get_subdev_hostdata() and 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 v4l2_subdev pointer. For i2c devices this is easy: you call
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i2c_get_clientdata(). For other busses 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 v4l2_subdev contains function pointers that sub-device drivers can
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implement (or leave NULL if it is not applicable). Since sub-devices can do
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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:: none
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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 v4l2_subdev struct using:
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.. code-block:: none
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v4l2_subdev_init(sd, &ops);
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Afterwards you need to initialize subdev->name with a unique name and set the
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module owner. This is done for you if you use the i2c helper functions.
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If integration with the media framework is needed, you must initialize the
|
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media_entity struct embedded in the v4l2_subdev struct (entity field) by
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calling media_entity_pads_init(), if the entity has pads:
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.. code-block:: none
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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 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:: none
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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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v4l2_subdev_pad_ops instead of v4l2_subdev_video_ops.
|
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|
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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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|
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If link_validate op is not set, the default function
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v4l2_subdev_link_validate_default() is used instead. This function ensures
|
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that width, height and the media bus pixel code are equal on both source and
|
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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:
|
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|
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.. code-block:: none
|
||||
|
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int err = v4l2_device_register_subdev(v4l2_dev, sd);
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|
||||
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
|
||||
^^^^^^^^^^^^^^^^
|
||||
|
||||
|
|
|
|||
Loading…
Reference in New Issue