[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>

2 files changed, 256 insertions, 257 deletions

diff --git a/Documentation/media/kapi/v4l2-framework.rst b/Documentation/media/kapi/v4l2-framework.rst
index 1fdc96bd7411..a7f64b38af5c 100644
--- a/Documentation/media/kapi/v4l2-framework.rst
+++ b/Documentation/media/kapi/v4l2-framework.rst
@@ -80,263 +80,6 @@ The V4L2 framework also optionally integrates with the media framework. If a
 driver sets the struct v4l2_device mdev field, sub-devices and video nodes
 will automatically appear in the media framework as entities.
 
-struct v4l2_subdev
-------------------
-
-Many drivers need to communicate with sub-devices. These devices can do all
-sort of tasks, but most commonly they handle audio and/or video muxing,
-encoding or decoding. For webcams common sub-devices are sensors and camera
-controllers.
-
-Usually these are I2C devices, but not necessarily. In order to provide the
-driver with a consistent interface to these sub-devices the v4l2_subdev struct
-(v4l2-subdev.h) was created.
-
-Each sub-device driver must have a v4l2_subdev struct. This struct can be
-stand-alone for simple sub-devices or it might be embedded in a larger struct
-if more state information needs to be stored. Usually there is a low-level
-device struct (e.g. i2c_client) that contains the device data as setup
-by the kernel. It is recommended to store that pointer in the private
-data of v4l2_subdev using v4l2_set_subdevdata(). That makes it easy to go
-from a v4l2_subdev to the actual low-level bus-specific device data.
-
-You also need a way to go from the low-level struct to v4l2_subdev. For the
-common i2c_client struct the i2c_set_clientdata() call is used to store a
-v4l2_subdev pointer, for other busses you may have to use other methods.
-
-Bridges might also need to store per-subdev private data, such as a pointer to
-bridge-specific per-subdev private data. The v4l2_subdev structure provides
-host private data for that purpose that can be accessed with
-v4l2_get_subdev_hostdata() and v4l2_set_subdev_hostdata().
-
-From the bridge driver perspective you load the sub-device module and somehow
-obtain the v4l2_subdev pointer. For i2c devices this is easy: you call
-i2c_get_clientdata(). For other busses something similar needs to be done.
-Helper functions exists for sub-devices on an I2C bus that do most of this
-tricky work for you.
-
-Each v4l2_subdev contains function pointers that sub-device drivers can
-implement (or leave NULL if it is not applicable). Since sub-devices can do
-so many different things and you do not want to end up with a huge ops struct
-of which only a handful of ops are commonly implemented, the function pointers
-are sorted according to category and each category has its own ops struct.
-
-The top-level ops struct contains pointers to the category ops structs, which
-may be NULL if the subdev driver does not support anything from that category.
-
-It looks like this:
-
-.. code-block:: none
-
-	struct v4l2_subdev_core_ops {
-		int (*log_status)(struct v4l2_subdev *sd);
-		int (*init)(struct v4l2_subdev *sd, u32 val);
-		...
-	};
-
-	struct v4l2_subdev_tuner_ops {
-		...
-	};
-
-	struct v4l2_subdev_audio_ops {
-		...
-	};
-
-	struct v4l2_subdev_video_ops {
-		...
-	};
-
-	struct v4l2_subdev_pad_ops {
-		...
-	};
-
-	struct v4l2_subdev_ops {
-		const struct v4l2_subdev_core_ops  *core;
-		const struct v4l2_subdev_tuner_ops *tuner;
-		const struct v4l2_subdev_audio_ops *audio;
-		const struct v4l2_subdev_video_ops *video;
-		const struct v4l2_subdev_pad_ops *video;
-	};
-
-The core ops are common to all subdevs, the other categories are implemented
-depending on the sub-device. E.g. a video device is unlikely to support the
-audio ops and vice versa.
-
-This setup limits the number of function pointers while still making it easy
-to add new ops and categories.
-
-A sub-device driver initializes the v4l2_subdev struct using:
-
-.. code-block:: none
-
-	v4l2_subdev_init(sd, &ops);
-
-Afterwards you need to initialize subdev->name with a unique name and set the
-module owner. This is done for you if you use the i2c helper functions.
-
-If integration with the media framework is needed, you must initialize the
-media_entity struct embedded in the v4l2_subdev struct (entity field) by
-calling media_entity_pads_init(), if the entity has pads:
-
-.. code-block:: none
-
-	struct media_pad *pads = &my_sd->pads;
-	int err;
-
-	err = media_entity_pads_init(&sd->entity, npads, pads);
-
-The pads array must have been previously initialized. There is no need to
-manually set the struct media_entity function and name fields, but the
-revision field must be initialized if needed.
-
-A reference to the entity will be automatically acquired/released when the
-subdev device node (if any) is opened/closed.
-
-Don't forget to cleanup the media entity before the sub-device is destroyed:
-
-.. code-block:: none
-
-	media_entity_cleanup(&sd->entity);
-
-If the subdev driver intends to process video and integrate with the media
-framework, it must implement format related functionality using
-v4l2_subdev_pad_ops instead of v4l2_subdev_video_ops.
-
-In that case, the subdev driver may set the link_validate field to provide
-its own link validation function. The link validation function is called for
-every link in the pipeline where both of the ends of the links are V4L2
-sub-devices. The driver is still responsible for validating the correctness
-of the format configuration between sub-devices and video nodes.
-
-If link_validate op is not set, the default function
-v4l2_subdev_link_validate_default() is used instead. This function ensures
-that width, height and the media bus pixel code are equal on both source and
-sink of the link. Subdev drivers are also free to use this function to
-perform the checks mentioned above in addition to their own checks.
-
-There are currently two ways to register subdevices with the V4L2 core. The
-first (traditional) possibility is to have subdevices registered by bridge
-drivers. This can be done when the bridge driver has the complete information
-about subdevices connected to it and knows exactly when to register them. This
-is typically the case for internal subdevices, like video data processing units
-within SoCs or complex PCI(e) boards, camera sensors in USB cameras or connected
-to SoCs, which pass information about them to bridge drivers, usually in their
-platform data.
-
-There are however also situations where subdevices have to be registered
-asynchronously to bridge devices. An example of such a configuration is a Device
-Tree based system where information about subdevices is made available to the
-system independently from the bridge devices, e.g. when subdevices are defined
-in DT as I2C device nodes. The API used in this second case is described further
-below.
-
-Using one or the other registration method only affects the probing process, the
-run-time bridge-subdevice interaction is in both cases the same.
-
-In the synchronous case a device (bridge) driver needs to register the
-v4l2_subdev with the v4l2_device:
-
-.. code-block:: none
-
-	int err = v4l2_device_register_subdev(v4l2_dev, sd);
-
-This can fail if the subdev module disappeared before it could be registered.
-After this function was called successfully the subdev->dev field points to
-the v4l2_device.
-
-If the v4l2_device parent device has a non-NULL mdev field, the sub-device
-entity will be automatically registered with the media device.
-
-You can unregister a sub-device using:
-
-.. code-block:: none
-
-	v4l2_device_unregister_subdev(sd);
-
-Afterwards the subdev module can be unloaded and sd->dev == NULL.
-
-You can call an ops function either directly:
-
-.. code-block:: none
-
-	err = sd->ops->core->g_std(sd, &norm);
-
-but it is better and easier to use this macro:
-
-.. code-block:: none
-
-	err = v4l2_subdev_call(sd, core, g_std, &norm);
-
-The macro will to the right NULL pointer checks and returns -ENODEV if subdev
-is NULL, -ENOIOCTLCMD if either subdev->core or subdev->core->g_std is
-NULL, or the actual result of the subdev->ops->core->g_std ops.
-
-It is also possible to call all or a subset of the sub-devices:
-
-.. code-block:: none
-
-	v4l2_device_call_all(v4l2_dev, 0, core, g_std, &norm);
-
-Any subdev that does not support this ops is skipped and error results are
-ignored. If you want to check for errors use this:
-
-.. code-block:: none
-
-	err = v4l2_device_call_until_err(v4l2_dev, 0, core, g_std, &norm);
-
-Any error except -ENOIOCTLCMD will exit the loop with that error. If no
-errors (except -ENOIOCTLCMD) occurred, then 0 is returned.
-
-The second argument to both calls is a group ID. If 0, then all subdevs are
-called. If non-zero, then only those whose group ID match that value will
-be called. Before a bridge driver registers a subdev it can set sd->grp_id
-to whatever value it wants (it's 0 by default). This value is owned by the
-bridge driver and the sub-device driver will never modify or use it.
-
-The group ID gives the bridge driver more control how callbacks are called.
-For example, there may be multiple audio chips on a board, each capable of
-changing the volume. But usually only one will actually be used when the
-user want to change the volume. You can set the group ID for that subdev to
-e.g. AUDIO_CONTROLLER and specify that as the group ID value when calling
-v4l2_device_call_all(). That ensures that it will only go to the subdev
-that needs it.
-
-If the sub-device needs to notify its v4l2_device parent of an event, then
-it can call v4l2_subdev_notify(sd, notification, arg). This macro checks
-whether there is a notify() callback defined and returns -ENODEV if not.
-Otherwise the result of the notify() call is returned.
-
-The advantage of using v4l2_subdev is that it is a generic struct and does
-not contain any knowledge about the underlying hardware. So a driver might
-contain several subdevs that use an I2C bus, but also a subdev that is
-controlled through GPIO pins. This distinction is only relevant when setting
-up the device, but once the subdev is registered it is completely transparent.
-
-
-In the asynchronous case subdevice probing can be invoked independently of the
-bridge driver availability. The subdevice driver then has to verify whether all
-the requirements for a successful probing are satisfied. This can include a
-check for a master clock availability. If any of the conditions aren't satisfied
-the driver might decide to return -EPROBE_DEFER to request further reprobing
-attempts. Once all conditions are met the subdevice shall be registered using
-the v4l2_async_register_subdev() function. Unregistration is performed using
-the v4l2_async_unregister_subdev() call. Subdevices registered this way are
-stored in a global list of subdevices, ready to be picked up by bridge drivers.
-
-Bridge drivers in turn have to register a notifier object with an array of
-subdevice descriptors that the bridge device needs for its operation. This is
-performed using the v4l2_async_notifier_register() call. To unregister the
-notifier the driver has to call v4l2_async_notifier_unregister(). The former of
-the two functions takes two arguments: a pointer to struct v4l2_device and a
-pointer to struct v4l2_async_notifier. The latter contains a pointer to an array
-of pointers to subdevice descriptors of type struct v4l2_async_subdev type. The
-V4L2 core will then use these descriptors to match asynchronously registered
-subdevices to them. If a match is detected the .bound() notifier callback is
-called. After all subdevices have been located the .complete() callback is
-called. When a subdevice is removed from the system the .unbind() method is
-called. All three callbacks are optional.
-
-
 V4L2 sub-device userspace API
 -----------------------------
 
diff --git a/Documentation/media/kapi/v4l2-subdev.rst b/Documentation/media/kapi/v4l2-subdev.rst
index 1b262aa7e250..3e2b25b47ff4 100644
--- a/Documentation/media/kapi/v4l2-subdev.rst
+++ b/Documentation/media/kapi/v4l2-subdev.rst
@@ -1,3 +1,259 @@
+V4L2 sub-devices
+----------------
+
+Many drivers need to communicate with sub-devices. These devices can do all
+sort of tasks, but most commonly they handle audio and/or video muxing,
+encoding or decoding. For webcams common sub-devices are sensors and camera
+controllers.
+
+Usually these are I2C devices, but not necessarily. In order to provide the
+driver with a consistent interface to these sub-devices the v4l2_subdev struct
+(v4l2-subdev.h) was created.
+
+Each sub-device driver must have a v4l2_subdev struct. This struct can be
+stand-alone for simple sub-devices or it might be embedded in a larger struct
+if more state information needs to be stored. Usually there is a low-level
+device struct (e.g. i2c_client) that contains the device data as setup
+by the kernel. It is recommended to store that pointer in the private
+data of v4l2_subdev using v4l2_set_subdevdata(). That makes it easy to go
+from a v4l2_subdev to the actual low-level bus-specific device data.
+
+You also need a way to go from the low-level struct to v4l2_subdev. For the
+common i2c_client struct the i2c_set_clientdata() call is used to store a
+v4l2_subdev pointer, for other busses you may have to use other methods.
+
+Bridges might also need to store per-subdev private data, such as a pointer to
+bridge-specific per-subdev private data. The v4l2_subdev structure provides
+host private data for that purpose that can be accessed with
+v4l2_get_subdev_hostdata() and v4l2_set_subdev_hostdata().
+
+From the bridge driver perspective you load the sub-device module and somehow
+obtain the v4l2_subdev pointer. For i2c devices this is easy: you call
+i2c_get_clientdata(). For other busses something similar needs to be done.
+Helper functions exists for sub-devices on an I2C bus that do most of this
+tricky work for you.
+
+Each v4l2_subdev contains function pointers that sub-device drivers can
+implement (or leave NULL if it is not applicable). Since sub-devices can do
+so many different things and you do not want to end up with a huge ops struct
+of which only a handful of ops are commonly implemented, the function pointers
+are sorted according to category and each category has its own ops struct.
+
+The top-level ops struct contains pointers to the category ops structs, which
+may be NULL if the subdev driver does not support anything from that category.
+
+It looks like this:
+
+.. code-block:: none
+
+	struct v4l2_subdev_core_ops {
+		int (*log_status)(struct v4l2_subdev *sd);
+		int (*init)(struct v4l2_subdev *sd, u32 val);
+		...
+	};
+
+	struct v4l2_subdev_tuner_ops {
+		...
+	};
+
+	struct v4l2_subdev_audio_ops {
+		...
+	};
+
+	struct v4l2_subdev_video_ops {
+		...
+	};
+
+	struct v4l2_subdev_pad_ops {
+		...
+	};
+
+	struct v4l2_subdev_ops {
+		const struct v4l2_subdev_core_ops  *core;
+		const struct v4l2_subdev_tuner_ops *tuner;
+		const struct v4l2_subdev_audio_ops *audio;
+		const struct v4l2_subdev_video_ops *video;
+		const struct v4l2_subdev_pad_ops *video;
+	};
+
+The core ops are common to all subdevs, the other categories are implemented
+depending on the sub-device. E.g. a video device is unlikely to support the
+audio ops and vice versa.
+
+This setup limits the number of function pointers while still making it easy
+to add new ops and categories.
+
+A sub-device driver initializes the v4l2_subdev struct using:
+
+.. code-block:: none
+
+	v4l2_subdev_init(sd, &ops);
+
+Afterwards you need to initialize subdev->name with a unique name and set the
+module owner. This is done for you if you use the i2c helper functions.
+
+If integration with the media framework is needed, you must initialize the
+media_entity struct embedded in the v4l2_subdev struct (entity field) by
+calling media_entity_pads_init(), if the entity has pads:
+
+.. code-block:: none
+
+	struct media_pad *pads = &my_sd->pads;
+	int err;
+
+	err = media_entity_pads_init(&sd->entity, npads, pads);
+
+The pads array must have been previously initialized. There is no need to
+manually set the struct media_entity function and name fields, but the
+revision field must be initialized if needed.
+
+A reference to the entity will be automatically acquired/released when the
+subdev device node (if any) is opened/closed.
+
+Don't forget to cleanup the media entity before the sub-device is destroyed:
+
+.. code-block:: none
+
+	media_entity_cleanup(&sd->entity);
+
+If the subdev driver intends to process video and integrate with the media
+framework, it must implement format related functionality using
+v4l2_subdev_pad_ops instead of v4l2_subdev_video_ops.
+
+In that case, the subdev driver may set the link_validate field to provide
+its own link validation function. The link validation function is called for
+every link in the pipeline where both of the ends of the links are V4L2
+sub-devices. The driver is still responsible for validating the correctness
+of the format configuration between sub-devices and video nodes.
+
+If link_validate op is not set, the default function
+v4l2_subdev_link_validate_default() is used instead. This function ensures
+that width, height and the media bus pixel code are equal on both source and
+sink of the link. Subdev drivers are also free to use this function to
+perform the checks mentioned above in addition to their own checks.
+
+There are currently two ways to register subdevices with the V4L2 core. The
+first (traditional) possibility is to have subdevices registered by bridge
+drivers. This can be done when the bridge driver has the complete information
+about subdevices connected to it and knows exactly when to register them. This
+is typically the case for internal subdevices, like video data processing units
+within SoCs or complex PCI(e) boards, camera sensors in USB cameras or connected
+to SoCs, which pass information about them to bridge drivers, usually in their
+platform data.
+
+There are however also situations where subdevices have to be registered
+asynchronously to bridge devices. An example of such a configuration is a Device
+Tree based system where information about subdevices is made available to the
+system independently from the bridge devices, e.g. when subdevices are defined
+in DT as I2C device nodes. The API used in this second case is described further
+below.
+
+Using one or the other registration method only affects the probing process, the
+run-time bridge-subdevice interaction is in both cases the same.
+
+In the synchronous case a device (bridge) driver needs to register the
+v4l2_subdev with the v4l2_device:
+
+.. code-block:: none
+
+	int err = v4l2_device_register_subdev(v4l2_dev, sd);
+
+This can fail if the subdev module disappeared before it could be registered.
+After this function was called successfully the subdev->dev field points to
+the v4l2_device.
+
+If the v4l2_device parent device has a non-NULL mdev field, the sub-device
+entity will be automatically registered with the media device.
+
+You can unregister a sub-device using:
+
+.. code-block:: none
+
+	v4l2_device_unregister_subdev(sd);
+
+Afterwards the subdev module can be unloaded and sd->dev == NULL.
+
+You can call an ops function either directly:
+
+.. code-block:: none
+
+	err = sd->ops->core->g_std(sd, &norm);
+
+but it is better and easier to use this macro:
+
+.. code-block:: none
+
+	err = v4l2_subdev_call(sd, core, g_std, &norm);
+
+The macro will to the right NULL pointer checks and returns -ENODEV if subdev
+is NULL, -ENOIOCTLCMD if either subdev->core or subdev->core->g_std is
+NULL, or the actual result of the subdev->ops->core->g_std ops.
+
+It is also possible to call all or a subset of the sub-devices:
+
+.. code-block:: none
+
+	v4l2_device_call_all(v4l2_dev, 0, core, g_std, &norm);
+
+Any subdev that does not support this ops is skipped and error results are
+ignored. If you want to check for errors use this:
+
+.. code-block:: none
+
+	err = v4l2_device_call_until_err(v4l2_dev, 0, core, g_std, &norm);
+
+Any error except -ENOIOCTLCMD will exit the loop with that error. If no
+errors (except -ENOIOCTLCMD) occurred, then 0 is returned.
+
+The second argument to both calls is a group ID. If 0, then all subdevs are
+called. If non-zero, then only those whose group ID match that value will
+be called. Before a bridge driver registers a subdev it can set sd->grp_id
+to whatever value it wants (it's 0 by default). This value is owned by the
+bridge driver and the sub-device driver will never modify or use it.
+
+The group ID gives the bridge driver more control how callbacks are called.
+For example, there may be multiple audio chips on a board, each capable of
+changing the volume. But usually only one will actually be used when the
+user want to change the volume. You can set the group ID for that subdev to
+e.g. AUDIO_CONTROLLER and specify that as the group ID value when calling
+v4l2_device_call_all(). That ensures that it will only go to the subdev
+that needs it.
+
+If the sub-device needs to notify its v4l2_device parent of an event, then
+it can call v4l2_subdev_notify(sd, notification, arg). This macro checks
+whether there is a notify() callback defined and returns -ENODEV if not.
+Otherwise the result of the notify() call is returned.
+
+The advantage of using v4l2_subdev is that it is a generic struct and does
+not contain any knowledge about the underlying hardware. So a driver might
+contain several subdevs that use an I2C bus, but also a subdev that is
+controlled through GPIO pins. This distinction is only relevant when setting
+up the device, but once the subdev is registered it is completely transparent.
+
+
+In the asynchronous case subdevice probing can be invoked independently of the
+bridge driver availability. The subdevice driver then has to verify whether all
+the requirements for a successful probing are satisfied. This can include a
+check for a master clock availability. If any of the conditions aren't satisfied
+the driver might decide to return -EPROBE_DEFER to request further reprobing
+attempts. Once all conditions are met the subdevice shall be registered using
+the v4l2_async_register_subdev() function. Unregistration is performed using
+the v4l2_async_unregister_subdev() call. Subdevices registered this way are
+stored in a global list of subdevices, ready to be picked up by bridge drivers.
+
+Bridge drivers in turn have to register a notifier object with an array of
+subdevice descriptors that the bridge device needs for its operation. This is
+performed using the v4l2_async_notifier_register() call. To unregister the
+notifier the driver has to call v4l2_async_notifier_unregister(). The former of
+the two functions takes two arguments: a pointer to struct v4l2_device and a
+pointer to struct v4l2_async_notifier. The latter contains a pointer to an array
+of pointers to subdevice descriptors of type struct v4l2_async_subdev type. The
+V4L2 core will then use these descriptors to match asynchronously registered
+subdevices to them. If a match is detected the .bound() notifier callback is
+called. After all subdevices have been located the .complete() callback is
+called. When a subdevice is removed from the system the .unbind() method is
+called. All three callbacks are optional.
+
 V4L2 subdev kAPI
 ^^^^^^^^^^^^^^^^