[media] doc-rst: move framework docs to kAPI documentation

Those documentation are part of the kAPI one. Move to the right
place.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

2 files changed, 1911 insertions, 0 deletions

diff --git a/Documentation/media/kapi/v4l2-controls.rst b/Documentation/media/kapi/v4l2-controls.rst
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+Introduction
+============
+
+The V4L2 control API seems simple enough, but quickly becomes very hard to
+implement correctly in drivers. But much of the code needed to handle controls
+is actually not driver specific and can be moved to the V4L core framework.
+
+After all, the only part that a driver developer is interested in is:
+
+1) How do I add a control?
+2) How do I set the control's value? (i.e. s_ctrl)
+
+And occasionally:
+
+3) How do I get the control's value? (i.e. g_volatile_ctrl)
+4) How do I validate the user's proposed control value? (i.e. try_ctrl)
+
+All the rest is something that can be done centrally.
+
+The control framework was created in order to implement all the rules of the
+V4L2 specification with respect to controls in a central place. And to make
+life as easy as possible for the driver developer.
+
+Note that the control framework relies on the presence of a struct v4l2_device
+for V4L2 drivers and struct v4l2_subdev for sub-device drivers.
+
+
+Objects in the framework
+========================
+
+There are two main objects:
+
+The v4l2_ctrl object describes the control properties and keeps track of the
+control's value (both the current value and the proposed new value).
+
+v4l2_ctrl_handler is the object that keeps track of controls. It maintains a
+list of v4l2_ctrl objects that it owns and another list of references to
+controls, possibly to controls owned by other handlers.
+
+
+Basic usage for V4L2 and sub-device drivers
+===========================================
+
+1) Prepare the driver:
+
+1.1) Add the handler to your driver's top-level struct:
+
+	struct foo_dev {
+		...
+		struct v4l2_ctrl_handler ctrl_handler;
+		...
+	};
+
+	struct foo_dev *foo;
+
+1.2) Initialize the handler:
+
+	v4l2_ctrl_handler_init(&foo->ctrl_handler, nr_of_controls);
+
+  The second argument is a hint telling the function how many controls this
+  handler is expected to handle. It will allocate a hashtable based on this
+  information. It is a hint only.
+
+1.3) Hook the control handler into the driver:
+
+1.3.1) For V4L2 drivers do this:
+
+	struct foo_dev {
+		...
+		struct v4l2_device v4l2_dev;
+		...
+		struct v4l2_ctrl_handler ctrl_handler;
+		...
+	};
+
+	foo->v4l2_dev.ctrl_handler = &foo->ctrl_handler;
+
+  Where foo->v4l2_dev is of type struct v4l2_device.
+
+  Finally, remove all control functions from your v4l2_ioctl_ops (if any):
+  vidioc_queryctrl, vidioc_query_ext_ctrl, vidioc_querymenu, vidioc_g_ctrl,
+  vidioc_s_ctrl, vidioc_g_ext_ctrls, vidioc_try_ext_ctrls and vidioc_s_ext_ctrls.
+  Those are now no longer needed.
+
+1.3.2) For sub-device drivers do this:
+
+	struct foo_dev {
+		...
+		struct v4l2_subdev sd;
+		...
+		struct v4l2_ctrl_handler ctrl_handler;
+		...
+	};
+
+	foo->sd.ctrl_handler = &foo->ctrl_handler;
+
+  Where foo->sd is of type struct v4l2_subdev.
+
+  And set all core control ops in your struct v4l2_subdev_core_ops to these
+  helpers:
+
+	.queryctrl = v4l2_subdev_queryctrl,
+	.querymenu = v4l2_subdev_querymenu,
+	.g_ctrl = v4l2_subdev_g_ctrl,
+	.s_ctrl = v4l2_subdev_s_ctrl,
+	.g_ext_ctrls = v4l2_subdev_g_ext_ctrls,
+	.try_ext_ctrls = v4l2_subdev_try_ext_ctrls,
+	.s_ext_ctrls = v4l2_subdev_s_ext_ctrls,
+
+  Note: this is a temporary solution only. Once all V4L2 drivers that depend
+  on subdev drivers are converted to the control framework these helpers will
+  no longer be needed.
+
+1.4) Clean up the handler at the end:
+
+	v4l2_ctrl_handler_free(&foo->ctrl_handler);
+
+
+2) Add controls:
+
+You add non-menu controls by calling v4l2_ctrl_new_std:
+
+	struct v4l2_ctrl *v4l2_ctrl_new_std(struct v4l2_ctrl_handler *hdl,
+			const struct v4l2_ctrl_ops *ops,
+			u32 id, s32 min, s32 max, u32 step, s32 def);
+
+Menu and integer menu controls are added by calling v4l2_ctrl_new_std_menu:
+
+	struct v4l2_ctrl *v4l2_ctrl_new_std_menu(struct v4l2_ctrl_handler *hdl,
+			const struct v4l2_ctrl_ops *ops,
+			u32 id, s32 max, s32 skip_mask, s32 def);
+
+Menu controls with a driver specific menu are added by calling
+v4l2_ctrl_new_std_menu_items:
+
+       struct v4l2_ctrl *v4l2_ctrl_new_std_menu_items(
+                       struct v4l2_ctrl_handler *hdl,
+                       const struct v4l2_ctrl_ops *ops, u32 id, s32 max,
+                       s32 skip_mask, s32 def, const char * const *qmenu);
+
+Integer menu controls with a driver specific menu can be added by calling
+v4l2_ctrl_new_int_menu:
+
+	struct v4l2_ctrl *v4l2_ctrl_new_int_menu(struct v4l2_ctrl_handler *hdl,
+			const struct v4l2_ctrl_ops *ops,
+			u32 id, s32 max, s32 def, const s64 *qmenu_int);
+
+These functions are typically called right after the v4l2_ctrl_handler_init:
+
+	static const s64 exp_bias_qmenu[] = {
+	       -2, -1, 0, 1, 2
+	};
+	static const char * const test_pattern[] = {
+		"Disabled",
+		"Vertical Bars",
+		"Solid Black",
+		"Solid White",
+	};
+
+	v4l2_ctrl_handler_init(&foo->ctrl_handler, nr_of_controls);
+	v4l2_ctrl_new_std(&foo->ctrl_handler, &foo_ctrl_ops,
+			V4L2_CID_BRIGHTNESS, 0, 255, 1, 128);
+	v4l2_ctrl_new_std(&foo->ctrl_handler, &foo_ctrl_ops,
+			V4L2_CID_CONTRAST, 0, 255, 1, 128);
+	v4l2_ctrl_new_std_menu(&foo->ctrl_handler, &foo_ctrl_ops,
+			V4L2_CID_POWER_LINE_FREQUENCY,
+			V4L2_CID_POWER_LINE_FREQUENCY_60HZ, 0,
+			V4L2_CID_POWER_LINE_FREQUENCY_DISABLED);
+	v4l2_ctrl_new_int_menu(&foo->ctrl_handler, &foo_ctrl_ops,
+			V4L2_CID_EXPOSURE_BIAS,
+			ARRAY_SIZE(exp_bias_qmenu) - 1,
+			ARRAY_SIZE(exp_bias_qmenu) / 2 - 1,
+			exp_bias_qmenu);
+	v4l2_ctrl_new_std_menu_items(&foo->ctrl_handler, &foo_ctrl_ops,
+			V4L2_CID_TEST_PATTERN, ARRAY_SIZE(test_pattern) - 1, 0,
+			0, test_pattern);
+	...
+	if (foo->ctrl_handler.error) {
+		int err = foo->ctrl_handler.error;
+
+		v4l2_ctrl_handler_free(&foo->ctrl_handler);
+		return err;
+	}
+
+The v4l2_ctrl_new_std function returns the v4l2_ctrl pointer to the new
+control, but if you do not need to access the pointer outside the control ops,
+then there is no need to store it.
+
+The v4l2_ctrl_new_std function will fill in most fields based on the control
+ID except for the min, max, step and default values. These are passed in the
+last four arguments. These values are driver specific while control attributes
+like type, name, flags are all global. The control's current value will be set
+to the default value.
+
+The v4l2_ctrl_new_std_menu function is very similar but it is used for menu
+controls. There is no min argument since that is always 0 for menu controls,
+and instead of a step there is a skip_mask argument: if bit X is 1, then menu
+item X is skipped.
+
+The v4l2_ctrl_new_int_menu function creates a new standard integer menu
+control with driver-specific items in the menu. It differs from
+v4l2_ctrl_new_std_menu in that it doesn't have the mask argument and takes
+as the last argument an array of signed 64-bit integers that form an exact
+menu item list.
+
+The v4l2_ctrl_new_std_menu_items function is very similar to
+v4l2_ctrl_new_std_menu but takes an extra parameter qmenu, which is the driver
+specific menu for an otherwise standard menu control. A good example for this
+control is the test pattern control for capture/display/sensors devices that
+have the capability to generate test patterns. These test patterns are hardware
+specific, so the contents of the menu will vary from device to device.
+
+Note that if something fails, the function will return NULL or an error and
+set ctrl_handler->error to the error code. If ctrl_handler->error was already
+set, then it will just return and do nothing. This is also true for
+v4l2_ctrl_handler_init if it cannot allocate the internal data structure.
+
+This makes it easy to init the handler and just add all controls and only check
+the error code at the end. Saves a lot of repetitive error checking.
+
+It is recommended to add controls in ascending control ID order: it will be
+a bit faster that way.
+
+3) Optionally force initial control setup:
+
+	v4l2_ctrl_handler_setup(&foo->ctrl_handler);
+
+This will call s_ctrl for all controls unconditionally. Effectively this
+initializes the hardware to the default control values. It is recommended
+that you do this as this ensures that both the internal data structures and
+the hardware are in sync.
+
+4) Finally: implement the v4l2_ctrl_ops
+
+	static const struct v4l2_ctrl_ops foo_ctrl_ops = {
+		.s_ctrl = foo_s_ctrl,
+	};
+
+Usually all you need is s_ctrl:
+
+	static int foo_s_ctrl(struct v4l2_ctrl *ctrl)
+	{
+		struct foo *state = container_of(ctrl->handler, struct foo, ctrl_handler);
+
+		switch (ctrl->id) {
+		case V4L2_CID_BRIGHTNESS:
+			write_reg(0x123, ctrl->val);
+			break;
+		case V4L2_CID_CONTRAST:
+			write_reg(0x456, ctrl->val);
+			break;
+		}
+		return 0;
+	}
+
+The control ops are called with the v4l2_ctrl pointer as argument.
+The new control value has already been validated, so all you need to do is
+to actually update the hardware registers.
+
+You're done! And this is sufficient for most of the drivers we have. No need
+to do any validation of control values, or implement QUERYCTRL, QUERY_EXT_CTRL
+and QUERYMENU. And G/S_CTRL as well as G/TRY/S_EXT_CTRLS are automatically supported.
+
+
+==============================================================================
+
+The remainder of this document deals with more advanced topics and scenarios.
+In practice the basic usage as described above is sufficient for most drivers.
+
+===============================================================================
+
+
+Inheriting Controls
+===================
+
+When a sub-device is registered with a V4L2 driver by calling
+v4l2_device_register_subdev() and the ctrl_handler fields of both v4l2_subdev
+and v4l2_device are set, then the controls of the subdev will become
+automatically available in the V4L2 driver as well. If the subdev driver
+contains controls that already exist in the V4L2 driver, then those will be
+skipped (so a V4L2 driver can always override a subdev control).
+
+What happens here is that v4l2_device_register_subdev() calls
+v4l2_ctrl_add_handler() adding the controls of the subdev to the controls
+of v4l2_device.
+
+
+Accessing Control Values
+========================
+
+The following union is used inside the control framework to access control
+values:
+
+union v4l2_ctrl_ptr {
+	s32 *p_s32;
+	s64 *p_s64;
+	char *p_char;
+	void *p;
+};
+
+The v4l2_ctrl struct contains these fields that can be used to access both
+current and new values:
+
+	s32 val;
+	struct {
+		s32 val;
+	} cur;
+
+
+	union v4l2_ctrl_ptr p_new;
+	union v4l2_ctrl_ptr p_cur;
+
+If the control has a simple s32 type type, then:
+
+	&ctrl->val == ctrl->p_new.p_s32
+	&ctrl->cur.val == ctrl->p_cur.p_s32
+
+For all other types use ctrl->p_cur.p<something>. Basically the val
+and cur.val fields can be considered an alias since these are used so often.
+
+Within the control ops you can freely use these. The val and cur.val speak for
+themselves. The p_char pointers point to character buffers of length
+ctrl->maximum + 1, and are always 0-terminated.
+
+Unless the control is marked volatile the p_cur field points to the the
+current cached control value. When you create a new control this value is made
+identical to the default value. After calling v4l2_ctrl_handler_setup() this
+value is passed to the hardware. It is generally a good idea to call this
+function.
+
+Whenever a new value is set that new value is automatically cached. This means
+that most drivers do not need to implement the g_volatile_ctrl() op. The
+exception is for controls that return a volatile register such as a signal
+strength read-out that changes continuously. In that case you will need to
+implement g_volatile_ctrl like this:
+
+	static int foo_g_volatile_ctrl(struct v4l2_ctrl *ctrl)
+	{
+		switch (ctrl->id) {
+		case V4L2_CID_BRIGHTNESS:
+			ctrl->val = read_reg(0x123);
+			break;
+		}
+	}
+
+Note that you use the 'new value' union as well in g_volatile_ctrl. In general
+controls that need to implement g_volatile_ctrl are read-only controls. If they
+are not, a V4L2_EVENT_CTRL_CH_VALUE will not be generated when the control
+changes.
+
+To mark a control as volatile you have to set V4L2_CTRL_FLAG_VOLATILE:
+
+	ctrl = v4l2_ctrl_new_std(&sd->ctrl_handler, ...);
+	if (ctrl)
+		ctrl->flags |= V4L2_CTRL_FLAG_VOLATILE;
+
+For try/s_ctrl the new values (i.e. as passed by the user) are filled in and
+you can modify them in try_ctrl or set them in s_ctrl. The 'cur' union
+contains the current value, which you can use (but not change!) as well.
+
+If s_ctrl returns 0 (OK), then the control framework will copy the new final
+values to the 'cur' union.
+
+While in g_volatile/s/try_ctrl you can access the value of all controls owned
+by the same handler since the handler's lock is held. If you need to access
+the value of controls owned by other handlers, then you have to be very careful
+not to introduce deadlocks.
+
+Outside of the control ops you have to go through to helper functions to get
+or set a single control value safely in your driver:
+
+	s32 v4l2_ctrl_g_ctrl(struct v4l2_ctrl *ctrl);
+	int v4l2_ctrl_s_ctrl(struct v4l2_ctrl *ctrl, s32 val);
+
+These functions go through the control framework just as VIDIOC_G/S_CTRL ioctls
+do. Don't use these inside the control ops g_volatile/s/try_ctrl, though, that
+will result in a deadlock since these helpers lock the handler as well.
+
+You can also take the handler lock yourself:
+
+	mutex_lock(&state->ctrl_handler.lock);
+	pr_info("String value is '%s'\n", ctrl1->p_cur.p_char);
+	pr_info("Integer value is '%s'\n", ctrl2->cur.val);
+	mutex_unlock(&state->ctrl_handler.lock);
+
+
+Menu Controls
+=============
+
+The v4l2_ctrl struct contains this union:
+
+	union {
+		u32 step;
+		u32 menu_skip_mask;
+	};
+
+For menu controls menu_skip_mask is used. What it does is that it allows you
+to easily exclude certain menu items. This is used in the VIDIOC_QUERYMENU
+implementation where you can return -EINVAL if a certain menu item is not
+present. Note that VIDIOC_QUERYCTRL always returns a step value of 1 for
+menu controls.
+
+A good example is the MPEG Audio Layer II Bitrate menu control where the
+menu is a list of standardized possible bitrates. But in practice hardware
+implementations will only support a subset of those. By setting the skip
+mask you can tell the framework which menu items should be skipped. Setting
+it to 0 means that all menu items are supported.
+
+You set this mask either through the v4l2_ctrl_config struct for a custom
+control, or by calling v4l2_ctrl_new_std_menu().
+
+
+Custom Controls
+===============
+
+Driver specific controls can be created using v4l2_ctrl_new_custom():
+
+	static const struct v4l2_ctrl_config ctrl_filter = {
+		.ops = &ctrl_custom_ops,
+		.id = V4L2_CID_MPEG_CX2341X_VIDEO_SPATIAL_FILTER,
+		.name = "Spatial Filter",
+		.type = V4L2_CTRL_TYPE_INTEGER,
+		.flags = V4L2_CTRL_FLAG_SLIDER,
+		.max = 15,
+		.step = 1,
+	};
+
+	ctrl = v4l2_ctrl_new_custom(&foo->ctrl_handler, &ctrl_filter, NULL);
+
+The last argument is the priv pointer which can be set to driver-specific
+private data.
+
+The v4l2_ctrl_config struct also has a field to set the is_private flag.
+
+If the name field is not set, then the framework will assume this is a standard
+control and will fill in the name, type and flags fields accordingly.
+
+
+Active and Grabbed Controls
+===========================
+
+If you get more complex relationships between controls, then you may have to
+activate and deactivate controls. For example, if the Chroma AGC control is
+on, then the Chroma Gain control is inactive. That is, you may set it, but
+the value will not be used by the hardware as long as the automatic gain
+control is on. Typically user interfaces can disable such input fields.
+
+You can set the 'active' status using v4l2_ctrl_activate(). By default all
+controls are active. Note that the framework does not check for this flag.
+It is meant purely for GUIs. The function is typically called from within
+s_ctrl.
+
+The other flag is the 'grabbed' flag. A grabbed control means that you cannot
+change it because it is in use by some resource. Typical examples are MPEG
+bitrate controls that cannot be changed while capturing is in progress.
+
+If a control is set to 'grabbed' using v4l2_ctrl_grab(), then the framework
+will return -EBUSY if an attempt is made to set this control. The
+v4l2_ctrl_grab() function is typically called from the driver when it
+starts or stops streaming.
+
+
+Control Clusters
+================
+
+By default all controls are independent from the others. But in more
+complex scenarios you can get dependencies from one control to another.
+In that case you need to 'cluster' them:
+
+	struct foo {
+		struct v4l2_ctrl_handler ctrl_handler;
+#define AUDIO_CL_VOLUME (0)
+#define AUDIO_CL_MUTE   (1)
+		struct v4l2_ctrl *audio_cluster[2];
+		...
+	};
+
+	state->audio_cluster[AUDIO_CL_VOLUME] =
+		v4l2_ctrl_new_std(&state->ctrl_handler, ...);
+	state->audio_cluster[AUDIO_CL_MUTE] =
+		v4l2_ctrl_new_std(&state->ctrl_handler, ...);
+	v4l2_ctrl_cluster(ARRAY_SIZE(state->audio_cluster), state->audio_cluster);
+
+From now on whenever one or more of the controls belonging to the same
+cluster is set (or 'gotten', or 'tried'), only the control ops of the first
+control ('volume' in this example) is called. You effectively create a new
+composite control. Similar to how a 'struct' works in C.
+
+So when s_ctrl is called with V4L2_CID_AUDIO_VOLUME as argument, you should set
+all two controls belonging to the audio_cluster:
+
+	static int foo_s_ctrl(struct v4l2_ctrl *ctrl)
+	{
+		struct foo *state = container_of(ctrl->handler, struct foo, ctrl_handler);
+
+		switch (ctrl->id) {
+		case V4L2_CID_AUDIO_VOLUME: {
+			struct v4l2_ctrl *mute = ctrl->cluster[AUDIO_CL_MUTE];
+
+			write_reg(0x123, mute->val ? 0 : ctrl->val);
+			break;
+		}
+		case V4L2_CID_CONTRAST:
+			write_reg(0x456, ctrl->val);
+			break;
+		}
+		return 0;
+	}
+
+In the example above the following are equivalent for the VOLUME case:
+
+	ctrl == ctrl->cluster[AUDIO_CL_VOLUME] == state->audio_cluster[AUDIO_CL_VOLUME]
+	ctrl->cluster[AUDIO_CL_MUTE] == state->audio_cluster[AUDIO_CL_MUTE]
+
+In practice using cluster arrays like this becomes very tiresome. So instead
+the following equivalent method is used:
+
+	struct {
+		/* audio cluster */
+		struct v4l2_ctrl *volume;
+		struct v4l2_ctrl *mute;
+	};
+
+The anonymous struct is used to clearly 'cluster' these two control pointers,
+but it serves no other purpose. The effect is the same as creating an
+array with two control pointers. So you can just do:
+
+	state->volume = v4l2_ctrl_new_std(&state->ctrl_handler, ...);
+	state->mute = v4l2_ctrl_new_std(&state->ctrl_handler, ...);
+	v4l2_ctrl_cluster(2, &state->volume);
+
+And in foo_s_ctrl you can use these pointers directly: state->mute->val.
+
+Note that controls in a cluster may be NULL. For example, if for some
+reason mute was never added (because the hardware doesn't support that
+particular feature), then mute will be NULL. So in that case we have a
+cluster of 2 controls, of which only 1 is actually instantiated. The
+only restriction is that the first control of the cluster must always be
+present, since that is the 'master' control of the cluster. The master
+control is the one that identifies the cluster and that provides the
+pointer to the v4l2_ctrl_ops struct that is used for that cluster.
+
+Obviously, all controls in the cluster array must be initialized to either
+a valid control or to NULL.
+
+In rare cases you might want to know which controls of a cluster actually
+were set explicitly by the user. For this you can check the 'is_new' flag of
+each control. For example, in the case of a volume/mute cluster the 'is_new'
+flag of the mute control would be set if the user called VIDIOC_S_CTRL for
+mute only. If the user would call VIDIOC_S_EXT_CTRLS for both mute and volume
+controls, then the 'is_new' flag would be 1 for both controls.
+
+The 'is_new' flag is always 1 when called from v4l2_ctrl_handler_setup().
+
+
+Handling autogain/gain-type Controls with Auto Clusters
+=======================================================
+
+A common type of control cluster is one that handles 'auto-foo/foo'-type
+controls. Typical examples are autogain/gain, autoexposure/exposure,
+autowhitebalance/red balance/blue balance. In all cases you have one control
+that determines whether another control is handled automatically by the hardware,
+or whether it is under manual control from the user.
+
+If the cluster is in automatic mode, then the manual controls should be
+marked inactive and volatile. When the volatile controls are read the
+g_volatile_ctrl operation should return the value that the hardware's automatic
+mode set up automatically.
+
+If the cluster is put in manual mode, then the manual controls should become
+active again and the volatile flag is cleared (so g_volatile_ctrl is no longer
+called while in manual mode). In addition just before switching to manual mode
+the current values as determined by the auto mode are copied as the new manual
+values.
+
+Finally the V4L2_CTRL_FLAG_UPDATE should be set for the auto control since
+changing that control affects the control flags of the manual controls.
+
+In order to simplify this a special variation of v4l2_ctrl_cluster was
+introduced:
+
+void v4l2_ctrl_auto_cluster(unsigned ncontrols, struct v4l2_ctrl **controls,
+			u8 manual_val, bool set_volatile);
+
+The first two arguments are identical to v4l2_ctrl_cluster. The third argument
+tells the framework which value switches the cluster into manual mode. The
+last argument will optionally set V4L2_CTRL_FLAG_VOLATILE for the non-auto controls.
+If it is false, then the manual controls are never volatile. You would typically
+use that if the hardware does not give you the option to read back to values as
+determined by the auto mode (e.g. if autogain is on, the hardware doesn't allow
+you to obtain the current gain value).
+
+The first control of the cluster is assumed to be the 'auto' control.
+
+Using this function will ensure that you don't need to handle all the complex
+flag and volatile handling.
+
+
+VIDIOC_LOG_STATUS Support
+=========================
+
+This ioctl allow you to dump the current status of a driver to the kernel log.
+The v4l2_ctrl_handler_log_status(ctrl_handler, prefix) can be used to dump the
+value of the controls owned by the given handler to the log. You can supply a
+prefix as well. If the prefix didn't end with a space, then ': ' will be added
+for you.
+
+
+Different Handlers for Different Video Nodes
+============================================
+
+Usually the V4L2 driver has just one control handler that is global for
+all video nodes. But you can also specify different control handlers for
+different video nodes. You can do that by manually setting the ctrl_handler
+field of struct video_device.
+
+That is no problem if there are no subdevs involved but if there are, then
+you need to block the automatic merging of subdev controls to the global
+control handler. You do that by simply setting the ctrl_handler field in
+struct v4l2_device to NULL. Now v4l2_device_register_subdev() will no longer
+merge subdev controls.
+
+After each subdev was added, you will then have to call v4l2_ctrl_add_handler
+manually to add the subdev's control handler (sd->ctrl_handler) to the desired
+control handler. This control handler may be specific to the video_device or
+for a subset of video_device's. For example: the radio device nodes only have
+audio controls, while the video and vbi device nodes share the same control
+handler for the audio and video controls.
+
+If you want to have one handler (e.g. for a radio device node) have a subset
+of another handler (e.g. for a video device node), then you should first add
+the controls to the first handler, add the other controls to the second
+handler and finally add the first handler to the second. For example:
+
+	v4l2_ctrl_new_std(&radio_ctrl_handler, &radio_ops, V4L2_CID_AUDIO_VOLUME, ...);
+	v4l2_ctrl_new_std(&radio_ctrl_handler, &radio_ops, V4L2_CID_AUDIO_MUTE, ...);
+	v4l2_ctrl_new_std(&video_ctrl_handler, &video_ops, V4L2_CID_BRIGHTNESS, ...);
+	v4l2_ctrl_new_std(&video_ctrl_handler, &video_ops, V4L2_CID_CONTRAST, ...);
+	v4l2_ctrl_add_handler(&video_ctrl_handler, &radio_ctrl_handler, NULL);
+
+The last argument to v4l2_ctrl_add_handler() is a filter function that allows
+you to filter which controls will be added. Set it to NULL if you want to add
+all controls.
+
+Or you can add specific controls to a handler:
+
+	volume = v4l2_ctrl_new_std(&video_ctrl_handler, &ops, V4L2_CID_AUDIO_VOLUME, ...);
+	v4l2_ctrl_new_std(&video_ctrl_handler, &ops, V4L2_CID_BRIGHTNESS, ...);
+	v4l2_ctrl_new_std(&video_ctrl_handler, &ops, V4L2_CID_CONTRAST, ...);
+
+What you should not do is make two identical controls for two handlers.
+For example:
+
+	v4l2_ctrl_new_std(&radio_ctrl_handler, &radio_ops, V4L2_CID_AUDIO_MUTE, ...);
+	v4l2_ctrl_new_std(&video_ctrl_handler, &video_ops, V4L2_CID_AUDIO_MUTE, ...);
+
+This would be bad since muting the radio would not change the video mute
+control. The rule is to have one control for each hardware 'knob' that you
+can twiddle.
+
+
+Finding Controls
+================
+
+Normally you have created the controls yourself and you can store the struct
+v4l2_ctrl pointer into your own struct.
+
+But sometimes you need to find a control from another handler that you do
+not own. For example, if you have to find a volume control from a subdev.
+
+You can do that by calling v4l2_ctrl_find:
+
+	struct v4l2_ctrl *volume;
+
+	volume = v4l2_ctrl_find(sd->ctrl_handler, V4L2_CID_AUDIO_VOLUME);
+
+Since v4l2_ctrl_find will lock the handler you have to be careful where you
+use it. For example, this is not a good idea:
+
+	struct v4l2_ctrl_handler ctrl_handler;
+
+	v4l2_ctrl_new_std(&ctrl_handler, &video_ops, V4L2_CID_BRIGHTNESS, ...);
+	v4l2_ctrl_new_std(&ctrl_handler, &video_ops, V4L2_CID_CONTRAST, ...);
+
+...and in video_ops.s_ctrl:
+
+	case V4L2_CID_BRIGHTNESS:
+		contrast = v4l2_find_ctrl(&ctrl_handler, V4L2_CID_CONTRAST);
+		...
+
+When s_ctrl is called by the framework the ctrl_handler.lock is already taken, so
+attempting to find another control from the same handler will deadlock.
+
+It is recommended not to use this function from inside the control ops.
+
+
+Inheriting Controls
+===================
+
+When one control handler is added to another using v4l2_ctrl_add_handler, then
+by default all controls from one are merged to the other. But a subdev might
+have low-level controls that make sense for some advanced embedded system, but
+not when it is used in consumer-level hardware. In that case you want to keep
+those low-level controls local to the subdev. You can do this by simply
+setting the 'is_private' flag of the control to 1:
+
+	static const struct v4l2_ctrl_config ctrl_private = {
+		.ops = &ctrl_custom_ops,
+		.id = V4L2_CID_...,
+		.name = "Some Private Control",
+		.type = V4L2_CTRL_TYPE_INTEGER,
+		.max = 15,
+		.step = 1,
+		.is_private = 1,
+	};
+
+	ctrl = v4l2_ctrl_new_custom(&foo->ctrl_handler, &ctrl_private, NULL);
+
+These controls will now be skipped when v4l2_ctrl_add_handler is called.
+
+
+V4L2_CTRL_TYPE_CTRL_CLASS Controls
+==================================
+
+Controls of this type can be used by GUIs to get the name of the control class.
+A fully featured GUI can make a dialog with multiple tabs with each tab
+containing the controls belonging to a particular control class. The name of
+each tab can be found by querying a special control with ID <control class | 1>.
+
+Drivers do not have to care about this. The framework will automatically add
+a control of this type whenever the first control belonging to a new control
+class is added.
+
+
+Adding Notify Callbacks
+=======================
+
+Sometimes the platform or bridge driver needs to be notified when a control
+from a sub-device driver changes. You can set a notify callback by calling
+this function:
+
+void v4l2_ctrl_notify(struct v4l2_ctrl *ctrl,
+	void (*notify)(struct v4l2_ctrl *ctrl, void *priv), void *priv);
+
+Whenever the give control changes value the notify callback will be called
+with a pointer to the control and the priv pointer that was passed with
+v4l2_ctrl_notify. Note that the control's handler lock is held when the
+notify function is called.
+
+There can be only one notify function per control handler. Any attempt
+to set another notify function will cause a WARN_ON.
diff --git a/Documentation/media/kapi/v4l2-framework.rst b/Documentation/media/kapi/v4l2-framework.rst
new file mode 100644
index 000000000000..cbefc7902f5f
--- /dev/null
+++ b/Documentation/media/kapi/v4l2-framework.rst
@@ -0,0 +1,1160 @@
+Overview of the V4L2 driver framework
+=====================================
+
+This text documents the various structures provided by the V4L2 framework and
+their relationships.
+
+
+Introduction
+------------
+
+The V4L2 drivers tend to be very complex due to the complexity of the
+hardware: most devices have multiple ICs, export multiple device nodes in
+/dev, and create also non-V4L2 devices such as DVB, ALSA, FB, I2C and input
+(IR) devices.
+
+Especially the fact that V4L2 drivers have to setup supporting ICs to
+do audio/video muxing/encoding/decoding makes it more complex than most.
+Usually these ICs are connected to the main bridge driver through one or
+more I2C busses, but other busses can also be used. Such devices are
+called 'sub-devices'.
+
+For a long time the framework was limited to the video_device struct for
+creating V4L device nodes and video_buf for handling the video buffers
+(note that this document does not discuss the video_buf framework).
+
+This meant that all drivers had to do the setup of device instances and
+connecting to sub-devices themselves. Some of this is quite complicated
+to do right and many drivers never did do it correctly.
+
+There is also a lot of common code that could never be refactored due to
+the lack of a framework.
+
+So this framework sets up the basic building blocks that all drivers
+need and this same framework should make it much easier to refactor
+common code into utility functions shared by all drivers.
+
+A good example to look at as a reference is the v4l2-pci-skeleton.c
+source that is available in samples/v4l/. It is a skeleton driver for
+a PCI capture card, and demonstrates how to use the V4L2 driver
+framework. It can be used as a template for real PCI video capture driver.
+
+Structure of a driver
+---------------------
+
+All drivers have the following structure:
+
+1) A struct for each device instance containing the device state.
+
+2) A way of initializing and commanding sub-devices (if any).
+
+3) Creating V4L2 device nodes (/dev/videoX, /dev/vbiX and /dev/radioX)
+   and keeping track of device-node specific data.
+
+4) Filehandle-specific structs containing per-filehandle data;
+
+5) video buffer handling.
+
+This is a rough schematic of how it all relates:
+
+    device instances
+      |
+      +-sub-device instances
+      |
+      \-V4L2 device nodes
+	  |
+	  \-filehandle instances
+
+
+Structure of the framework
+--------------------------
+
+The framework closely resembles the driver structure: it has a v4l2_device
+struct for the device instance data, a v4l2_subdev struct to refer to
+sub-device instances, the video_device struct stores V4L2 device node data
+and the v4l2_fh struct keeps track of filehandle instances.
+
+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_device
+------------------
+
+Each device instance is represented by a struct v4l2_device (v4l2-device.h).
+Very simple devices can just allocate this struct, but most of the time you
+would embed this struct inside a larger struct.
+
+You must register the device instance:
+
+	v4l2_device_register(struct device *dev, struct v4l2_device *v4l2_dev);
+
+Registration will initialize the v4l2_device struct. If the dev->driver_data
+field is NULL, it will be linked to v4l2_dev.
+
+Drivers that want integration with the media device framework need to set
+dev->driver_data manually to point to the driver-specific device structure
+that embed the struct v4l2_device instance. This is achieved by a
+dev_set_drvdata() call before registering the V4L2 device instance. They must
+also set the struct v4l2_device mdev field to point to a properly initialized
+and registered media_device instance.
+
+If v4l2_dev->name is empty then it will be set to a value derived from dev
+(driver name followed by the bus_id, to be precise). If you set it up before
+calling v4l2_device_register then it will be untouched. If dev is NULL, then
+you *must* setup v4l2_dev->name before calling v4l2_device_register.
+
+You can use v4l2_device_set_name() to set the name based on a driver name and
+a driver-global atomic_t instance. This will generate names like ivtv0, ivtv1,
+etc. If the name ends with a digit, then it will insert a dash: cx18-0,
+cx18-1, etc. This function returns the instance number.
+
+The first 'dev' argument is normally the struct device pointer of a pci_dev,
+usb_interface or platform_device. It is rare for dev to be NULL, but it happens
+with ISA devices or when one device creates multiple PCI devices, thus making
+it impossible to associate v4l2_dev with a particular parent.
+
+You can also supply a notify() callback that can be called by sub-devices to
+notify you of events. Whether you need to set this depends on the sub-device.
+Any notifications a sub-device supports must be defined in a header in
+include/media/<subdevice>.h.
+
+You unregister with:
+
+	v4l2_device_unregister(struct v4l2_device *v4l2_dev);
+
+If the dev->driver_data field points to v4l2_dev, it will be reset to NULL.
+Unregistering will also automatically unregister all subdevs from the device.
+
+If you have a hotpluggable device (e.g. a USB device), then when a disconnect
+happens the parent device becomes invalid. Since v4l2_device has a pointer to
+that parent device it has to be cleared as well to mark that the parent is
+gone. To do this call:
+
+	v4l2_device_disconnect(struct v4l2_device *v4l2_dev);
+
+This does *not* unregister the subdevs, so you still need to call the
+v4l2_device_unregister() function for that. If your driver is not hotpluggable,
+then there is no need to call v4l2_device_disconnect().
+
+Sometimes you need to iterate over all devices registered by a specific
+driver. This is usually the case if multiple device drivers use the same
+hardware. E.g. the ivtvfb driver is a framebuffer driver that uses the ivtv
+hardware. The same is true for alsa drivers for example.
+
+You can iterate over all registered devices as follows:
+
+static int callback(struct device *dev, void *p)
+{
+	struct v4l2_device *v4l2_dev = dev_get_drvdata(dev);
+
+	/* test if this device was inited */
+	if (v4l2_dev == NULL)
+		return 0;
+	...
+	return 0;
+}
+
+int iterate(void *p)
+{
+	struct device_driver *drv;
+	int err;
+
+	/* Find driver 'ivtv' on the PCI bus.
+	   pci_bus_type is a global. For USB busses use usb_bus_type. */
+	drv = driver_find("ivtv", &pci_bus_type);
+	/* iterate over all ivtv device instances */
+	err = driver_for_each_device(drv, NULL, p, callback);
+	put_driver(drv);
+	return err;
+}
+
+Sometimes you need to keep a running counter of the device instance. This is
+commonly used to map a device instance to an index of a module option array.
+
+The recommended approach is as follows:
+
+static atomic_t drv_instance = ATOMIC_INIT(0);
+
+static int drv_probe(struct pci_dev *pdev, const struct pci_device_id *pci_id)
+{
+	...
+	state->instance = atomic_inc_return(&drv_instance) - 1;
+}
+
+If you have multiple device nodes then it can be difficult to know when it is
+safe to unregister v4l2_device for hotpluggable devices. For this purpose
+v4l2_device has refcounting support. The refcount is increased whenever
+video_register_device is called and it is decreased whenever that device node
+is released. When the refcount reaches zero, then the v4l2_device release()
+callback is called. You can do your final cleanup there.
+
+If other device nodes (e.g. ALSA) are created, then you can increase and
+decrease the refcount manually as well by calling:
+
+void v4l2_device_get(struct v4l2_device *v4l2_dev);
+
+or:
+
+int v4l2_device_put(struct v4l2_device *v4l2_dev);
+
+Since the initial refcount is 1 you also need to call v4l2_device_put in the
+disconnect() callback (for USB devices) or in the remove() callback (for e.g.
+PCI devices), otherwise the refcount will never reach 0.
+
+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:
+
+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:
+
+	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:
+
+	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:
+
+	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:
+
+	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:
+
+	v4l2_device_unregister_subdev(sd);
+
+Afterwards the subdev module can be unloaded and sd->dev == NULL.
+
+You can call an ops function either directly:
+
+	err = sd->ops->core->g_std(sd, &norm);
+
+but it is better and easier to use this macro:
+
+	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:
+
+	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:
+
+	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
+-----------------------------
+
+Beside exposing a kernel API through the v4l2_subdev_ops structure, V4L2
+sub-devices can also be controlled directly by userspace applications.
+
+Device nodes named v4l-subdevX 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 v4l2_device driver can create device nodes
+for all registered sub-devices marked with V4L2_SUBDEV_FL_HAS_DEVNODE by calling
+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
+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
+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 v4l2_subdev::flags and initialize
+	v4l2_subdev::nevents to events queue depth before registering the
+	sub-device. After registration events can be queued as usual on the
+	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 v4l2_subdev support to an I2C driver is to
+embed the 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 v4l2_subdev directly.
+
+A typical state struct would look like this (where 'chipname' is replaced by
+the name of the chip):
+
+struct chipname_state {
+	struct v4l2_subdev sd;
+	...  /* additional state fields */
+};
+
+Initialize the v4l2_subdev struct as follows:
+
+	v4l2_i2c_subdev_init(&state->sd, client, subdev_ops);
+
+This function will fill in all the fields of v4l2_subdev and ensure that the
+v4l2_subdev and i2c_client both point to one another.
+
+You should also add a helper inline function to go from a v4l2_subdev pointer
+to a chipname_state struct:
+
+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 v4l2_subdev struct to the i2c_client struct:
+
+	struct i2c_client *client = v4l2_get_subdevdata(sd);
+
+And this to go from an i2c_client to a v4l2_subdev struct:
+
+	struct v4l2_subdev *sd = i2c_get_clientdata(client);
+
+Make sure to call v4l2_device_unregister_subdev(sd) 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 v4l2_device_unregister_subdev(sd)
+from the remove() callback ensures that this is always done correctly.
+
+
+The bridge driver also has some helper functions it can use:
+
+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 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 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 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:
+
+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:
+
+struct v4l2_subdev *sd = v4l2_i2c_new_subdev_cfg(v4l2_dev, adapter,
+	       "module_foo", "chipid", 0, NULL, 0, I2C_ADDRS(0x10));
+
+v4l2_i2c_new_subdev_board uses an 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
+v4l2_i2c_new_(probed_)subdev functions will call s_config as well, but with
+irq set to 0 and platform_data set to NULL.
+
+struct video_device
+-------------------
+
+The actual device nodes in the /dev directory are created using the
+video_device struct (v4l2-dev.h). This struct can either be allocated
+dynamically or embedded in a larger struct.
+
+To allocate it dynamically use:
+
+	struct video_device *vdev = video_device_alloc();
+
+	if (vdev == NULL)
+		return -ENOMEM;
+
+	vdev->release = video_device_release;
+
+If you embed it in a larger struct, then you must set the release()
+callback to your own function:
+
+	struct video_device *vdev = &my_vdev->vdev;
+
+	vdev->release = my_vdev_release;
+
+The release callback must be set and it is called when the last user
+of the video device exits.
+
+The default video_device_release() callback just calls kfree to free the
+allocated memory.
+
+There is also a video_device_release_empty() function that does nothing
+(is empty) and can be used if the struct is embedded and there is nothing
+to do when it is released.
+
+You should also set these fields:
+
+- v4l2_dev: must be set to the v4l2_device parent device.
+
+- name: set to something descriptive and unique.
+
+- vfl_dir: set this to VFL_DIR_RX for capture devices (VFL_DIR_RX has value 0,
+  so this is normally already the default), set to VFL_DIR_TX for output
+  devices and VFL_DIR_M2M for mem2mem (codec) devices.
+
+- fops: set to the v4l2_file_operations struct.
+
+- ioctl_ops: if you use the v4l2_ioctl_ops to simplify ioctl maintenance
+  (highly recommended to use this and it might become compulsory in the
+  future!), then set this to your v4l2_ioctl_ops struct. The vfl_type and
+  vfl_dir fields are used to disable ops that do not match the type/dir
+  combination. E.g. VBI ops are disabled for non-VBI nodes, and output ops
+  are disabled for a capture device. This makes it possible to provide
+  just one v4l2_ioctl_ops struct for both vbi and video nodes.
+
+- lock: leave to NULL if you want to do all the locking in the driver.
+  Otherwise you give it a pointer to a struct mutex_lock and before the
+  unlocked_ioctl file operation is called this lock will be taken by the
+  core and released afterwards. See the next section for more details.
+
+- queue: a pointer to the struct vb2_queue associated with this device node.
+  If queue is non-NULL, and queue->lock is non-NULL, then queue->lock is
+  used for the queuing ioctls (VIDIOC_REQBUFS, CREATE_BUFS, QBUF, DQBUF,
+  QUERYBUF, PREPARE_BUF, STREAMON and STREAMOFF) instead of the lock above.
+  That way the vb2 queuing framework does not have to wait for other ioctls.
+  This queue pointer is also used by the vb2 helper functions to check for
+  queuing ownership (i.e. is the filehandle calling it allowed to do the
+  operation).
+
+- prio: keeps track of the priorities. Used to implement VIDIOC_G/S_PRIORITY.
+  If left to NULL, then it will use the struct v4l2_prio_state in v4l2_device.
+  If you want to have a separate priority state per (group of) device node(s),
+  then you can point it to your own struct v4l2_prio_state.
+
+- dev_parent: you only set this if v4l2_device was registered with NULL as
+  the parent device struct. This only happens in cases where one hardware
+  device has multiple PCI devices that all share the same v4l2_device core.
+
+  The cx88 driver is an example of this: one core v4l2_device struct, but
+  it is used by both a raw video PCI device (cx8800) and a MPEG PCI device
+  (cx8802). Since the v4l2_device cannot be associated with two PCI devices
+  at the same time it is setup without a parent device. But when the struct
+  video_device is initialized you *do* know which parent PCI device to use and
+  so you set dev_device to the correct PCI device.
+
+If you use v4l2_ioctl_ops, then you should set .unlocked_ioctl to video_ioctl2
+in your v4l2_file_operations struct.
+
+Do not use .ioctl! This is deprecated and will go away in the future.
+
+In some cases you want to tell the core that a function you had specified in
+your v4l2_ioctl_ops should be ignored. You can mark such ioctls by calling this
+function before video_device_register is called:
+
+void v4l2_disable_ioctl(struct video_device *vdev, unsigned int cmd);
+
+This tends to be needed if based on external factors (e.g. which card is
+being used) you want to turns off certain features in v4l2_ioctl_ops without
+having to make a new struct.
+
+The v4l2_file_operations struct is a subset of file_operations. The main
+difference is that the inode argument is omitted since it is never used.
+
+If integration with the media framework is needed, you must initialize the
+media_entity struct embedded in the video_device struct (entity field) by
+calling media_entity_pads_init():
+
+	struct media_pad *pad = &my_vdev->pad;
+	int err;
+
+	err = media_entity_pads_init(&vdev->entity, 1, pad);
+
+The pads array must have been previously initialized. There is no need to
+manually set the struct media_entity type and name fields.
+
+A reference to the entity will be automatically acquired/released when the
+video device is opened/closed.
+
+ioctls and locking
+------------------
+
+The V4L core provides optional locking services. The main service is the
+lock field in struct video_device, which is a pointer to a mutex. If you set
+this pointer, then that will be used by unlocked_ioctl to serialize all ioctls.
+
+If you are using the videobuf2 framework, then there is a second lock that you
+can set: video_device->queue->lock. If set, then this lock will be used instead
+of video_device->lock to serialize all queuing ioctls (see the previous section
+for the full list of those ioctls).
+
+The advantage of using a different lock for the queuing ioctls is that for some
+drivers (particularly USB drivers) certain commands such as setting controls
+can take a long time, so you want to use a separate lock for the buffer queuing
+ioctls. That way your VIDIOC_DQBUF doesn't stall because the driver is busy
+changing the e.g. exposure of the webcam.
+
+Of course, you can always do all the locking yourself by leaving both lock
+pointers at NULL.
+
+If you use the old videobuf then you must pass the video_device lock to the
+videobuf queue initialize function: if videobuf has to wait for a frame to
+arrive, then it will temporarily unlock the lock and relock it afterwards. If
+your driver also waits in the code, then you should do the same to allow other
+processes to access the device node while the first process is waiting for
+something.
+
+In the case of videobuf2 you will need to implement the wait_prepare and
+wait_finish callbacks to unlock/lock if applicable. If you use the queue->lock
+pointer, then you can use the helper functions vb2_ops_wait_prepare/finish.
+
+The implementation of a hotplug disconnect should also take the lock from
+video_device before calling v4l2_device_disconnect. If you are also using
+video_device->queue->lock, then you have to first lock video_device->queue->lock
+followed by video_device->lock. That way you can be sure no ioctl is running
+when you call v4l2_device_disconnect.
+
+video_device registration
+-------------------------
+
+Next you register the video device: this will create the character device
+for you.
+
+	err = video_register_device(vdev, VFL_TYPE_GRABBER, -1);
+	if (err) {
+		video_device_release(vdev); /* or kfree(my_vdev); */
+		return err;
+	}
+
+If the v4l2_device parent device has a non-NULL mdev field, the video device
+entity will be automatically registered with the media device.
+
+Which device is registered depends on the type argument. The following
+types exist:
+
+VFL_TYPE_GRABBER: videoX for video input/output devices
+VFL_TYPE_VBI: vbiX for vertical blank data (i.e. closed captions, teletext)
+VFL_TYPE_RADIO: radioX for radio tuners
+VFL_TYPE_SDR: swradioX for Software Defined Radio tuners
+
+The last argument gives you a certain amount of control over the device
+device node number used (i.e. the X in videoX). Normally you will pass -1
+to let the v4l2 framework pick the first free number. But sometimes users
+want to select a specific node number. It is common that drivers allow
+the user to select a specific device node number through a driver module
+option. That number is then passed to this function and video_register_device
+will attempt to select that device node number. If that number was already
+in use, then the next free device node number will be selected and it
+will send a warning to the kernel log.
+
+Another use-case is if a driver creates many devices. In that case it can
+be useful to place different video devices in separate ranges. For example,
+video capture devices start at 0, video output devices start at 16.
+So you can use the last argument to specify a minimum device node number
+and the v4l2 framework will try to pick the first free number that is equal
+or higher to what you passed. If that fails, then it will just pick the
+first free number.
+
+Since in this case you do not care about a warning about not being able
+to select the specified device node number, you can call the function
+video_register_device_no_warn() instead.
+
+Whenever a device node is created some attributes are also created for you.
+If you look in /sys/class/video4linux you see the devices. Go into e.g.
+video0 and you will see 'name', 'dev_debug' and 'index' attributes. The 'name'
+attribute is the 'name' field of the video_device struct. The 'dev_debug' attribute
+can be used to enable core debugging. See the next section for more detailed
+information on this.
+
+The 'index' attribute is the index of the device node: for each call to
+video_register_device() the index is just increased by 1. The first video
+device node you register always starts with index 0.
+
+Users can setup udev rules that utilize the index attribute to make fancy
+device names (e.g. 'mpegX' for MPEG video capture device nodes).
+
+After the device was successfully registered, then you can use these fields:
+
+- vfl_type: the device type passed to video_register_device.
+- minor: the assigned device minor number.
+- num: the device node number (i.e. the X in videoX).
+- index: the device index number.
+
+If the registration failed, then you need to call video_device_release()
+to free the allocated video_device struct, or free your own struct if the
+video_device was embedded in it. The vdev->release() callback will never
+be called if the registration failed, nor should you ever attempt to
+unregister the device if the registration failed.
+
+video device debugging
+----------------------
+
+The 'dev_debug' attribute that is created for each video, vbi, radio or swradio
+device in /sys/class/video4linux/<devX>/ allows you to enable logging of
+file operations.
+
+It is a bitmask and the following bits can be set:
+
+0x01: Log the ioctl name and error code. VIDIOC_(D)QBUF ioctls are only logged
+      if bit 0x08 is also set.
+0x02: Log the ioctl name arguments and error code. VIDIOC_(D)QBUF ioctls are
+      only logged if bit 0x08 is also set.
+0x04: Log the file operations open, release, read, write, mmap and
+      get_unmapped_area. The read and write operations are only logged if
+      bit 0x08 is also set.
+0x08: Log the read and write file operations and the VIDIOC_QBUF and
+      VIDIOC_DQBUF ioctls.
+0x10: Log the poll file operation.
+
+video_device cleanup
+--------------------
+
+When the video device nodes have to be removed, either during the unload
+of the driver or because the USB device was disconnected, then you should
+unregister them:
+
+	video_unregister_device(vdev);
+
+This will remove the device nodes from sysfs (causing udev to remove them
+from /dev).
+
+After video_unregister_device() returns no new opens can be done. However,
+in the case of USB devices some application might still have one of these
+device nodes open. So after the unregister all file operations (except
+release, of course) will return an error as well.
+
+When the last user of the video device node exits, then the vdev->release()
+callback is called and you can do the final cleanup there.
+
+Don't forget to cleanup the media entity associated with the video device if
+it has been initialized:
+
+	media_entity_cleanup(&vdev->entity);
+
+This can be done from the release callback.
+
+
+video_device helper functions
+-----------------------------
+
+There are a few useful helper functions:
+
+- file/video_device private data
+
+You can set/get driver private data in the video_device struct using:
+
+void *video_get_drvdata(struct video_device *vdev);
+void video_set_drvdata(struct video_device *vdev, void *data);
+
+Note that you can safely call video_set_drvdata() before calling
+video_register_device().
+
+And this function:
+
+struct video_device *video_devdata(struct file *file);
+
+returns the video_device belonging to the file struct.
+
+The video_drvdata function combines video_get_drvdata with video_devdata:
+
+void *video_drvdata(struct file *file);
+
+You can go from a video_device struct to the v4l2_device struct using:
+
+struct v4l2_device *v4l2_dev = vdev->v4l2_dev;
+
+- Device node name
+
+The video_device node kernel name can be retrieved using
+
+const char *video_device_node_name(struct video_device *vdev);
+
+The name is used as a hint by userspace tools such as udev. The function
+should be used where possible instead of accessing the video_device::num and
+video_device::minor fields.
+
+
+video buffer helper functions
+-----------------------------
+
+The v4l2 core API provides a set of standard methods (called "videobuf")
+for dealing with video buffers. Those methods allow a driver to implement
+read(), mmap() and overlay() in a consistent way.  There are currently
+methods for using video buffers on devices that supports DMA with
+scatter/gather method (videobuf-dma-sg), DMA with linear access
+(videobuf-dma-contig), and vmalloced buffers, mostly used on USB drivers
+(videobuf-vmalloc).
+
+Please see Documentation/video4linux/videobuf for more information on how
+to use the videobuf layer.
+
+struct v4l2_fh
+--------------
+
+struct v4l2_fh provides a way to easily keep file handle specific data
+that is used by the V4L2 framework. New drivers must use struct v4l2_fh
+since it is also used to implement priority handling (VIDIOC_G/S_PRIORITY).
+
+The users of v4l2_fh (in the V4L2 framework, not the driver) know
+whether a driver uses v4l2_fh as its file->private_data pointer by
+testing the V4L2_FL_USES_V4L2_FH bit in video_device->flags. This bit is
+set whenever v4l2_fh_init() is called.
+
+struct v4l2_fh is allocated as a part of the driver's own file handle
+structure and file->private_data is set to it in the driver's open
+function by the driver.
+
+In many cases the struct v4l2_fh will be embedded in a larger structure.
+In that case you should call v4l2_fh_init+v4l2_fh_add in open() and
+v4l2_fh_del+v4l2_fh_exit in release().
+
+Drivers can extract their own file handle structure by using the container_of
+macro. Example:
+
+struct my_fh {
+	int blah;
+	struct v4l2_fh fh;
+};
+
+...
+
+int my_open(struct file *file)
+{
+	struct my_fh *my_fh;
+	struct video_device *vfd;
+	int ret;
+
+	...
+
+	my_fh = kzalloc(sizeof(*my_fh), GFP_KERNEL);
+
+	...
+
+	v4l2_fh_init(&my_fh->fh, vfd);
+
+	...
+
+	file->private_data = &my_fh->fh;
+	v4l2_fh_add(&my_fh->fh);
+	return 0;
+}
+
+int my_release(struct file *file)
+{
+	struct v4l2_fh *fh = file->private_data;
+	struct my_fh *my_fh = container_of(fh, struct my_fh, fh);
+
+	...
+	v4l2_fh_del(&my_fh->fh);
+	v4l2_fh_exit(&my_fh->fh);
+	kfree(my_fh);
+	return 0;
+}
+
+Below is a short description of the v4l2_fh functions used:
+
+void v4l2_fh_init(struct v4l2_fh *fh, struct video_device *vdev)
+
+  Initialise the file handle. This *MUST* be performed in the driver's
+  v4l2_file_operations->open() handler.
+
+void v4l2_fh_add(struct v4l2_fh *fh)
+
+  Add a v4l2_fh to video_device file handle list. Must be called once the
+  file handle is completely initialized.
+
+void v4l2_fh_del(struct v4l2_fh *fh)
+
+  Unassociate the file handle from video_device(). The file handle
+  exit function may now be called.
+
+void v4l2_fh_exit(struct v4l2_fh *fh)
+
+  Uninitialise the file handle. After uninitialisation the v4l2_fh
+  memory can be freed.
+
+
+If struct v4l2_fh is not embedded, then you can use these helper functions:
+
+int v4l2_fh_open(struct file *filp)
+
+  This allocates a struct v4l2_fh, initializes it and adds it to the struct
+  video_device associated with the file struct.
+
+int v4l2_fh_release(struct file *filp)
+
+  This deletes it from the struct video_device associated with the file
+  struct, uninitialised the v4l2_fh and frees it.
+
+These two functions can be plugged into the v4l2_file_operation's open() and
+release() ops.
+
+
+Several drivers need to do something when the first file handle is opened and
+when the last file handle closes. Two helper functions were added to check
+whether the v4l2_fh struct is the only open filehandle of the associated
+device node:
+
+int v4l2_fh_is_singular(struct v4l2_fh *fh)
+
+  Returns 1 if the file handle is the only open file handle, else 0.
+
+int v4l2_fh_is_singular_file(struct file *filp)
+
+  Same, but it calls v4l2_fh_is_singular with filp->private_data.
+
+
+V4L2 events
+-----------
+
+The V4L2 events provide a generic way to pass events to user space.
+The driver must use v4l2_fh to be able to support V4L2 events.
+
+Events are defined by a type and an optional ID. The ID may refer to a V4L2
+object such as a control ID. If unused, then the ID is 0.
+
+When the user subscribes to an event the driver will allocate a number of
+kevent structs for that event. So every (type, ID) event tuple will have
+its own set of kevent structs. This guarantees that if a driver is generating
+lots of events of one type in a short time, then that will not overwrite
+events of another type.
+
+But if you get more events of one type than the number of kevents that were
+reserved, then the oldest event will be dropped and the new one added.
+
+Furthermore, the internal struct v4l2_subscribed_event has merge() and
+replace() callbacks which drivers can set. These callbacks are called when
+a new event is raised and there is no more room. The replace() callback
+allows you to replace the payload of the old event with that of the new event,
+merging any relevant data from the old payload into the new payload that
+replaces it. It is called when this event type has only one kevent struct
+allocated. The merge() callback allows you to merge the oldest event payload
+into that of the second-oldest event payload. It is called when there are two
+or more kevent structs allocated.
+
+This way no status information is lost, just the intermediate steps leading
+up to that state.
+
+A good example of these replace/merge callbacks is in v4l2-event.c:
+ctrls_replace() and ctrls_merge() callbacks for the control event.
+
+Note: these callbacks can be called from interrupt context, so they must be
+fast.
+
+Useful functions:
+
+void v4l2_event_queue(struct video_device *vdev, const struct v4l2_event *ev)
+
+  Queue events to video device. The driver's only responsibility is to fill
+  in the type and the data fields. The other fields will be filled in by
+  V4L2.
+
+int v4l2_event_subscribe(struct v4l2_fh *fh,
+			 struct v4l2_event_subscription *sub, unsigned elems,
+			 const struct v4l2_subscribed_event_ops *ops)
+
+  The video_device->ioctl_ops->vidioc_subscribe_event must check the driver
+  is able to produce events with specified event id. Then it calls
+  v4l2_event_subscribe() to subscribe the event.
+
+  The elems argument is the size of the event queue for this event. If it is 0,
+  then the framework will fill in a default value (this depends on the event
+  type).
+
+  The ops argument allows the driver to specify a number of callbacks:
+  * add:     called when a new listener gets added (subscribing to the same
+             event twice will only cause this callback to get called once)
+  * del:     called when a listener stops listening
+  * replace: replace event 'old' with event 'new'.
+  * merge:   merge event 'old' into event 'new'.
+  All 4 callbacks are optional, if you don't want to specify any callbacks
+  the ops argument itself maybe NULL.
+
+int v4l2_event_unsubscribe(struct v4l2_fh *fh,
+			   struct v4l2_event_subscription *sub)
+
+  vidioc_unsubscribe_event in struct v4l2_ioctl_ops. A driver may use
+  v4l2_event_unsubscribe() directly unless it wants to be involved in
+  unsubscription process.
+
+  The special type V4L2_EVENT_ALL may be used to unsubscribe all events. The
+  drivers may want to handle this in a special way.
+
+int v4l2_event_pending(struct v4l2_fh *fh)
+
+  Returns the number of pending events. Useful when implementing poll.
+
+Events are delivered to user space through the poll system call. The driver
+can use v4l2_fh->wait (a wait_queue_head_t) as the argument for poll_wait().
+
+There are standard and private events. New standard events must use the
+smallest available event type. The drivers must allocate their events from
+their own class starting from class base. Class base is
+V4L2_EVENT_PRIVATE_START + n * 1000 where n is the lowest available number.
+The first event type in the class is reserved for future use, so the first
+available event type is 'class base + 1'.
+
+An example on how the V4L2 events may be used can be found in the OMAP
+3 ISP driver (drivers/media/platform/omap3isp).
+
+A subdev can directly send an event to the v4l2_device notify function with
+V4L2_DEVICE_NOTIFY_EVENT. This allows the bridge to map the subdev that sends
+the event to the video node(s) associated with the subdev that need to be
+informed about such an event.
+
+V4L2 clocks
+-----------
+
+Many subdevices, like camera sensors, TV decoders and encoders, need a clock
+signal to be supplied by the system. Often this clock is supplied by the
+respective bridge device. The Linux kernel provides a Common Clock Framework for
+this purpose. However, it is not (yet) available on all architectures. Besides,
+the nature of the multi-functional (clock, data + synchronisation, I2C control)
+connection of subdevices to the system might impose special requirements on the
+clock API usage. E.g. V4L2 has to support clock provider driver unregistration
+while a subdevice driver is holding a reference to the clock. For these reasons
+a V4L2 clock helper API has been developed and is provided to bridge and
+subdevice drivers.
+
+The API consists of two parts: two functions to register and unregister a V4L2
+clock source: v4l2_clk_register() and v4l2_clk_unregister() and calls to control
+a clock object, similar to the respective generic clock API calls:
+v4l2_clk_get(), v4l2_clk_put(), v4l2_clk_enable(), v4l2_clk_disable(),
+v4l2_clk_get_rate(), and v4l2_clk_set_rate(). Clock suppliers have to provide
+clock operations that will be called when clock users invoke respective API
+methods.
+
+It is expected that once the CCF becomes available on all relevant
+architectures this API will be removed.