Adding upstream version 6.1.76.upstream/6.1.76

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 823 insertions, 0 deletions

diff --git a/Documentation/driver-api/media/v4l2-controls.rst b/Documentation/driver-api/media/v4l2-controls.rst
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+.. SPDX-License-Identifier: GPL-2.0
+
+V4L2 Controls
+=============
+
+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
+:c:type:`v4l2_device` for V4L2 drivers and struct v4l2_subdev for
+sub-device drivers.
+
+
+Objects in the framework
+------------------------
+
+There are two main objects:
+
+The :c:type:`v4l2_ctrl` object describes the control properties and keeps
+track of the control's value (both the current value and the proposed new
+value).
+
+:c:type:`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:
+
+.. code-block:: c
+
+	#include <media/v4l2-ctrls.h>
+
+1.1) Add the handler to your driver's top-level struct:
+
+For V4L2 drivers:
+
+.. code-block:: c
+
+	struct foo_dev {
+		...
+		struct v4l2_device v4l2_dev;
+		...
+		struct v4l2_ctrl_handler ctrl_handler;
+		...
+	};
+
+For sub-device drivers:
+
+.. code-block:: c
+
+	struct foo_dev {
+		...
+		struct v4l2_subdev sd;
+		...
+		struct v4l2_ctrl_handler ctrl_handler;
+		...
+	};
+
+1.2) Initialize the handler:
+
+.. code-block:: c
+
+	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:
+
+For V4L2 drivers:
+
+.. code-block:: c
+
+	foo->v4l2_dev.ctrl_handler = &foo->ctrl_handler;
+
+For sub-device drivers:
+
+.. code-block:: c
+
+	foo->sd.ctrl_handler = &foo->ctrl_handler;
+
+1.4) Clean up the handler at the end:
+
+.. code-block:: c
+
+	v4l2_ctrl_handler_free(&foo->ctrl_handler);
+
+
+2) Add controls:
+
+You add non-menu controls by calling :c:func:`v4l2_ctrl_new_std`:
+
+.. code-block:: c
+
+	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
+:c:func:`v4l2_ctrl_new_std_menu`:
+
+.. code-block:: c
+
+	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
+:c:func:`v4l2_ctrl_new_std_menu_items`:
+
+.. code-block:: c
+
+       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);
+
+Standard compound controls can be added by calling
+:c:func:`v4l2_ctrl_new_std_compound`:
+
+.. code-block:: c
+
+       struct v4l2_ctrl *v4l2_ctrl_new_std_compound(struct v4l2_ctrl_handler *hdl,
+                       const struct v4l2_ctrl_ops *ops, u32 id,
+                       const union v4l2_ctrl_ptr p_def);
+
+Integer menu controls with a driver specific menu can be added by calling
+:c:func:`v4l2_ctrl_new_int_menu`:
+
+.. code-block:: c
+
+	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
+:c:func:`v4l2_ctrl_handler_init`:
+
+.. code-block:: c
+
+	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 :c:func:`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 :c:func:`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 :c:func:`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 :c:func:`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 :c:func:`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:
+
+.. code-block:: c
+
+	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 :c:type:`v4l2_ctrl_ops`
+
+.. code-block:: c
+
+	static const struct v4l2_ctrl_ops foo_ctrl_ops = {
+		.s_ctrl = foo_s_ctrl,
+	};
+
+Usually all you need is s_ctrl:
+
+.. code-block:: c
+
+	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.
+
+
+.. note::
+
+   The remainder sections deal with more advanced controls topics and scenarios.
+   In practice the basic usage as described above is sufficient for most drivers.
+
+
+Inheriting Sub-device 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:
+
+.. code-block:: c
+
+	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:
+
+.. code-block:: c
+
+	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, then:
+
+.. code-block:: c
+
+	&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
+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:
+
+.. code-block:: c
+
+	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:
+
+.. code-block:: c
+
+	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:
+
+.. code-block:: c
+
+	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:
+
+.. code-block:: c
+
+	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:
+
+.. code-block:: c
+
+	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():
+
+.. code-block:: c
+
+	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:
+
+.. code-block:: c
+
+	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:
+
+.. code-block:: c
+
+	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:
+
+.. code-block:: c
+
+	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:
+
+.. code-block:: c
+
+	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:
+
+.. code-block:: c
+
+	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:
+
+.. code-block:: c
+
+	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:
+
+.. code-block:: c
+
+	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:
+
+.. code-block:: c
+
+	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:
+
+.. code-block:: c
+
+	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:
+
+.. code-block:: c
+
+	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:
+
+.. code-block:: c
+
+	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:
+
+.. code-block:: c
+
+	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.
+
+
+Preventing Controls inheritance
+-------------------------------
+
+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:
+
+.. code-block:: c
+
+	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:
+
+.. code-block:: c
+
+	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.
+
+v4l2_ctrl functions and data structures
+---------------------------------------
+
+.. kernel-doc:: include/media/v4l2-ctrls.h