1.12. V4L2 Controls

1.12.1. 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:

  1. How do I get the control’s value? (i.e. g_volatile_ctrl)
  2. 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.

1.12.2. 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.

1.12.3. 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.

1.4) Clean up the handler at the end:

v4l2_ctrl_handler_free(&foo->ctrl_handler);
  1. 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.

  1. 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.

  1. 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.

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.

1.12.4. 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.

1.12.5. 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);

1.12.7. 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.

1.12.8. 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.

1.12.9. 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().

1.12.10. 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.

1.12.11. 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.

1.12.12. 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.

1.12.13. 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.

1.12.14. 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.

1.12.15. 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.

1.12.16. 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.

1.12.17. v4l2_ctrl functions and data structures

union v4l2_ctrl_ptr

A pointer to a control value.

Definition

union v4l2_ctrl_ptr {
  s32 *p_s32;
  s64 *p_s64;
  u8 *p_u8;
  u16 *p_u16;
  u32 *p_u32;
  char *p_char;
  struct v4l2_ctrl_mpeg2_slice_params *p_mpeg2_slice_params;
  struct v4l2_ctrl_mpeg2_quantization *p_mpeg2_quantization;
  void *p;
};

Members

p_s32
Pointer to a 32-bit signed value.
p_s64
Pointer to a 64-bit signed value.
p_u8
Pointer to a 8-bit unsigned value.
p_u16
Pointer to a 16-bit unsigned value.
p_u32
Pointer to a 32-bit unsigned value.
p_char
Pointer to a string.
p_mpeg2_slice_params
Pointer to a MPEG2 slice parameters structure.
p_mpeg2_quantization
Pointer to a MPEG2 quantization data structure.
p
Pointer to a compound value.
struct v4l2_ctrl_ops

The control operations that the driver has to provide.

Definition

struct v4l2_ctrl_ops {
  int (*g_volatile_ctrl)(struct v4l2_ctrl *ctrl);
  int (*try_ctrl)(struct v4l2_ctrl *ctrl);
  int (*s_ctrl)(struct v4l2_ctrl *ctrl);
};

Members

g_volatile_ctrl
Get a new value for this control. Generally only relevant for volatile (and usually read-only) controls such as a control that returns the current signal strength which changes continuously. If not set, then the currently cached value will be returned.
try_ctrl
Test whether the control’s value is valid. Only relevant when the usual min/max/step checks are not sufficient.
s_ctrl
Actually set the new control value. s_ctrl is compulsory. The ctrl->handler->lock is held when these ops are called, so no one else can access controls owned by that handler.
struct v4l2_ctrl_type_ops

The control type operations that the driver has to provide.

Definition

struct v4l2_ctrl_type_ops {
  bool (*equal)(const struct v4l2_ctrl *ctrl, u32 idx,union v4l2_ctrl_ptr ptr1, union v4l2_ctrl_ptr ptr2);
  void (*init)(const struct v4l2_ctrl *ctrl, u32 idx, union v4l2_ctrl_ptr ptr);
  void (*log)(const struct v4l2_ctrl *ctrl);
  int (*validate)(const struct v4l2_ctrl *ctrl, u32 idx, union v4l2_ctrl_ptr ptr);
};

Members

equal
return true if both values are equal.
init
initialize the value.
log
log the value.
validate
validate the value. Return 0 on success and a negative value otherwise.
v4l2_ctrl_notify_fnc

Typedef: typedef for a notify argument with a function that should be called when a control value has changed.

Syntax

void v4l2_ctrl_notify_fnc (struct v4l2_ctrl * ctrl, void * priv);

Parameters

struct v4l2_ctrl * ctrl
pointer to struct v4l2_ctrl
void * priv
control private data

Description

This typedef definition is used as an argument to v4l2_ctrl_notify() and as an argument at struct v4l2_ctrl_handler.

struct v4l2_ctrl

The control structure.

Definition

struct v4l2_ctrl {
  struct list_head node;
  struct list_head ev_subs;
  struct v4l2_ctrl_handler *handler;
  struct v4l2_ctrl **cluster;
  unsigned int ncontrols;
  unsigned int done:1;
  unsigned int is_new:1;
  unsigned int has_changed:1;
  unsigned int is_private:1;
  unsigned int is_auto:1;
  unsigned int is_int:1;
  unsigned int is_string:1;
  unsigned int is_ptr:1;
  unsigned int is_array:1;
  unsigned int has_volatiles:1;
  unsigned int call_notify:1;
  unsigned int manual_mode_value:8;
  const struct v4l2_ctrl_ops *ops;
  const struct v4l2_ctrl_type_ops *type_ops;
  u32 id;
  const char *name;
  enum v4l2_ctrl_type type;
  s64 minimum, maximum, default_value;
  u32 elems;
  u32 elem_size;
  u32 dims[V4L2_CTRL_MAX_DIMS];
  u32 nr_of_dims;
  union {
    u64 step;
    u64 menu_skip_mask;
  };
  union {
    const char * const *qmenu;
    const s64 *qmenu_int;
  };
  unsigned long flags;
  void *priv;
  s32 val;
  struct {
    s32 val;
  } cur;
  union v4l2_ctrl_ptr p_new;
  union v4l2_ctrl_ptr p_cur;
};

Members

node
The list node.
ev_subs
The list of control event subscriptions.
handler
The handler that owns the control.
cluster
Point to start of cluster array.
ncontrols
Number of controls in cluster array.
done
Internal flag: set for each processed control.
is_new
Set when the user specified a new value for this control. It is also set when called from v4l2_ctrl_handler_setup(). Drivers should never set this flag.
has_changed
Set when the current value differs from the new value. Drivers should never use this flag.
is_private
If set, then this control is private to its handler and it will not be added to any other handlers. Drivers can set this flag.
is_auto
If set, then this control selects whether the other cluster members are in ‘automatic’ mode or ‘manual’ mode. This is used for autogain/gain type clusters. Drivers should never set this flag directly.
is_int
If set, then this control has a simple integer value (i.e. it uses ctrl->val).
is_string
If set, then this control has type V4L2_CTRL_TYPE_STRING.
is_ptr
If set, then this control is an array and/or has type >= V4L2_CTRL_COMPOUND_TYPES and/or has type V4L2_CTRL_TYPE_STRING. In other words, struct v4l2_ext_control uses field p to point to the data.
is_array
If set, then this control contains an N-dimensional array.
has_volatiles
If set, then one or more members of the cluster are volatile. Drivers should never touch this flag.
call_notify
If set, then call the handler’s notify function whenever the control’s value changes.
manual_mode_value
If the is_auto flag is set, then this is the value of the auto control that determines if that control is in manual mode. So if the value of the auto control equals this value, then the whole cluster is in manual mode. Drivers should never set this flag directly.
ops
The control ops.
type_ops
The control type ops.
id
The control ID.
name
The control name.
type
The control type.
minimum
The control’s minimum value.
maximum
The control’s maximum value.
default_value
The control’s default value.
elems
The number of elements in the N-dimensional array.
elem_size
The size in bytes of the control.
dims
The size of each dimension.
nr_of_dims
The number of dimensions in dims.
{unnamed_union}
anonymous
step
The control’s step value for non-menu controls.
menu_skip_mask
The control’s skip mask for menu controls. This makes it easy to skip menu items that are not valid. If bit X is set, then menu item X is skipped. Of course, this only works for menus with <= 32 menu items. There are no menus that come close to that number, so this is OK. Should we ever need more, then this will have to be extended to a u64 or a bit array.
{unnamed_union}
anonymous
qmenu
A const char * array for all menu items. Array entries that are empty strings (“”) correspond to non-existing menu items (this is in addition to the menu_skip_mask above). The last entry must be NULL. Used only if the type is V4L2_CTRL_TYPE_MENU.
qmenu_int
A 64-bit integer array for with integer menu items. The size of array must be equal to the menu size, e. g.: ceil(\frac{maximum - minimum}{step}) + 1. Used only if the type is V4L2_CTRL_TYPE_INTEGER_MENU.
flags
The control’s flags.
priv
The control’s private pointer. For use by the driver. It is untouched by the control framework. Note that this pointer is not freed when the control is deleted. Should this be needed then a new internal bitfield can be added to tell the framework to free this pointer.
val
The control’s new s32 value.
cur
Structure to store the current value.
cur.val
The control’s current value, if the type is represented via a u32 integer (see enum v4l2_ctrl_type).
p_new
The control’s new value represented via a union which provides a standard way of accessing control types through a pointer.
p_cur
The control’s current value represented via a union which provides a standard way of accessing control types through a pointer.
struct v4l2_ctrl_ref

The control reference.

Definition

struct v4l2_ctrl_ref {
  struct list_head node;
  struct v4l2_ctrl_ref *next;
  struct v4l2_ctrl *ctrl;
  struct v4l2_ctrl_helper *helper;
  bool from_other_dev;
  bool req_done;
  struct v4l2_ctrl_ref *req;
  union v4l2_ctrl_ptr p_req;
};

Members

node
List node for the sorted list.
next
Single-link list node for the hash.
ctrl
The actual control information.
helper
Pointer to helper struct. Used internally in prepare_ext_ctrls function at v4l2-ctrl.c.
from_other_dev
If true, then ctrl was defined in another device than the struct v4l2_ctrl_handler.
req_done
Internal flag: if the control handler containing this control reference is bound to a media request, then this is set when the control has been applied. This prevents applying controls from a cluster with multiple controls twice (when the first control of a cluster is applied, they all are).
req
If set, this refers to another request that sets this control.
p_req
If the control handler containing this control reference is bound to a media request, then this points to the value of the control that should be applied when the request is executed, or to the value of the control at the time that the request was completed.

Description

Each control handler has a list of these refs. The list_head is used to keep a sorted-by-control-ID list of all controls, while the next pointer is used to link the control in the hash’s bucket.

struct v4l2_ctrl_handler

The control handler keeps track of all the controls: both the controls owned by the handler and those inherited from other handlers.

Definition

struct v4l2_ctrl_handler {
  struct mutex _lock;
  struct mutex *lock;
  struct list_head ctrls;
  struct list_head ctrl_refs;
  struct v4l2_ctrl_ref *cached;
  struct v4l2_ctrl_ref **buckets;
  v4l2_ctrl_notify_fnc notify;
  void *notify_priv;
  u16 nr_of_buckets;
  int error;
  bool request_is_queued;
  struct list_head requests;
  struct list_head requests_queued;
  struct media_request_object req_obj;
};

Members

_lock
Default for “lock”.
lock
Lock to control access to this handler and its controls. May be replaced by the user right after init.
ctrls
The list of controls owned by this handler.
ctrl_refs
The list of control references.
cached
The last found control reference. It is common that the same control is needed multiple times, so this is a simple optimization.
buckets
Buckets for the hashing. Allows for quick control lookup.
notify
A notify callback that is called whenever the control changes value. Note that the handler’s lock is held when the notify function is called!
notify_priv
Passed as argument to the v4l2_ctrl notify callback.
nr_of_buckets
Total number of buckets in the array.
error
The error code of the first failed control addition.
request_is_queued
True if the request was queued.
requests
List to keep track of open control handler request objects. For the parent control handler (req_obj.req == NULL) this is the list header. When the parent control handler is removed, it has to unbind and put all these requests since they refer to the parent.
requests_queued
List of the queued requests. This determines the order in which these controls are applied. Once the request is completed it is removed from this list.
req_obj
The struct media_request_object, used to link into a struct media_request. This request object has a refcount.
struct v4l2_ctrl_config

Control configuration structure.

Definition

struct v4l2_ctrl_config {
  const struct v4l2_ctrl_ops *ops;
  const struct v4l2_ctrl_type_ops *type_ops;
  u32 id;
  const char *name;
  enum v4l2_ctrl_type type;
  s64 min;
  s64 max;
  u64 step;
  s64 def;
  u32 dims[V4L2_CTRL_MAX_DIMS];
  u32 elem_size;
  u32 flags;
  u64 menu_skip_mask;
  const char * const *qmenu;
  const s64 *qmenu_int;
  unsigned int is_private:1;
};

Members

ops
The control ops.
type_ops
The control type ops. Only needed for compound controls.
id
The control ID.
name
The control name.
type
The control type.
min
The control’s minimum value.
max
The control’s maximum value.
step
The control’s step value for non-menu controls.
def
The control’s default value.
dims
The size of each dimension.
elem_size
The size in bytes of the control.
flags
The control’s flags.
menu_skip_mask
The control’s skip mask for menu controls. This makes it easy to skip menu items that are not valid. If bit X is set, then menu item X is skipped. Of course, this only works for menus with <= 64 menu items. There are no menus that come close to that number, so this is OK. Should we ever need more, then this will have to be extended to a bit array.
qmenu
A const char * array for all menu items. Array entries that are empty strings (“”) correspond to non-existing menu items (this is in addition to the menu_skip_mask above). The last entry must be NULL.
qmenu_int
A const s64 integer array for all menu items of the type V4L2_CTRL_TYPE_INTEGER_MENU.
is_private
If set, then this control is private to its handler and it will not be added to any other handlers.
void v4l2_ctrl_fill(u32 id, const char ** name, enum v4l2_ctrl_type * type, s64 * min, s64 * max, u64 * step, s64 * def, u32 * flags)

Fill in the control fields based on the control ID.

Parameters

u32 id
ID of the control
const char ** name
pointer to be filled with a string with the name of the control
enum v4l2_ctrl_type * type
pointer for storing the type of the control
s64 * min
pointer for storing the minimum value for the control
s64 * max
pointer for storing the maximum value for the control
u64 * step
pointer for storing the control step
s64 * def
pointer for storing the default value for the control
u32 * flags
pointer for storing the flags to be used on the control

Description

This works for all standard V4L2 controls. For non-standard controls it will only fill in the given arguments and name content will be set to NULL.

This function will overwrite the contents of name, type and flags. The contents of min, max, step and def may be modified depending on the type.

Note

Do not use in drivers! It is used internally for backwards compatibility control handling only. Once all drivers are converted to use the new control framework this function will no longer be exported.

int v4l2_ctrl_handler_init_class(struct v4l2_ctrl_handler * hdl, unsigned int nr_of_controls_hint, struct lock_class_key * key, const char * name)

Initialize the control handler.

Parameters

struct v4l2_ctrl_handler * hdl
The control handler.
unsigned int nr_of_controls_hint
A hint of how many controls this handler is expected to refer to. This is the total number, so including any inherited controls. It doesn’t have to be precise, but if it is way off, then you either waste memory (too many buckets are allocated) or the control lookup becomes slower (not enough buckets are allocated, so there are more slow list lookups). It will always work, though.
struct lock_class_key * key
Used by the lock validator if CONFIG_LOCKDEP is set.
const char * name
Used by the lock validator if CONFIG_LOCKDEP is set.

Description

Attention

Never use this call directly, always use the v4l2_ctrl_handler_init() macro that hides the key and name arguments.

Return

returns an error if the buckets could not be allocated. This error will also be stored in hdl->error.

v4l2_ctrl_handler_init(hdl, nr_of_controls_hint)

helper function to create a static struct lock_class_key and calls v4l2_ctrl_handler_init_class()

Parameters

hdl
The control handler.
nr_of_controls_hint
A hint of how many controls this handler is expected to refer to. This is the total number, so including any inherited controls. It doesn’t have to be precise, but if it is way off, then you either waste memory (too many buckets are allocated) or the control lookup becomes slower (not enough buckets are allocated, so there are more slow list lookups). It will always work, though.

Description

This helper function creates a static struct lock_class_key and calls v4l2_ctrl_handler_init_class(), providing a proper name for the lock validador.

Use this helper function to initialize a control handler.

void v4l2_ctrl_handler_free(struct v4l2_ctrl_handler * hdl)

Free all controls owned by the handler and free the control list.

Parameters

struct v4l2_ctrl_handler * hdl
The control handler.

Description

Does nothing if hdl == NULL.

void v4l2_ctrl_lock(struct v4l2_ctrl * ctrl)

Helper function to lock the handler associated with the control.

Parameters

struct v4l2_ctrl * ctrl
The control to lock.
void v4l2_ctrl_unlock(struct v4l2_ctrl * ctrl)

Helper function to unlock the handler associated with the control.

Parameters

struct v4l2_ctrl * ctrl
The control to unlock.
int __v4l2_ctrl_handler_setup(struct v4l2_ctrl_handler * hdl)

Call the s_ctrl op for all controls belonging to the handler to initialize the hardware to the current control values. The caller is responsible for acquiring the control handler mutex on behalf of __v4l2_ctrl_handler_setup().

Parameters

struct v4l2_ctrl_handler * hdl
The control handler.

Description

Button controls will be skipped, as are read-only controls.

If hdl == NULL, then this just returns 0.

int v4l2_ctrl_handler_setup(struct v4l2_ctrl_handler * hdl)

Call the s_ctrl op for all controls belonging to the handler to initialize the hardware to the current control values.

Parameters

struct v4l2_ctrl_handler * hdl
The control handler.

Description

Button controls will be skipped, as are read-only controls.

If hdl == NULL, then this just returns 0.

void v4l2_ctrl_handler_log_status(struct v4l2_ctrl_handler * hdl, const char * prefix)

Log all controls owned by the handler.

Parameters

struct v4l2_ctrl_handler * hdl
The control handler.
const char * prefix
The prefix to use when logging the control values. If the prefix does not end with a space, then ”: ” will be added after the prefix. If prefix == NULL, then no prefix will be used.

Description

For use with VIDIOC_LOG_STATUS.

Does nothing if hdl == NULL.

struct v4l2_ctrl * v4l2_ctrl_new_custom(struct v4l2_ctrl_handler * hdl, const struct v4l2_ctrl_config * cfg, void * priv)

Allocate and initialize a new custom V4L2 control.

Parameters

struct v4l2_ctrl_handler * hdl
The control handler.
const struct v4l2_ctrl_config * cfg
The control’s configuration data.
void * priv
The control’s driver-specific private data.

Description

If the v4l2_ctrl struct could not be allocated then NULL is returned and hdl->error is set to the error code (if it wasn’t set already).

struct v4l2_ctrl * v4l2_ctrl_new_std(struct v4l2_ctrl_handler * hdl, const struct v4l2_ctrl_ops * ops, u32 id, s64 min, s64 max, u64 step, s64 def)

Allocate and initialize a new standard V4L2 non-menu control.

Parameters

struct v4l2_ctrl_handler * hdl
The control handler.
const struct v4l2_ctrl_ops * ops
The control ops.
u32 id
The control ID.
s64 min
The control’s minimum value.
s64 max
The control’s maximum value.
u64 step
The control’s step value
s64 def
The control’s default value.

Description

If the v4l2_ctrl struct could not be allocated, or the control ID is not known, then NULL is returned and hdl->error is set to the appropriate error code (if it wasn’t set already).

If id refers to a menu control, then this function will return NULL.

Use v4l2_ctrl_new_std_menu() when adding menu controls.

struct v4l2_ctrl * v4l2_ctrl_new_std_menu(struct v4l2_ctrl_handler * hdl, const struct v4l2_ctrl_ops * ops, u32 id, u8 max, u64 mask, u8 def)

Allocate and initialize a new standard V4L2 menu control.

Parameters

struct v4l2_ctrl_handler * hdl
The control handler.
const struct v4l2_ctrl_ops * ops
The control ops.
u32 id
The control ID.
u8 max
The control’s maximum value.
u64 mask
The control’s skip mask for menu controls. This makes it easy to skip menu items that are not valid. If bit X is set, then menu item X is skipped. Of course, this only works for menus with <= 64 menu items. There are no menus that come close to that number, so this is OK. Should we ever need more, then this will have to be extended to a bit array.
u8 def
The control’s default value.

Description

Same as v4l2_ctrl_new_std(), but min is set to 0 and the mask value determines which menu items are to be skipped.

If id refers to a non-menu control, then this function will return NULL.

struct v4l2_ctrl * v4l2_ctrl_new_std_menu_items(struct v4l2_ctrl_handler * hdl, const struct v4l2_ctrl_ops * ops, u32 id, u8 max, u64 mask, u8 def, const char *const * qmenu)

Create a new standard V4L2 menu control with driver specific menu.

Parameters

struct v4l2_ctrl_handler * hdl
The control handler.
const struct v4l2_ctrl_ops * ops
The control ops.
u32 id
The control ID.
u8 max
The control’s maximum value.
u64 mask
The control’s skip mask for menu controls. This makes it easy to skip menu items that are not valid. If bit X is set, then menu item X is skipped. Of course, this only works for menus with <= 64 menu items. There are no menus that come close to that number, so this is OK. Should we ever need more, then this will have to be extended to a bit array.
u8 def
The control’s default value.
const char *const * qmenu
The new menu.

Description

Same as v4l2_ctrl_new_std_menu(), but qmenu will be the driver specific menu of this control.

struct v4l2_ctrl * v4l2_ctrl_new_int_menu(struct v4l2_ctrl_handler * hdl, const struct v4l2_ctrl_ops * ops, u32 id, u8 max, u8 def, const s64 * qmenu_int)

Create a new standard V4L2 integer menu control.

Parameters

struct v4l2_ctrl_handler * hdl
The control handler.
const struct v4l2_ctrl_ops * ops
The control ops.
u32 id
The control ID.
u8 max
The control’s maximum value.
u8 def
The control’s default value.
const s64 * qmenu_int
The control’s menu entries.

Description

Same as v4l2_ctrl_new_std_menu(), but mask is set to 0 and it additionaly takes as an argument an array of integers determining the menu items.

If id refers to a non-integer-menu control, then this function will return NULL.

v4l2_ctrl_filter

Typedef: Typedef to define the filter function to be used when adding a control handler.

Syntax

bool v4l2_ctrl_filter (const struct v4l2_ctrl * ctrl);

Parameters

const struct v4l2_ctrl * ctrl
pointer to struct v4l2_ctrl.
int v4l2_ctrl_add_handler(struct v4l2_ctrl_handler * hdl, struct v4l2_ctrl_handler * add, v4l2_ctrl_filter filter, bool from_other_dev)

Add all controls from handler add to handler hdl.

Parameters

struct v4l2_ctrl_handler * hdl
The control handler.
struct v4l2_ctrl_handler * add
The control handler whose controls you want to add to the hdl control handler.
v4l2_ctrl_filter filter
This function will filter which controls should be added.
bool from_other_dev
If true, then the controls in add were defined in another device than hdl.

Description

Does nothing if either of the two handlers is a NULL pointer. If filter is NULL, then all controls are added. Otherwise only those controls for which filter returns true will be added. In case of an error hdl->error will be set to the error code (if it wasn’t set already).

bool v4l2_ctrl_radio_filter(const struct v4l2_ctrl * ctrl)

Standard filter for radio controls.

Parameters

const struct v4l2_ctrl * ctrl
The control that is filtered.

Description

This will return true for any controls that are valid for radio device nodes. Those are all of the V4L2_CID_AUDIO_* user controls and all FM transmitter class controls.

This function is to be used with v4l2_ctrl_add_handler().

void v4l2_ctrl_cluster(unsigned int ncontrols, struct v4l2_ctrl ** controls)

Mark all controls in the cluster as belonging to that cluster.

Parameters

unsigned int ncontrols
The number of controls in this cluster.
struct v4l2_ctrl ** controls
The cluster control array of size ncontrols.
void v4l2_ctrl_auto_cluster(unsigned int ncontrols, struct v4l2_ctrl ** controls, u8 manual_val, bool set_volatile)

Mark all controls in the cluster as belonging to that cluster and set it up for autofoo/foo-type handling.

Parameters

unsigned int ncontrols
The number of controls in this cluster.
struct v4l2_ctrl ** controls
The cluster control array of size ncontrols. The first control must be the ‘auto’ control (e.g. autogain, autoexposure, etc.)
u8 manual_val
The value for the first control in the cluster that equals the manual setting.
bool set_volatile
If true, then all controls except the first auto control will be volatile.

Description

Use for control groups where one control selects some automatic feature and the other controls are only active whenever the automatic feature is turned off (manual mode). Typical examples: autogain vs gain, auto-whitebalance vs red and blue balance, etc.

The behavior of such controls is as follows:

When the autofoo control is set to automatic, then any manual controls are set to inactive and any reads will call g_volatile_ctrl (if the control was marked volatile).

When the autofoo control is set to manual, then any manual controls will be marked active, and any reads will just return the current value without going through g_volatile_ctrl.

In addition, this function will set the V4L2_CTRL_FLAG_UPDATE flag on the autofoo control and V4L2_CTRL_FLAG_INACTIVE on the foo control(s) if autofoo is in auto mode.

struct v4l2_ctrl * v4l2_ctrl_find(struct v4l2_ctrl_handler * hdl, u32 id)

Find a control with the given ID.

Parameters

struct v4l2_ctrl_handler * hdl
The control handler.
u32 id
The control ID to find.

Description

If hdl == NULL this will return NULL as well. Will lock the handler so do not use from inside v4l2_ctrl_ops.

void v4l2_ctrl_activate(struct v4l2_ctrl * ctrl, bool active)

Make the control active or inactive.

Parameters

struct v4l2_ctrl * ctrl
The control to (de)activate.
bool active
True if the control should become active.

Description

This sets or clears the V4L2_CTRL_FLAG_INACTIVE flag atomically. Does nothing if ctrl == NULL. This will usually be called from within the s_ctrl op. The V4L2_EVENT_CTRL event will be generated afterwards.

This function assumes that the control handler is locked.

void __v4l2_ctrl_grab(struct v4l2_ctrl * ctrl, bool grabbed)

Unlocked variant of v4l2_ctrl_grab.

Parameters

struct v4l2_ctrl * ctrl
The control to (de)activate.
bool grabbed
True if the control should become grabbed.

Description

This sets or clears the V4L2_CTRL_FLAG_GRABBED flag atomically. Does nothing if ctrl == NULL. The V4L2_EVENT_CTRL event will be generated afterwards. This will usually be called when starting or stopping streaming in the driver.

This function assumes that the control handler is locked by the caller.

void v4l2_ctrl_grab(struct v4l2_ctrl * ctrl, bool grabbed)

Mark the control as grabbed or not grabbed.

Parameters

struct v4l2_ctrl * ctrl
The control to (de)activate.
bool grabbed
True if the control should become grabbed.

Description

This sets or clears the V4L2_CTRL_FLAG_GRABBED flag atomically. Does nothing if ctrl == NULL. The V4L2_EVENT_CTRL event will be generated afterwards. This will usually be called when starting or stopping streaming in the driver.

This function assumes that the control handler is not locked and will take the lock itself.

int __v4l2_ctrl_modify_range(struct v4l2_ctrl * ctrl, s64 min, s64 max, u64 step, s64 def)

Unlocked variant of v4l2_ctrl_modify_range()

Parameters

struct v4l2_ctrl * ctrl
The control to update.
s64 min
The control’s minimum value.
s64 max
The control’s maximum value.
u64 step
The control’s step value
s64 def
The control’s default value.

Description

Update the range of a control on the fly. This works for control types INTEGER, BOOLEAN, MENU, INTEGER MENU and BITMASK. For menu controls the step value is interpreted as a menu_skip_mask.

An error is returned if one of the range arguments is invalid for this control type.

The caller is responsible for acquiring the control handler mutex on behalf of __v4l2_ctrl_modify_range().

int v4l2_ctrl_modify_range(struct v4l2_ctrl * ctrl, s64 min, s64 max, u64 step, s64 def)

Update the range of a control.

Parameters

struct v4l2_ctrl * ctrl
The control to update.
s64 min
The control’s minimum value.
s64 max
The control’s maximum value.
u64 step
The control’s step value
s64 def
The control’s default value.

Description

Update the range of a control on the fly. This works for control types INTEGER, BOOLEAN, MENU, INTEGER MENU and BITMASK. For menu controls the step value is interpreted as a menu_skip_mask.

An error is returned if one of the range arguments is invalid for this control type.

This function assumes that the control handler is not locked and will take the lock itself.

void v4l2_ctrl_notify(struct v4l2_ctrl * ctrl, v4l2_ctrl_notify_fnc notify, void * priv)

Function to set a notify callback for a control.

Parameters

struct v4l2_ctrl * ctrl
The control.
v4l2_ctrl_notify_fnc notify
The callback function.
void * priv
The callback private handle, passed as argument to the callback.

Description

This function sets a callback function for the control. If ctrl is NULL, then it will do nothing. If notify is NULL, then the notify callback will be removed.

There can be only one notify. If another already exists, then a WARN_ON will be issued and the function will do nothing.

const char * v4l2_ctrl_get_name(u32 id)

Get the name of the control

Parameters

u32 id
The control ID.

Description

This function returns the name of the given control ID or NULL if it isn’t a known control.

const char * const * v4l2_ctrl_get_menu(u32 id)

Get the menu string array of the control

Parameters

u32 id
The control ID.

Description

This function returns the NULL-terminated menu string array name of the given control ID or NULL if it isn’t a known menu control.

const s64 * v4l2_ctrl_get_int_menu(u32 id, u32 * len)

Get the integer menu array of the control

Parameters

u32 id
The control ID.
u32 * len
The size of the integer array.

Description

This function returns the integer array of the given control ID or NULL if it if it isn’t a known integer menu control.

s32 v4l2_ctrl_g_ctrl(struct v4l2_ctrl * ctrl)

Helper function to get the control’s value from within a driver.

Parameters

struct v4l2_ctrl * ctrl
The control.

Description

This returns the control’s value safely by going through the control framework. This function will lock the control’s handler, so it cannot be used from within the v4l2_ctrl_ops functions.

This function is for integer type controls only.

int __v4l2_ctrl_s_ctrl(struct v4l2_ctrl * ctrl, s32 val)

Unlocked variant of v4l2_ctrl_s_ctrl().

Parameters

struct v4l2_ctrl * ctrl
The control.
s32 val
TheControls name new value.

Description

This sets the control’s new value safely by going through the control framework. This function assumes the control’s handler is already locked, allowing it to be used from within the v4l2_ctrl_ops functions.

This function is for integer type controls only.

int v4l2_ctrl_s_ctrl(struct v4l2_ctrl * ctrl, s32 val)

Helper function to set the control’s value from within a driver.

Parameters

struct v4l2_ctrl * ctrl
The control.
s32 val
The new value.

Description

This sets the control’s new value safely by going through the control framework. This function will lock the control’s handler, so it cannot be used from within the v4l2_ctrl_ops functions.

This function is for integer type controls only.

s64 v4l2_ctrl_g_ctrl_int64(struct v4l2_ctrl * ctrl)

Helper function to get a 64-bit control’s value from within a driver.

Parameters

struct v4l2_ctrl * ctrl
The control.

Description

This returns the control’s value safely by going through the control framework. This function will lock the control’s handler, so it cannot be used from within the v4l2_ctrl_ops functions.

This function is for 64-bit integer type controls only.

int __v4l2_ctrl_s_ctrl_int64(struct v4l2_ctrl * ctrl, s64 val)

Unlocked variant of v4l2_ctrl_s_ctrl_int64().

Parameters

struct v4l2_ctrl * ctrl
The control.
s64 val
The new value.

Description

This sets the control’s new value safely by going through the control framework. This function assumes the control’s handler is already locked, allowing it to be used from within the v4l2_ctrl_ops functions.

This function is for 64-bit integer type controls only.

int v4l2_ctrl_s_ctrl_int64(struct v4l2_ctrl * ctrl, s64 val)

Helper function to set a 64-bit control’s value from within a driver.

Parameters

struct v4l2_ctrl * ctrl
The control.
s64 val
The new value.

Description

This sets the control’s new value safely by going through the control framework. This function will lock the control’s handler, so it cannot be used from within the v4l2_ctrl_ops functions.

This function is for 64-bit integer type controls only.

int __v4l2_ctrl_s_ctrl_string(struct v4l2_ctrl * ctrl, const char * s)

Unlocked variant of v4l2_ctrl_s_ctrl_string().

Parameters

struct v4l2_ctrl * ctrl
The control.
const char * s
The new string.

Description

This sets the control’s new string safely by going through the control framework. This function assumes the control’s handler is already locked, allowing it to be used from within the v4l2_ctrl_ops functions.

This function is for string type controls only.

int v4l2_ctrl_s_ctrl_string(struct v4l2_ctrl * ctrl, const char * s)

Helper function to set a control’s string value from within a driver.

Parameters

struct v4l2_ctrl * ctrl
The control.
const char * s
The new string. Controls name This sets the control’s new string safely by going through the control framework. This function will lock the control’s handler, so it cannot be used from within the v4l2_ctrl_ops functions.

Description

This function is for string type controls only.

void v4l2_ctrl_replace(struct v4l2_event * old, const struct v4l2_event * new)

Function to be used as a callback to struct v4l2_subscribed_event_ops replace()

Parameters

struct v4l2_event * old
pointer to struct v4l2_event with the reported event;
const struct v4l2_event * new
pointer to struct v4l2_event with the modified event;
void v4l2_ctrl_merge(const struct v4l2_event * old, struct v4l2_event * new)

Function to be used as a callback to struct v4l2_subscribed_event_ops merge()

Parameters

const struct v4l2_event * old
pointer to struct v4l2_event with the reported event;
struct v4l2_event * new
pointer to struct v4l2_event with the merged event;
int v4l2_ctrl_log_status(struct file * file, void * fh)

helper function to implement VIDIOC_LOG_STATUS ioctl

Parameters

struct file * file
pointer to struct file
void * fh
unused. Kept just to be compatible to the arguments expected by struct v4l2_ioctl_ops.vidioc_log_status.

Description

Can be used as a vidioc_log_status function that just dumps all controls associated with the filehandle.

int v4l2_ctrl_subscribe_event(struct v4l2_fh * fh, const struct v4l2_event_subscription * sub)

Subscribes to an event

Parameters

struct v4l2_fh * fh
pointer to struct v4l2_fh
const struct v4l2_event_subscription * sub
pointer to struct v4l2_event_subscription

Description

Can be used as a vidioc_subscribe_event function that just subscribes control events.

__poll_t v4l2_ctrl_poll(struct file * file, struct poll_table_struct * wait)

function to be used as a callback to the poll() That just polls for control events.

Parameters

struct file * file
pointer to struct file
struct poll_table_struct * wait
pointer to struct poll_table_struct
void v4l2_ctrl_request_setup(struct media_request * req, struct v4l2_ctrl_handler * parent)

helper function to apply control values in a request

Parameters

struct media_request * req
The request
struct v4l2_ctrl_handler * parent
The parent control handler (‘priv’ in media_request_object_find())

Description

This is a helper function to call the control handler’s s_ctrl callback with the control values contained in the request. Do note that this approach of applying control values in a request is only applicable to memory-to-memory devices.

void v4l2_ctrl_request_complete(struct media_request * req, struct v4l2_ctrl_handler * parent)

Complete a control handler request object

Parameters

struct media_request * req
The request
struct v4l2_ctrl_handler * parent
The parent control handler (‘priv’ in media_request_object_find())

Description

This function is to be called on each control handler that may have had a request object associated with it, i.e. control handlers of a driver that supports requests.

The function first obtains the values of any volatile controls in the control handler and attach them to the request. Then, the function completes the request object.

struct v4l2_ctrl_handler * v4l2_ctrl_request_hdl_find(struct media_request * req, struct v4l2_ctrl_handler * parent)

Find the control handler in the request

Parameters

struct media_request * req
The request
struct v4l2_ctrl_handler * parent
The parent control handler (‘priv’ in media_request_object_find())

Description

This function finds the control handler in the request. It may return NULL if not found. When done, you must call v4l2_ctrl_request_put_hdl() with the returned handler pointer.

If the request is not in state VALIDATING or QUEUED, then this function will always return NULL.

Note that in state VALIDATING the req_queue_mutex is held, so no objects can be added or deleted from the request.

In state QUEUED it is the driver that will have to ensure this.

void v4l2_ctrl_request_hdl_put(struct v4l2_ctrl_handler * hdl)

Put the control handler

Parameters

struct v4l2_ctrl_handler * hdl
Put this control handler

Description

This function released the control handler previously obtained from’ v4l2_ctrl_request_hdl_find().

struct v4l2_ctrl * v4l2_ctrl_request_hdl_ctrl_find(struct v4l2_ctrl_handler * hdl, u32 id)

Find a control with the given ID.

Parameters

struct v4l2_ctrl_handler * hdl
The control handler from the request.
u32 id
The ID of the control to find.

Description

This function returns a pointer to the control if this control is part of the request or NULL otherwise.

int v4l2_queryctrl(struct v4l2_ctrl_handler * hdl, struct v4l2_queryctrl * qc)

Helper function to implement VIDIOC_QUERYCTRL ioctl

Parameters

struct v4l2_ctrl_handler * hdl
pointer to struct v4l2_ctrl_handler
struct v4l2_queryctrl * qc
pointer to struct v4l2_queryctrl

Description

If hdl == NULL then they will all return -EINVAL.

int v4l2_query_ext_ctrl(struct v4l2_ctrl_handler * hdl, struct v4l2_query_ext_ctrl * qc)

Helper function to implement VIDIOC_QUERY_EXT_CTRL ioctl

Parameters

struct v4l2_ctrl_handler * hdl
pointer to struct v4l2_ctrl_handler
struct v4l2_query_ext_ctrl * qc
pointer to struct v4l2_query_ext_ctrl

Description

If hdl == NULL then they will all return -EINVAL.

int v4l2_querymenu(struct v4l2_ctrl_handler * hdl, struct v4l2_querymenu * qm)

Helper function to implement VIDIOC_QUERYMENU ioctl

Parameters

struct v4l2_ctrl_handler * hdl
pointer to struct v4l2_ctrl_handler
struct v4l2_querymenu * qm
pointer to struct v4l2_querymenu

Description

If hdl == NULL then they will all return -EINVAL.

int v4l2_g_ctrl(struct v4l2_ctrl_handler * hdl, struct v4l2_control * ctrl)

Helper function to implement VIDIOC_G_CTRL ioctl

Parameters

struct v4l2_ctrl_handler * hdl
pointer to struct v4l2_ctrl_handler
struct v4l2_control * ctrl
pointer to struct v4l2_control

Description

If hdl == NULL then they will all return -EINVAL.

int v4l2_s_ctrl(struct v4l2_fh * fh, struct v4l2_ctrl_handler * hdl, struct v4l2_control * ctrl)

Helper function to implement VIDIOC_S_CTRL ioctl

Parameters

struct v4l2_fh * fh
pointer to struct v4l2_fh
struct v4l2_ctrl_handler * hdl
pointer to struct v4l2_ctrl_handler
struct v4l2_control * ctrl
pointer to struct v4l2_control

Description

If hdl == NULL then they will all return -EINVAL.

int v4l2_g_ext_ctrls(struct v4l2_ctrl_handler * hdl, struct media_device * mdev, struct v4l2_ext_controls * c)

Helper function to implement VIDIOC_G_EXT_CTRLS ioctl

Parameters

struct v4l2_ctrl_handler * hdl
pointer to struct v4l2_ctrl_handler
struct media_device * mdev
pointer to struct media_device
struct v4l2_ext_controls * c
pointer to struct v4l2_ext_controls

Description

If hdl == NULL then they will all return -EINVAL.

int v4l2_try_ext_ctrls(struct v4l2_ctrl_handler * hdl, struct media_device * mdev, struct v4l2_ext_controls * c)

Helper function to implement VIDIOC_TRY_EXT_CTRLS ioctl

Parameters

struct v4l2_ctrl_handler * hdl
pointer to struct v4l2_ctrl_handler
struct media_device * mdev
pointer to struct media_device
struct v4l2_ext_controls * c
pointer to struct v4l2_ext_controls

Description

If hdl == NULL then they will all return -EINVAL.

int v4l2_s_ext_ctrls(struct v4l2_fh * fh, struct v4l2_ctrl_handler * hdl, struct media_device * mdev, struct v4l2_ext_controls * c)

Helper function to implement VIDIOC_S_EXT_CTRLS ioctl

Parameters

struct v4l2_fh * fh
pointer to struct v4l2_fh
struct v4l2_ctrl_handler * hdl
pointer to struct v4l2_ctrl_handler
struct media_device * mdev
pointer to struct media_device
struct v4l2_ext_controls * c
pointer to struct v4l2_ext_controls

Description

If hdl == NULL then they will all return -EINVAL.

int v4l2_ctrl_subdev_subscribe_event(struct v4l2_subdev * sd, struct v4l2_fh * fh, struct v4l2_event_subscription * sub)

Helper function to implement as a struct v4l2_subdev_core_ops subscribe_event function that just subscribes control events.

Parameters

struct v4l2_subdev * sd
pointer to struct v4l2_subdev
struct v4l2_fh * fh
pointer to struct v4l2_fh
struct v4l2_event_subscription * sub
pointer to struct v4l2_event_subscription
int v4l2_ctrl_subdev_log_status(struct v4l2_subdev * sd)

Log all controls owned by subdev’s control handler.

Parameters

struct v4l2_subdev * sd
pointer to struct v4l2_subdev