Devices typically have a number of user-settable controls such as brightness, saturation and so on, which would be presented to the user on a graphical user interface. But, different devices will have different controls available, and furthermore, the range of possible values, and the default value will vary from device to device. The control ioctls provide the information and a mechanism to create a nice user interface for these controls that will work correctly with any device. All controls are accessed using an ID value. V4L2 defines several IDs for specific purposes. Drivers can also implement their own custom controls using V4L2_CID_PRIVATE_BASE and higher values. The pre-defined control IDs have the prefix V4L2_CID_, and are listed in . The ID is used when querying the attributes of a control, and when getting or setting the current value. Generally applications should present controls to the user without assumptions about their purpose. Each control comes with a name string the user is supposed to understand. When the purpose is non-intuitive the driver writer should provide a user manual, a user interface plug-in or a driver specific panel application. Predefined IDs were introduced to change a few controls programmatically, for example to mute a device during a channel switch. Drivers may enumerate different controls after switching the current video input or output, tuner or modulator, or audio input or output. Different in the sense of other bounds, another default and current value, step size or other menu items. A control with a certain custom ID can also change name and type. It will be more convenient for applications if drivers make use of the V4L2_CTRL_FLAG_DISABLED flag, but that was never required. Control values are stored globally, they do not change when switching except to stay within the reported bounds. They also do not change ⪚ when the device is opened or closed, when the tuner radio frequency is changed or generally never without application request. Since V4L2 specifies no event mechanism, panel applications intended to cooperate with other panel applications (be they built into a larger application, as a TV viewer) may need to regularly poll control values to update their user interface. Applications could call an ioctl to request events. After another process called &VIDIOC-S-CTRL; or another ioctl changing shared properties the &func-select; function would indicate readability until any ioctl (querying the properties) is called. All controls use machine endianness. &cs-def; ID Type Description V4L2_CID_BASE First predefined ID, equal to V4L2_CID_BRIGHTNESS. V4L2_CID_USER_BASE Synonym of V4L2_CID_BASE. V4L2_CID_BRIGHTNESS integer Picture brightness, or more precisely, the black level. V4L2_CID_CONTRAST integer Picture contrast or luma gain. V4L2_CID_SATURATION integer Picture color saturation or chroma gain. V4L2_CID_HUE integer Hue or color balance. V4L2_CID_AUDIO_VOLUME integer Overall audio volume. Note some drivers also provide an OSS or ALSA mixer interface. V4L2_CID_AUDIO_BALANCE integer Audio stereo balance. Minimum corresponds to all the way left, maximum to right. V4L2_CID_AUDIO_BASS integer Audio bass adjustment. V4L2_CID_AUDIO_TREBLE integer Audio treble adjustment. V4L2_CID_AUDIO_MUTE boolean Mute audio, &ie; set the volume to zero, however without affecting V4L2_CID_AUDIO_VOLUME. Like ALSA drivers, V4L2 drivers must mute at load time to avoid excessive noise. Actually the entire device should be reset to a low power consumption state. V4L2_CID_AUDIO_LOUDNESS boolean Loudness mode (bass boost). V4L2_CID_BLACK_LEVEL integer Another name for brightness (not a synonym of V4L2_CID_BRIGHTNESS). This control is deprecated and should not be used in new drivers and applications. V4L2_CID_AUTO_WHITE_BALANCE boolean Automatic white balance (cameras). V4L2_CID_DO_WHITE_BALANCE button This is an action control. When set (the value is ignored), the device will do a white balance and then hold the current setting. Contrast this with the boolean V4L2_CID_AUTO_WHITE_BALANCE, which, when activated, keeps adjusting the white balance. V4L2_CID_RED_BALANCE integer Red chroma balance. V4L2_CID_BLUE_BALANCE integer Blue chroma balance. V4L2_CID_GAMMA integer Gamma adjust. V4L2_CID_WHITENESS integer Whiteness for grey-scale devices. This is a synonym for V4L2_CID_GAMMA. This control is deprecated and should not be used in new drivers and applications. V4L2_CID_EXPOSURE integer Exposure (cameras). [Unit?] V4L2_CID_AUTOGAIN boolean Automatic gain/exposure control. V4L2_CID_GAIN integer Gain control. V4L2_CID_HFLIP boolean Mirror the picture horizontally. V4L2_CID_VFLIP boolean Mirror the picture vertically. V4L2_CID_HCENTER_DEPRECATED (formerly V4L2_CID_HCENTER) integer Horizontal image centering. This control is deprecated. New drivers and applications should use the Camera class controls V4L2_CID_PAN_ABSOLUTE, V4L2_CID_PAN_RELATIVE and V4L2_CID_PAN_RESET instead. V4L2_CID_VCENTER_DEPRECATED (formerly V4L2_CID_VCENTER) integer Vertical image centering. Centering is intended to physically adjust cameras. For image cropping see , for clipping . This control is deprecated. New drivers and applications should use the Camera class controls V4L2_CID_TILT_ABSOLUTE, V4L2_CID_TILT_RELATIVE and V4L2_CID_TILT_RESET instead. V4L2_CID_POWER_LINE_FREQUENCY enum Enables a power line frequency filter to avoid flicker. Possible values for enum v4l2_power_line_frequency are: V4L2_CID_POWER_LINE_FREQUENCY_DISABLED (0), V4L2_CID_POWER_LINE_FREQUENCY_50HZ (1) and V4L2_CID_POWER_LINE_FREQUENCY_60HZ (2). V4L2_CID_HUE_AUTO boolean Enables automatic hue control by the device. The effect of setting V4L2_CID_HUE while automatic hue control is enabled is undefined, drivers should ignore such request. V4L2_CID_WHITE_BALANCE_TEMPERATURE integer This control specifies the white balance settings as a color temperature in Kelvin. A driver should have a minimum of 2800 (incandescent) to 6500 (daylight). For more information about color temperature see Wikipedia. V4L2_CID_SHARPNESS integer Adjusts the sharpness filters in a camera. The minimum value disables the filters, higher values give a sharper picture. V4L2_CID_BACKLIGHT_COMPENSATION integer Adjusts the backlight compensation in a camera. The minimum value disables backlight compensation. V4L2_CID_CHROMA_AGC boolean Chroma automatic gain control. V4L2_CID_CHROMA_GAIN integer Adjusts the Chroma gain control (for use when chroma AGC is disabled). V4L2_CID_COLOR_KILLER boolean Enable the color killer (&ie; force a black & white image in case of a weak video signal). V4L2_CID_COLORFX enum Selects a color effect. Possible values for enum v4l2_colorfx are: V4L2_COLORFX_NONE (0), V4L2_COLORFX_BW (1), V4L2_COLORFX_SEPIA (2), V4L2_COLORFX_NEGATIVE (3), V4L2_COLORFX_EMBOSS (4), V4L2_COLORFX_SKETCH (5), V4L2_COLORFX_SKY_BLUE (6), V4L2_COLORFX_GRASS_GREEN (7), V4L2_COLORFX_SKIN_WHITEN (8) and V4L2_COLORFX_VIVID (9). V4L2_CID_ROTATE integer Rotates the image by specified angle. Common angles are 90, 270 and 180. Rotating the image to 90 and 270 will reverse the height and width of the display window. It is necessary to set the new height and width of the picture using the &VIDIOC-S-FMT; ioctl according to the rotation angle selected. V4L2_CID_BG_COLOR integer Sets the background color on the current output device. Background color needs to be specified in the RGB24 format. The supplied 32 bit value is interpreted as bits 0-7 Red color information, bits 8-15 Green color information, bits 16-23 Blue color information and bits 24-31 must be zero. V4L2_CID_ILLUMINATORS_1 V4L2_CID_ILLUMINATORS_2 boolean Switch on or off the illuminator 1 or 2 of the device (usually a microscope). V4L2_CID_LASTP1 End of the predefined control IDs (currently V4L2_CID_ILLUMINATORS_2 + 1). V4L2_CID_PRIVATE_BASE ID of the first custom (driver specific) control. Applications depending on particular custom controls should check the driver name and version, see . Applications can enumerate the available controls with the &VIDIOC-QUERYCTRL; and &VIDIOC-QUERYMENU; ioctls, get and set a control value with the &VIDIOC-G-CTRL; and &VIDIOC-S-CTRL; ioctls. Drivers must implement VIDIOC_QUERYCTRL, VIDIOC_G_CTRL and VIDIOC_S_CTRL when the device has one or more controls, VIDIOC_QUERYMENU when it has one or more menu type controls. &v4l2-queryctrl; queryctrl; &v4l2-querymenu; querymenu; static void enumerate_menu (void) { printf (" Menu items:\n"); memset (&querymenu, 0, sizeof (querymenu)); querymenu.id = queryctrl.id; for (querymenu.index = queryctrl.minimum; querymenu.index <= queryctrl.maximum; querymenu.index++) { if (0 == ioctl (fd, &VIDIOC-QUERYMENU;, &querymenu)) { printf (" %s\n", querymenu.name); } } } memset (&queryctrl, 0, sizeof (queryctrl)); for (queryctrl.id = V4L2_CID_BASE; queryctrl.id < V4L2_CID_LASTP1; queryctrl.id++) { if (0 == ioctl (fd, &VIDIOC-QUERYCTRL;, &queryctrl)) { if (queryctrl.flags & V4L2_CTRL_FLAG_DISABLED) continue; printf ("Control %s\n", queryctrl.name); if (queryctrl.type == V4L2_CTRL_TYPE_MENU) enumerate_menu (); } else { if (errno == EINVAL) continue; perror ("VIDIOC_QUERYCTRL"); exit (EXIT_FAILURE); } } for (queryctrl.id = V4L2_CID_PRIVATE_BASE;; queryctrl.id++) { if (0 == ioctl (fd, &VIDIOC-QUERYCTRL;, &queryctrl)) { if (queryctrl.flags & V4L2_CTRL_FLAG_DISABLED) continue; printf ("Control %s\n", queryctrl.name); if (queryctrl.type == V4L2_CTRL_TYPE_MENU) enumerate_menu (); } else { if (errno == EINVAL) break; perror ("VIDIOC_QUERYCTRL"); exit (EXIT_FAILURE); } } &v4l2-queryctrl; queryctrl; &v4l2-control; control; memset (&queryctrl, 0, sizeof (queryctrl)); queryctrl.id = V4L2_CID_BRIGHTNESS; if (-1 == ioctl (fd, &VIDIOC-QUERYCTRL;, &queryctrl)) { if (errno != EINVAL) { perror ("VIDIOC_QUERYCTRL"); exit (EXIT_FAILURE); } else { printf ("V4L2_CID_BRIGHTNESS is not supported\n"); } } else if (queryctrl.flags & V4L2_CTRL_FLAG_DISABLED) { printf ("V4L2_CID_BRIGHTNESS is not supported\n"); } else { memset (&control, 0, sizeof (control)); control.id = V4L2_CID_BRIGHTNESS; control.value = queryctrl.default_value; if (-1 == ioctl (fd, &VIDIOC-S-CTRL;, &control)) { perror ("VIDIOC_S_CTRL"); exit (EXIT_FAILURE); } } memset (&control, 0, sizeof (control)); control.id = V4L2_CID_CONTRAST; if (0 == ioctl (fd, &VIDIOC-G-CTRL;, &control)) { control.value += 1; /* The driver may clamp the value or return ERANGE, ignored here */ if (-1 == ioctl (fd, &VIDIOC-S-CTRL;, &control) && errno != ERANGE) { perror ("VIDIOC_S_CTRL"); exit (EXIT_FAILURE); } /* Ignore if V4L2_CID_CONTRAST is unsupported */ } else if (errno != EINVAL) { perror ("VIDIOC_G_CTRL"); exit (EXIT_FAILURE); } control.id = V4L2_CID_AUDIO_MUTE; control.value = TRUE; /* silence */ /* Errors ignored */ ioctl (fd, VIDIOC_S_CTRL, &control);

The control mechanism as originally designed was meant to be used for user settings (brightness, saturation, etc). However, it turned out to be a very useful model for implementing more complicated driver APIs where each driver implements only a subset of a larger API. The MPEG encoding API was the driving force behind designing and implementing this extended control mechanism: the MPEG standard is quite large and the currently supported hardware MPEG encoders each only implement a subset of this standard. Further more, many parameters relating to how the video is encoded into an MPEG stream are specific to the MPEG encoding chip since the MPEG standard only defines the format of the resulting MPEG stream, not how the video is actually encoded into that format. Unfortunately, the original control API lacked some features needed for these new uses and so it was extended into the (not terribly originally named) extended control API. Even though the MPEG encoding API was the first effort to use the Extended Control API, nowadays there are also other classes of Extended Controls, such as Camera Controls and FM Transmitter Controls. The Extended Controls API as well as all Extended Controls classes are described in the following text.

Three new ioctls are available: &VIDIOC-G-EXT-CTRLS;, &VIDIOC-S-EXT-CTRLS; and &VIDIOC-TRY-EXT-CTRLS;. These ioctls act on arrays of controls (as opposed to the &VIDIOC-G-CTRL; and &VIDIOC-S-CTRL; ioctls that act on a single control). This is needed since it is often required to atomically change several controls at once. Each of the new ioctls expects a pointer to a &v4l2-ext-controls;. This structure contains a pointer to the control array, a count of the number of controls in that array and a control class. Control classes are used to group similar controls into a single class. For example, control class V4L2_CTRL_CLASS_USER contains all user controls (&ie; all controls that can also be set using the old VIDIOC_S_CTRL ioctl). Control class V4L2_CTRL_CLASS_MPEG contains all controls relating to MPEG encoding, etc. All controls in the control array must belong to the specified control class. An error is returned if this is not the case. It is also possible to use an empty control array (count == 0) to check whether the specified control class is supported. The control array is a &v4l2-ext-control; array. The v4l2_ext_control structure is very similar to &v4l2-control;, except for the fact that it also allows for 64-bit values and pointers to be passed. It is important to realize that due to the flexibility of controls it is necessary to check whether the control you want to set actually is supported in the driver and what the valid range of values is. So use the &VIDIOC-QUERYCTRL; and &VIDIOC-QUERYMENU; ioctls to check this. Also note that it is possible that some of the menu indices in a control of type V4L2_CTRL_TYPE_MENU may not be supported (VIDIOC_QUERYMENU will return an error). A good example is the list of supported MPEG audio bitrates. Some drivers only support one or two bitrates, others support a wider range. All controls use machine endianness.

The recommended way to enumerate over the extended controls is by using &VIDIOC-QUERYCTRL; in combination with the V4L2_CTRL_FLAG_NEXT_CTRL flag: &v4l2-queryctrl; qctrl; qctrl.id = V4L2_CTRL_FLAG_NEXT_CTRL; while (0 == ioctl (fd, &VIDIOC-QUERYCTRL;, &qctrl)) { /* ... */ qctrl.id |= V4L2_CTRL_FLAG_NEXT_CTRL; } The initial control ID is set to 0 ORed with the V4L2_CTRL_FLAG_NEXT_CTRL flag. The VIDIOC_QUERYCTRL ioctl will return the first control with a higher ID than the specified one. When no such controls are found an error is returned. If you want to get all controls within a specific control class, then you can set the initial qctrl.id value to the control class and add an extra check to break out of the loop when a control of another control class is found: qctrl.id = V4L2_CTRL_CLASS_MPEG | V4L2_CTRL_FLAG_NEXT_CTRL; while (0 == ioctl (fd, &VIDIOC-QUERYCTRL;, &qctrl)) { if (V4L2_CTRL_ID2CLASS (qctrl.id) != V4L2_CTRL_CLASS_MPEG) break; /* ... */ qctrl.id |= V4L2_CTRL_FLAG_NEXT_CTRL; } The 32-bit qctrl.id value is subdivided into three bit ranges: the top 4 bits are reserved for flags (⪚ V4L2_CTRL_FLAG_NEXT_CTRL) and are not actually part of the ID. The remaining 28 bits form the control ID, of which the most significant 12 bits define the control class and the least significant 16 bits identify the control within the control class. It is guaranteed that these last 16 bits are always non-zero for controls. The range of 0x1000 and up are reserved for driver-specific controls. The macro V4L2_CTRL_ID2CLASS(id) returns the control class ID based on a control ID. If the driver does not support extended controls, then VIDIOC_QUERYCTRL will fail when used in combination with V4L2_CTRL_FLAG_NEXT_CTRL. In that case the old method of enumerating control should be used (see 1.8). But if it is supported, then it is guaranteed to enumerate over all controls, including driver-private controls.

It is possible to create control panels for a graphical user interface where the user can select the various controls. Basically you will have to iterate over all controls using the method described above. Each control class starts with a control of type V4L2_CTRL_TYPE_CTRL_CLASS. VIDIOC_QUERYCTRL will return the name of this control class which can be used as the title of a tab page within a control panel. The flags field of &v4l2-queryctrl; also contains hints on the behavior of the control. See the &VIDIOC-QUERYCTRL; documentation for more details.

Below all controls within the MPEG control class are described. First the generic controls, then controls specific for certain hardware.

ID Type Description V4L2_CID_MPEG_CLASS  class The MPEG class descriptor. Calling &VIDIOC-QUERYCTRL; for this control will return a description of this control class. This description can be used as the caption of a Tab page in a GUI, for example. V4L2_CID_MPEG_STREAM_TYPE  enum v4l2_mpeg_stream_type The MPEG-1, -2 or -4 output stream type. One cannot assume anything here. Each hardware MPEG encoder tends to support different subsets of the available MPEG stream types. The currently defined stream types are: V4L2_MPEG_STREAM_TYPE_MPEG2_PS  MPEG-2 program stream V4L2_MPEG_STREAM_TYPE_MPEG2_TS  MPEG-2 transport stream V4L2_MPEG_STREAM_TYPE_MPEG1_SS  MPEG-1 system stream V4L2_MPEG_STREAM_TYPE_MPEG2_DVD  MPEG-2 DVD-compatible stream V4L2_MPEG_STREAM_TYPE_MPEG1_VCD  MPEG-1 VCD-compatible stream V4L2_MPEG_STREAM_TYPE_MPEG2_SVCD  MPEG-2 SVCD-compatible stream V4L2_CID_MPEG_STREAM_PID_PMT  integer Program Map Table Packet ID for the MPEG transport stream (default 16) V4L2_CID_MPEG_STREAM_PID_AUDIO  integer Audio Packet ID for the MPEG transport stream (default 256) V4L2_CID_MPEG_STREAM_PID_VIDEO  integer Video Packet ID for the MPEG transport stream (default 260) V4L2_CID_MPEG_STREAM_PID_PCR  integer Packet ID for the MPEG transport stream carrying PCR fields (default 259) V4L2_CID_MPEG_STREAM_PES_ID_AUDIO  integer Audio ID for MPEG PES V4L2_CID_MPEG_STREAM_PES_ID_VIDEO  integer Video ID for MPEG PES V4L2_CID_MPEG_STREAM_VBI_FMT  enum v4l2_mpeg_stream_vbi_fmt Some cards can embed VBI data (⪚ Closed Caption, Teletext) into the MPEG stream. This control selects whether VBI data should be embedded, and if so, what embedding method should be used. The list of possible VBI formats depends on the driver. The currently defined VBI format types are: V4L2_MPEG_STREAM_VBI_FMT_NONE  No VBI in the MPEG stream V4L2_MPEG_STREAM_VBI_FMT_IVTV  VBI in private packets, IVTV format (documented in the kernel sources in the file Documentation/video4linux/cx2341x/README.vbi) V4L2_CID_MPEG_AUDIO_SAMPLING_FREQ  enum v4l2_mpeg_audio_sampling_freq MPEG Audio sampling frequency. Possible values are: V4L2_MPEG_AUDIO_SAMPLING_FREQ_44100  44.1 kHz V4L2_MPEG_AUDIO_SAMPLING_FREQ_48000  48 kHz V4L2_MPEG_AUDIO_SAMPLING_FREQ_32000  32 kHz V4L2_CID_MPEG_AUDIO_ENCODING  enum v4l2_mpeg_audio_encoding MPEG Audio encoding. Possible values are: V4L2_MPEG_AUDIO_ENCODING_LAYER_1  MPEG-1/2 Layer I encoding V4L2_MPEG_AUDIO_ENCODING_LAYER_2  MPEG-1/2 Layer II encoding V4L2_MPEG_AUDIO_ENCODING_LAYER_3  MPEG-1/2 Layer III encoding V4L2_MPEG_AUDIO_ENCODING_AAC  MPEG-2/4 AAC (Advanced Audio Coding) V4L2_MPEG_AUDIO_ENCODING_AC3  AC-3 aka ATSC A/52 encoding V4L2_CID_MPEG_AUDIO_L1_BITRATE  enum v4l2_mpeg_audio_l1_bitrate MPEG-1/2 Layer I bitrate. Possible values are: V4L2_MPEG_AUDIO_L1_BITRATE_32K  32 kbit/s V4L2_MPEG_AUDIO_L1_BITRATE_64K  64 kbit/s V4L2_MPEG_AUDIO_L1_BITRATE_96K  96 kbit/s V4L2_MPEG_AUDIO_L1_BITRATE_128K  128 kbit/s V4L2_MPEG_AUDIO_L1_BITRATE_160K  160 kbit/s V4L2_MPEG_AUDIO_L1_BITRATE_192K  192 kbit/s V4L2_MPEG_AUDIO_L1_BITRATE_224K  224 kbit/s V4L2_MPEG_AUDIO_L1_BITRATE_256K  256 kbit/s V4L2_MPEG_AUDIO_L1_BITRATE_288K  288 kbit/s V4L2_MPEG_AUDIO_L1_BITRATE_320K  320 kbit/s V4L2_MPEG_AUDIO_L1_BITRATE_352K  352 kbit/s V4L2_MPEG_AUDIO_L1_BITRATE_384K  384 kbit/s V4L2_MPEG_AUDIO_L1_BITRATE_416K  416 kbit/s V4L2_MPEG_AUDIO_L1_BITRATE_448K  448 kbit/s V4L2_CID_MPEG_AUDIO_L2_BITRATE  enum v4l2_mpeg_audio_l2_bitrate MPEG-1/2 Layer II bitrate. Possible values are: V4L2_MPEG_AUDIO_L2_BITRATE_32K  32 kbit/s V4L2_MPEG_AUDIO_L2_BITRATE_48K  48 kbit/s V4L2_MPEG_AUDIO_L2_BITRATE_56K  56 kbit/s V4L2_MPEG_AUDIO_L2_BITRATE_64K  64 kbit/s V4L2_MPEG_AUDIO_L2_BITRATE_80K  80 kbit/s V4L2_MPEG_AUDIO_L2_BITRATE_96K  96 kbit/s V4L2_MPEG_AUDIO_L2_BITRATE_112K  112 kbit/s V4L2_MPEG_AUDIO_L2_BITRATE_128K  128 kbit/s V4L2_MPEG_AUDIO_L2_BITRATE_160K  160 kbit/s V4L2_MPEG_AUDIO_L2_BITRATE_192K  192 kbit/s V4L2_MPEG_AUDIO_L2_BITRATE_224K  224 kbit/s V4L2_MPEG_AUDIO_L2_BITRATE_256K  256 kbit/s V4L2_MPEG_AUDIO_L2_BITRATE_320K  320 kbit/s V4L2_MPEG_AUDIO_L2_BITRATE_384K  384 kbit/s V4L2_CID_MPEG_AUDIO_L3_BITRATE  enum v4l2_mpeg_audio_l3_bitrate MPEG-1/2 Layer III bitrate. Possible values are: V4L2_MPEG_AUDIO_L3_BITRATE_32K  32 kbit/s V4L2_MPEG_AUDIO_L3_BITRATE_40K  40 kbit/s V4L2_MPEG_AUDIO_L3_BITRATE_48K  48 kbit/s V4L2_MPEG_AUDIO_L3_BITRATE_56K  56 kbit/s V4L2_MPEG_AUDIO_L3_BITRATE_64K  64 kbit/s V4L2_MPEG_AUDIO_L3_BITRATE_80K  80 kbit/s V4L2_MPEG_AUDIO_L3_BITRATE_96K  96 kbit/s V4L2_MPEG_AUDIO_L3_BITRATE_112K  112 kbit/s V4L2_MPEG_AUDIO_L3_BITRATE_128K  128 kbit/s V4L2_MPEG_AUDIO_L3_BITRATE_160K  160 kbit/s V4L2_MPEG_AUDIO_L3_BITRATE_192K  192 kbit/s V4L2_MPEG_AUDIO_L3_BITRATE_224K  224 kbit/s V4L2_MPEG_AUDIO_L3_BITRATE_256K  256 kbit/s V4L2_MPEG_AUDIO_L3_BITRATE_320K  320 kbit/s V4L2_CID_MPEG_AUDIO_AAC_BITRATE  integer AAC bitrate in bits per second. V4L2_CID_MPEG_AUDIO_AC3_BITRATE  enum v4l2_mpeg_audio_ac3_bitrate AC-3 bitrate. Possible values are: V4L2_MPEG_AUDIO_AC3_BITRATE_32K  32 kbit/s V4L2_MPEG_AUDIO_AC3_BITRATE_40K  40 kbit/s V4L2_MPEG_AUDIO_AC3_BITRATE_48K  48 kbit/s V4L2_MPEG_AUDIO_AC3_BITRATE_56K  56 kbit/s V4L2_MPEG_AUDIO_AC3_BITRATE_64K  64 kbit/s V4L2_MPEG_AUDIO_AC3_BITRATE_80K  80 kbit/s V4L2_MPEG_AUDIO_AC3_BITRATE_96K  96 kbit/s V4L2_MPEG_AUDIO_AC3_BITRATE_112K  112 kbit/s V4L2_MPEG_AUDIO_AC3_BITRATE_128K  128 kbit/s V4L2_MPEG_AUDIO_AC3_BITRATE_160K  160 kbit/s V4L2_MPEG_AUDIO_AC3_BITRATE_192K  192 kbit/s V4L2_MPEG_AUDIO_AC3_BITRATE_224K  224 kbit/s V4L2_MPEG_AUDIO_AC3_BITRATE_256K  256 kbit/s V4L2_MPEG_AUDIO_AC3_BITRATE_320K  320 kbit/s V4L2_MPEG_AUDIO_AC3_BITRATE_384K  384 kbit/s V4L2_MPEG_AUDIO_AC3_BITRATE_448K  448 kbit/s V4L2_MPEG_AUDIO_AC3_BITRATE_512K  512 kbit/s V4L2_MPEG_AUDIO_AC3_BITRATE_576K  576 kbit/s V4L2_MPEG_AUDIO_AC3_BITRATE_640K  640 kbit/s V4L2_CID_MPEG_AUDIO_MODE  enum v4l2_mpeg_audio_mode MPEG Audio mode. Possible values are: V4L2_MPEG_AUDIO_MODE_STEREO  Stereo V4L2_MPEG_AUDIO_MODE_JOINT_STEREO  Joint Stereo V4L2_MPEG_AUDIO_MODE_DUAL  Bilingual V4L2_MPEG_AUDIO_MODE_MONO  Mono V4L2_CID_MPEG_AUDIO_MODE_EXTENSION  enum v4l2_mpeg_audio_mode_extension Joint Stereo audio mode extension. In Layer I and II they indicate which subbands are in intensity stereo. All other subbands are coded in stereo. Layer III is not (yet) supported. Possible values are: V4L2_MPEG_AUDIO_MODE_EXTENSION_BOUND_4  Subbands 4-31 in intensity stereo V4L2_MPEG_AUDIO_MODE_EXTENSION_BOUND_8  Subbands 8-31 in intensity stereo V4L2_MPEG_AUDIO_MODE_EXTENSION_BOUND_12  Subbands 12-31 in intensity stereo V4L2_MPEG_AUDIO_MODE_EXTENSION_BOUND_16  Subbands 16-31 in intensity stereo V4L2_CID_MPEG_AUDIO_EMPHASIS  enum v4l2_mpeg_audio_emphasis Audio Emphasis. Possible values are: V4L2_MPEG_AUDIO_EMPHASIS_NONE  None V4L2_MPEG_AUDIO_EMPHASIS_50_DIV_15_uS  50/15 microsecond emphasis V4L2_MPEG_AUDIO_EMPHASIS_CCITT_J17  CCITT J.17 V4L2_CID_MPEG_AUDIO_CRC  enum v4l2_mpeg_audio_crc CRC method. Possible values are: V4L2_MPEG_AUDIO_CRC_NONE  None V4L2_MPEG_AUDIO_CRC_CRC16  16 bit parity check V4L2_CID_MPEG_AUDIO_MUTE  boolean Mutes the audio when capturing. This is not done by muting audio hardware, which can still produce a slight hiss, but in the encoder itself, guaranteeing a fixed and reproducible audio bitstream. 0 = unmuted, 1 = muted. V4L2_CID_MPEG_VIDEO_ENCODING  enum v4l2_mpeg_video_encoding MPEG Video encoding method. Possible values are: V4L2_MPEG_VIDEO_ENCODING_MPEG_1  MPEG-1 Video encoding V4L2_MPEG_VIDEO_ENCODING_MPEG_2  MPEG-2 Video encoding V4L2_MPEG_VIDEO_ENCODING_MPEG_4_AVC  MPEG-4 AVC (H.264) Video encoding V4L2_CID_MPEG_VIDEO_ASPECT  enum v4l2_mpeg_video_aspect Video aspect. Possible values are: V4L2_MPEG_VIDEO_ASPECT_1x1  V4L2_MPEG_VIDEO_ASPECT_4x3  V4L2_MPEG_VIDEO_ASPECT_16x9  V4L2_MPEG_VIDEO_ASPECT_221x100  V4L2_CID_MPEG_VIDEO_B_FRAMES  integer Number of B-Frames (default 2) V4L2_CID_MPEG_VIDEO_GOP_SIZE  integer GOP size (default 12) V4L2_CID_MPEG_VIDEO_GOP_CLOSURE  boolean GOP closure (default 1) V4L2_CID_MPEG_VIDEO_PULLDOWN  boolean Enable 3:2 pulldown (default 0) V4L2_CID_MPEG_VIDEO_BITRATE_MODE  enum v4l2_mpeg_video_bitrate_mode Video bitrate mode. Possible values are: V4L2_MPEG_VIDEO_BITRATE_MODE_VBR  Variable bitrate V4L2_MPEG_VIDEO_BITRATE_MODE_CBR  Constant bitrate V4L2_CID_MPEG_VIDEO_BITRATE  integer Video bitrate in bits per second. V4L2_CID_MPEG_VIDEO_BITRATE_PEAK  integer Peak video bitrate in bits per second. Must be larger or equal to the average video bitrate. It is ignored if the video bitrate mode is set to constant bitrate. V4L2_CID_MPEG_VIDEO_TEMPORAL_DECIMATION  integer For every captured frame, skip this many subsequent frames (default 0). V4L2_CID_MPEG_VIDEO_MUTE  boolean "Mutes" the video to a fixed color when capturing. This is useful for testing, to produce a fixed video bitstream. 0 = unmuted, 1 = muted. V4L2_CID_MPEG_VIDEO_MUTE_YUV  integer Sets the "mute" color of the video. The supplied 32-bit integer is interpreted as follows (bit 0 = least significant bit): Bit 0:7 V chrominance information Bit 8:15 U chrominance information Bit 16:23 Y luminance information Bit 24:31 Must be zero.

The following MPEG class controls deal with MPEG encoding settings that are specific to the Conexant CX23415 and CX23416 MPEG encoding chips. ID Type Description V4L2_CID_MPEG_CX2341X_VIDEO_SPATIAL_FILTER_MODE  enum v4l2_mpeg_cx2341x_video_spatial_filter_mode Sets the Spatial Filter mode (default MANUAL). Possible values are: V4L2_MPEG_CX2341X_VIDEO_SPATIAL_FILTER_MODE_MANUAL  Choose the filter manually V4L2_MPEG_CX2341X_VIDEO_SPATIAL_FILTER_MODE_AUTO  Choose the filter automatically V4L2_CID_MPEG_CX2341X_VIDEO_SPATIAL_FILTER  integer (0-15) The setting for the Spatial Filter. 0 = off, 15 = maximum. (Default is 0.) V4L2_CID_MPEG_CX2341X_VIDEO_LUMA_SPATIAL_FILTER_TYPE  enum v4l2_mpeg_cx2341x_video_luma_spatial_filter_type Select the algorithm to use for the Luma Spatial Filter (default 1D_HOR). Possible values: V4L2_MPEG_CX2341X_VIDEO_LUMA_SPATIAL_FILTER_TYPE_OFF  No filter V4L2_MPEG_CX2341X_VIDEO_LUMA_SPATIAL_FILTER_TYPE_1D_HOR  One-dimensional horizontal V4L2_MPEG_CX2341X_VIDEO_LUMA_SPATIAL_FILTER_TYPE_1D_VERT  One-dimensional vertical V4L2_MPEG_CX2341X_VIDEO_LUMA_SPATIAL_FILTER_TYPE_2D_HV_SEPARABLE  Two-dimensional separable V4L2_MPEG_CX2341X_VIDEO_LUMA_SPATIAL_FILTER_TYPE_2D_SYM_NON_SEPARABLE  Two-dimensional symmetrical non-separable V4L2_CID_MPEG_CX2341X_VIDEO_CHROMA_SPATIAL_FILTER_TYPE  enum v4l2_mpeg_cx2341x_video_chroma_spatial_filter_type Select the algorithm for the Chroma Spatial Filter (default 1D_HOR). Possible values are: V4L2_MPEG_CX2341X_VIDEO_CHROMA_SPATIAL_FILTER_TYPE_OFF  No filter V4L2_MPEG_CX2341X_VIDEO_CHROMA_SPATIAL_FILTER_TYPE_1D_HOR  One-dimensional horizontal V4L2_CID_MPEG_CX2341X_VIDEO_TEMPORAL_FILTER_MODE  enum v4l2_mpeg_cx2341x_video_temporal_filter_mode Sets the Temporal Filter mode (default MANUAL). Possible values are: V4L2_MPEG_CX2341X_VIDEO_TEMPORAL_FILTER_MODE_MANUAL  Choose the filter manually V4L2_MPEG_CX2341X_VIDEO_TEMPORAL_FILTER_MODE_AUTO  Choose the filter automatically V4L2_CID_MPEG_CX2341X_VIDEO_TEMPORAL_FILTER  integer (0-31) The setting for the Temporal Filter. 0 = off, 31 = maximum. (Default is 8 for full-scale capturing and 0 for scaled capturing.) V4L2_CID_MPEG_CX2341X_VIDEO_MEDIAN_FILTER_TYPE  enum v4l2_mpeg_cx2341x_video_median_filter_type Median Filter Type (default OFF). Possible values are: V4L2_MPEG_CX2341X_VIDEO_MEDIAN_FILTER_TYPE_OFF  No filter V4L2_MPEG_CX2341X_VIDEO_MEDIAN_FILTER_TYPE_HOR  Horizontal filter V4L2_MPEG_CX2341X_VIDEO_MEDIAN_FILTER_TYPE_VERT  Vertical filter V4L2_MPEG_CX2341X_VIDEO_MEDIAN_FILTER_TYPE_HOR_VERT  Horizontal and vertical filter V4L2_MPEG_CX2341X_VIDEO_MEDIAN_FILTER_TYPE_DIAG  Diagonal filter V4L2_CID_MPEG_CX2341X_VIDEO_LUMA_MEDIAN_FILTER_BOTTOM  integer (0-255) Threshold above which the luminance median filter is enabled (default 0) V4L2_CID_MPEG_CX2341X_VIDEO_LUMA_MEDIAN_FILTER_TOP  integer (0-255) Threshold below which the luminance median filter is enabled (default 255) V4L2_CID_MPEG_CX2341X_VIDEO_CHROMA_MEDIAN_FILTER_BOTTOM  integer (0-255) Threshold above which the chroma median filter is enabled (default 0) V4L2_CID_MPEG_CX2341X_VIDEO_CHROMA_MEDIAN_FILTER_TOP  integer (0-255) Threshold below which the chroma median filter is enabled (default 255) V4L2_CID_MPEG_CX2341X_STREAM_INSERT_NAV_PACKETS  boolean The CX2341X MPEG encoder can insert one empty MPEG-2 PES packet into the stream between every four video frames. The packet size is 2048 bytes, including the packet_start_code_prefix and stream_id fields. The stream_id is 0xBF (private stream 2). The payload consists of 0x00 bytes, to be filled in by the application. 0 = do not insert, 1 = insert packets.

The Camera class includes controls for mechanical (or equivalent digital) features of a device such as controllable lenses or sensors. ID Type Description V4L2_CID_CAMERA_CLASS  class The Camera class descriptor. Calling &VIDIOC-QUERYCTRL; for this control will return a description of this control class. V4L2_CID_EXPOSURE_AUTO  enum v4l2_exposure_auto_type Enables automatic adjustments of the exposure time and/or iris aperture. The effect of manual changes of the exposure time or iris aperture while these features are enabled is undefined, drivers should ignore such requests. Possible values are: V4L2_EXPOSURE_AUTO  Automatic exposure time, automatic iris aperture. V4L2_EXPOSURE_MANUAL  Manual exposure time, manual iris. V4L2_EXPOSURE_SHUTTER_PRIORITY  Manual exposure time, auto iris. V4L2_EXPOSURE_APERTURE_PRIORITY  Auto exposure time, manual iris. V4L2_CID_EXPOSURE_ABSOLUTE  integer Determines the exposure time of the camera sensor. The exposure time is limited by the frame interval. Drivers should interpret the values as 100 µs units, where the value 1 stands for 1/10000th of a second, 10000 for 1 second and 100000 for 10 seconds. V4L2_CID_EXPOSURE_AUTO_PRIORITY  boolean When V4L2_CID_EXPOSURE_AUTO is set to AUTO or APERTURE_PRIORITY, this control determines if the device may dynamically vary the frame rate. By default this feature is disabled (0) and the frame rate must remain constant. V4L2_CID_PAN_RELATIVE  integer This control turns the camera horizontally by the specified amount. The unit is undefined. A positive value moves the camera to the right (clockwise when viewed from above), a negative value to the left. A value of zero does not cause motion. This is a write-only control. V4L2_CID_TILT_RELATIVE  integer This control turns the camera vertically by the specified amount. The unit is undefined. A positive value moves the camera up, a negative value down. A value of zero does not cause motion. This is a write-only control. V4L2_CID_PAN_RESET  button When this control is set, the camera moves horizontally to the default position. V4L2_CID_TILT_RESET  button When this control is set, the camera moves vertically to the default position. V4L2_CID_PAN_ABSOLUTE  integer This control turns the camera horizontally to the specified position. Positive values move the camera to the right (clockwise when viewed from above), negative values to the left. Drivers should interpret the values as arc seconds, with valid values between -180 * 3600 and +180 * 3600 inclusive. V4L2_CID_TILT_ABSOLUTE  integer This control turns the camera vertically to the specified position. Positive values move the camera up, negative values down. Drivers should interpret the values as arc seconds, with valid values between -180 * 3600 and +180 * 3600 inclusive. V4L2_CID_FOCUS_ABSOLUTE  integer This control sets the focal point of the camera to the specified position. The unit is undefined. Positive values set the focus closer to the camera, negative values towards infinity. V4L2_CID_FOCUS_RELATIVE  integer This control moves the focal point of the camera by the specified amount. The unit is undefined. Positive values move the focus closer to the camera, negative values towards infinity. This is a write-only control. V4L2_CID_FOCUS_AUTO  boolean Enables automatic focus adjustments. The effect of manual focus adjustments while this feature is enabled is undefined, drivers should ignore such requests. V4L2_CID_ZOOM_ABSOLUTE  integer Specify the objective lens focal length as an absolute value. The zoom unit is driver-specific and its value should be a positive integer. V4L2_CID_ZOOM_RELATIVE  integer Specify the objective lens focal length relatively to the current value. Positive values move the zoom lens group towards the telephoto direction, negative values towards the wide-angle direction. The zoom unit is driver-specific. This is a write-only control. V4L2_CID_ZOOM_CONTINUOUS  integer Move the objective lens group at the specified speed until it reaches physical device limits or until an explicit request to stop the movement. A positive value moves the zoom lens group towards the telephoto direction. A value of zero stops the zoom lens group movement. A negative value moves the zoom lens group towards the wide-angle direction. The zoom speed unit is driver-specific. V4L2_CID_IRIS_ABSOLUTE  integer This control sets the camera's aperture to the specified value. The unit is undefined. Larger values open the iris wider, smaller values close it. V4L2_CID_IRIS_RELATIVE  integer This control modifies the camera's aperture by the specified amount. The unit is undefined. Positive values open the iris one step further, negative values close it one step further. This is a write-only control. V4L2_CID_PRIVACY  boolean Prevent video from being acquired by the camera. When this control is set to TRUE (1), no image can be captured by the camera. Common means to enforce privacy are mechanical obturation of the sensor and firmware image processing, but the device is not restricted to these methods. Devices that implement the privacy control must support read access and may support write access. V4L2_CID_BAND_STOP_FILTER  integer Switch the band-stop filter of a camera sensor on or off, or specify its strength. Such band-stop filters can be used, for example, to filter out the fluorescent light component.

The FM Transmitter (FM_TX) class includes controls for common features of FM transmissions capable devices. Currently this class includes parameters for audio compression, pilot tone generation, audio deviation limiter, RDS transmission and tuning power features. ID Type Description V4L2_CID_FM_TX_CLASS  class The FM_TX class descriptor. Calling &VIDIOC-QUERYCTRL; for this control will return a description of this control class. V4L2_CID_RDS_TX_DEVIATION  integer Configures RDS signal frequency deviation level in Hz. The range and step are driver-specific. V4L2_CID_RDS_TX_PI  integer Sets the RDS Programme Identification field for transmission. V4L2_CID_RDS_TX_PTY  integer Sets the RDS Programme Type field for transmission. This encodes up to 31 pre-defined programme types. V4L2_CID_RDS_TX_PS_NAME  string Sets the Programme Service name (PS_NAME) for transmission. It is intended for static display on a receiver. It is the primary aid to listeners in programme service identification and selection. In Annex E of , the RDS specification, there is a full description of the correct character encoding for Programme Service name strings. Also from RDS specification, PS is usually a single eight character text. However, it is also possible to find receivers which can scroll strings sized as 8 x N characters. So, this control must be configured with steps of 8 characters. The result is it must always contain a string with size multiple of 8. V4L2_CID_RDS_TX_RADIO_TEXT  string Sets the Radio Text info for transmission. It is a textual description of what is being broadcasted. RDS Radio Text can be applied when broadcaster wishes to transmit longer PS names, programme-related information or any other text. In these cases, RadioText should be used in addition to V4L2_CID_RDS_TX_PS_NAME. The encoding for Radio Text strings is also fully described in Annex E of . The length of Radio Text strings depends on which RDS Block is being used to transmit it, either 32 (2A block) or 64 (2B block). However, it is also possible to find receivers which can scroll strings sized as 32 x N or 64 x N characters. So, this control must be configured with steps of 32 or 64 characters. The result is it must always contain a string with size multiple of 32 or 64. V4L2_CID_AUDIO_LIMITER_ENABLED  boolean Enables or disables the audio deviation limiter feature. The limiter is useful when trying to maximize the audio volume, minimize receiver-generated distortion and prevent overmodulation. V4L2_CID_AUDIO_LIMITER_RELEASE_TIME  integer Sets the audio deviation limiter feature release time. Unit is in useconds. Step and range are driver-specific. V4L2_CID_AUDIO_LIMITER_DEVIATION  integer Configures audio frequency deviation level in Hz. The range and step are driver-specific. V4L2_CID_AUDIO_COMPRESSION_ENABLED  boolean Enables or disables the audio compression feature. This feature amplifies signals below the threshold by a fixed gain and compresses audio signals above the threshold by the ratio of Threshold/(Gain + Threshold). V4L2_CID_AUDIO_COMPRESSION_GAIN  integer Sets the gain for audio compression feature. It is a dB value. The range and step are driver-specific. V4L2_CID_AUDIO_COMPRESSION_THRESHOLD  integer Sets the threshold level for audio compression freature. It is a dB value. The range and step are driver-specific. V4L2_CID_AUDIO_COMPRESSION_ATTACK_TIME  integer Sets the attack time for audio compression feature. It is a useconds value. The range and step are driver-specific. V4L2_CID_AUDIO_COMPRESSION_RELEASE_TIME  integer Sets the release time for audio compression feature. It is a useconds value. The range and step are driver-specific. V4L2_CID_PILOT_TONE_ENABLED  boolean Enables or disables the pilot tone generation feature. V4L2_CID_PILOT_TONE_DEVIATION  integer Configures pilot tone frequency deviation level. Unit is in Hz. The range and step are driver-specific. V4L2_CID_PILOT_TONE_FREQUENCY  integer Configures pilot tone frequency value. Unit is in Hz. The range and step are driver-specific. V4L2_CID_TUNE_PREEMPHASIS  integer Configures the pre-emphasis value for broadcasting. A pre-emphasis filter is applied to the broadcast to accentuate the high audio frequencies. Depending on the region, a time constant of either 50 or 75 useconds is used. The enum v4l2_preemphasis defines possible values for pre-emphasis. Here they are: V4L2_PREEMPHASIS_DISABLED  No pre-emphasis is applied. V4L2_PREEMPHASIS_50_uS  A pre-emphasis of 50 uS is used. V4L2_PREEMPHASIS_75_uS  A pre-emphasis of 75 uS is used. V4L2_CID_TUNE_POWER_LEVEL  integer Sets the output power level for signal transmission. Unit is in dBuV. Range and step are driver-specific. V4L2_CID_TUNE_ANTENNA_CAPACITOR  integer This selects the value of antenna tuning capacitor manually or automatically if set to zero. Unit, range and step are driver-specific. For more details about RDS specification, refer to document, from CENELEC.

This is an experimental interface and may change in the future. The V4L2 flash controls are intended to provide generic access to flash controller devices. Flash controller devices are typically used in digital cameras. The interface can support both LED and xenon flash devices. As of writing this, there is no xenon flash driver using this interface.

Unsynchronised LED flash is controlled directly by the host as the sensor. The flash must be enabled by the host before the exposure of the image starts and disabled once it ends. The host is fully responsible for the timing of the flash. Example of such device: Nokia N900.

The synchronised LED flash is pre-programmed by the host (power and timeout) but controlled by the sensor through a strobe signal from the sensor to the flash. The sensor controls the flash duration and timing. This information typically must be made available to the sensor.

LED flash may be used as torch in conjunction with another use case involving camera or individually.

ID Type Description V4L2_CID_FLASH_CLASS class The FLASH class descriptor. V4L2_CID_FLASH_LED_MODE menu Defines the mode of the flash LED, the high-power white LED attached to the flash controller. Setting this control may not be possible in presence of some faults. See V4L2_CID_FLASH_FAULT. V4L2_FLASH_LED_MODE_NONE Off. V4L2_FLASH_LED_MODE_FLASH Flash mode. V4L2_FLASH_LED_MODE_TORCH Torch mode. See V4L2_CID_FLASH_TORCH_INTENSITY. V4L2_CID_FLASH_STROBE_SOURCE menu Defines the source of the flash LED strobe. V4L2_FLASH_STROBE_SOURCE_SOFTWARE The flash strobe is triggered by using the V4L2_CID_FLASH_STROBE control. V4L2_FLASH_STROBE_SOURCE_EXTERNAL The flash strobe is triggered by an external source. Typically this is a sensor, which makes it possible to synchronises the flash strobe start to exposure start. V4L2_CID_FLASH_STROBE button Strobe flash. Valid when V4L2_CID_FLASH_LED_MODE is set to V4L2_FLASH_LED_MODE_FLASH and V4L2_CID_FLASH_STROBE_SOURCE is set to V4L2_FLASH_STROBE_SOURCE_SOFTWARE. Setting this control may not be possible in presence of some faults. See V4L2_CID_FLASH_FAULT. V4L2_CID_FLASH_STROBE_STOP button Stop flash strobe immediately. V4L2_CID_FLASH_STROBE_STATUS boolean Strobe status: whether the flash is strobing at the moment or not. This is a read-only control. V4L2_CID_FLASH_TIMEOUT integer Hardware timeout for flash. The flash strobe is stopped after this period of time has passed from the start of the strobe. V4L2_CID_FLASH_INTENSITY integer Intensity of the flash strobe when the flash LED is in flash mode (V4L2_FLASH_LED_MODE_FLASH). The unit should be milliamps (mA) if possible. V4L2_CID_FLASH_TORCH_INTENSITY integer Intensity of the flash LED in torch mode (V4L2_FLASH_LED_MODE_TORCH). The unit should be milliamps (mA) if possible. Setting this control may not be possible in presence of some faults. See V4L2_CID_FLASH_FAULT. V4L2_CID_FLASH_INDICATOR_INTENSITY integer Intensity of the indicator LED. The indicator LED may be fully independent of the flash LED. The unit should be microamps (uA) if possible. V4L2_CID_FLASH_FAULT bitmask Faults related to the flash. The faults tell about specific problems in the flash chip itself or the LEDs attached to it. Faults may prevent further use of some of the flash controls. In particular, V4L2_CID_FLASH_LED_MODE is set to V4L2_FLASH_LED_MODE_NONE if the fault affects the flash LED. Exactly which faults have such an effect is chip dependent. Reading the faults resets the control and returns the chip to a usable state if possible. V4L2_FLASH_FAULT_OVER_VOLTAGE Flash controller voltage to the flash LED has exceeded the limit specific to the flash controller. V4L2_FLASH_FAULT_TIMEOUT The flash strobe was still on when the timeout set by the user --- V4L2_CID_FLASH_TIMEOUT control --- has expired. Not all flash controllers may set this in all such conditions. V4L2_FLASH_FAULT_OVER_TEMPERATURE The flash controller has overheated. V4L2_FLASH_FAULT_SHORT_CIRCUIT The short circuit protection of the flash controller has been triggered. V4L2_CID_FLASH_CHARGE boolean Enable or disable charging of the xenon flash capacitor. V4L2_CID_FLASH_READY boolean Is the flash ready to strobe? Xenon flashes require their capacitors charged before strobing. LED flashes often require a cooldown period after strobe during which another strobe will not be possible. This is a read-only control.