7.37. ioctl VIDIOC_G_PARM, VIDIOC_S_PARM¶
7.37.1. Name¶
VIDIOC_G_PARM - VIDIOC_S_PARM - Get or set streaming parameters
7.37.2. Synopsis¶
-
int
ioctl
(int fd, VIDIOC_G_PARM, v4l2_streamparm *argp)¶
-
int
ioctl
(int fd, VIDIOC_S_PARM, v4l2_streamparm *argp)¶
7.37.3. Arguments¶
fd
- File descriptor returned by open().
argp
- Pointer to struct
v4l2_streamparm
.
7.37.4. Description¶
The current video standard determines a nominal number of frames per second. If less than this number of frames is to be captured or output, applications can request frame skipping or duplicating on the driver side. This is especially useful when using the read() or write(), which are not augmented by timestamps or sequence counters, and to avoid unnecessary data copying.
Further these ioctls can be used to determine the number of buffers used internally by a driver in read/write mode. For implications see the section discussing the read() function.
To get and set the streaming parameters applications call the
VIDIOC_G_PARM and VIDIOC_S_PARM ioctl, respectively. They take a
pointer to a struct v4l2_streamparm
which contains a
union holding separate parameters for input and output devices.
-
v4l2_streamparm
¶
__u32 | type |
The buffer (stream) type, same as struct
v4l2_format type , set by the
application. See v4l2_buf_type |
|
union | parm |
||
struct v4l2_captureparm |
capture |
Parameters for capture devices, used when type is
V4L2_BUF_TYPE_VIDEO_CAPTURE . |
|
struct v4l2_outputparm |
output |
Parameters for output devices, used when type is
V4L2_BUF_TYPE_VIDEO_OUTPUT . |
|
__u8 | raw_data [200] |
A place holder for future extensions. |
-
v4l2_captureparm
¶
__u32 | capability |
See Streaming Parameters Capabilites. |
__u32 | capturemode |
Set by drivers and applications, see Capture Parameters Flags. |
struct v4l2_fract |
timeperframe |
This is the desired period between successive frames captured by the driver, in seconds. The field is intended to skip frames on the driver side, saving I/O bandwidth. Applications store here the desired frame period, drivers return
the actual frame period, which must be greater or equal to the
nominal frame period determined by the current video standard
(struct Drivers support this function only when they set the
|
__u32 | extendedmode |
Custom (driver specific) streaming parameters. When unused, applications and drivers must set this field to zero. Applications using this field should check the driver name and version, see Querying Capabilities. |
__u32 | readbuffers |
Applications set this field to the desired number of buffers used internally by the driver in read() mode. Drivers return the actual number of buffers. When an application requests zero buffers, drivers should just return the current setting rather than the minimum or an error code. For details see Read/Write. |
__u32 | reserved [4] |
Reserved for future extensions. Drivers and applications must set the array to zero. |
-
v4l2_outputparm
¶
__u32 | capability |
See Streaming Parameters Capabilites. |
__u32 | outputmode |
Set by drivers and applications, see Capture Parameters Flags. |
struct v4l2_fract |
timeperframe |
This is the desired period between successive frames output by the driver, in seconds. |
The field is intended to repeat frames on the driver side in write() mode (in streaming mode timestamps can be used to throttle the output), saving I/O bandwidth. Applications store here the desired frame period, drivers return
the actual frame period, which must be greater or equal to the
nominal frame period determined by the current video standard
(struct Drivers support this function only when they set the
|
||
__u32 | extendedmode |
Custom (driver specific) streaming parameters. When unused, applications and drivers must set this field to zero. Applications using this field should check the driver name and version, see Querying Capabilities. |
__u32 | writebuffers |
Applications set this field to the desired number of buffers used internally by the driver in write() mode. Drivers return the actual number of buffers. When an application requests zero buffers, drivers should just return the current setting rather than the minimum or an error code. For details see Read/Write. |
__u32 | reserved [4] |
Reserved for future extensions. Drivers and applications must set the array to zero. |
V4L2_CAP_TIMEPERFRAME |
0x1000 | The frame skipping/repeating controlled by the timeperframe
field is supported. |
V4L2_MODE_HIGHQUALITY |
0x0001 | High quality imaging mode. High quality mode is intended for still imaging applications. The idea is to get the best possible image quality that the hardware can deliver. It is not defined how the driver writer may achieve that; it will depend on the hardware and the ingenuity of the driver writer. High quality mode is a different mode from the regular motion video capture modes. In high quality mode:
|
7.37.5. Return Value¶
On success 0 is returned, on error -1 and the errno
variable is set
appropriately. The generic error codes are described at the
Generic Error Codes chapter.