7.51. ioctl VIDIOC_QUERYSTD, VIDIOC_SUBDEV_QUERYSTD¶
VIDIOC_QUERYSTD - VIDIOC_SUBDEV_QUERYSTD - Sense the video standard received by the current input
The hardware may be able to detect the current video standard automatically. To do so, applications call ioctl VIDIOC_QUERYSTD, VIDIOC_SUBDEV_QUERYSTD with a pointer to a v4l2_std_id type. The driver stores here a set of candidates, this can be a single flag or a set of supported standards if for example the hardware can only distinguish between 50 and 60 Hz systems. If no signal was detected, then the driver will return V4L2_STD_UNKNOWN. When detection is not possible or fails, the set must contain all standards supported by the current video input or output.
Drivers shall not switch the video standard
automatically if a new video standard is detected. Instead, drivers
should send the
V4L2_EVENT_SOURCE_CHANGE event (if they support
this) and expect that userspace will take action by calling
ioctl VIDIOC_QUERYSTD, VIDIOC_SUBDEV_QUERYSTD. The reason is that a new video standard can mean
different buffer sizes as well, and you cannot change buffer sizes on
the fly. In general, applications that receive the Source Change event
will have to call ioctl VIDIOC_QUERYSTD, VIDIOC_SUBDEV_QUERYSTD, and if the detected video
standard is valid they will have to stop streaming, set the new
standard, allocate new buffers and start streaming again.