Video4linux2 , get/set properties of images which are encoded by Camera - h.264

Question

I am trying to set properties of captured image in linux.
For example:
format, width, height, that could be achieved by:
VIDIOC_S_FMT/VIDIOC_G_FMT + struct v4l2_format fmt;
But, I am blocked in getting/setting more detail parameters:
like H264 key-frame period.
I found there are api to reach the goal.
that are v4l2_ext_controls, v4l2-ext-control and VIDIOC_G_EXT_CTRLS.
I have tried that, but that did not work in my example code.
My code is like this :
struct v4l2_ext_control extCtrl;
memset(&extCtrl, 0, sizeof(struct v4l2_ext_control));
extCtrl.id = V4L2_CID_MPEG_VIDEO_H264_I_PERIOD;
extCtrl.size = 0;
extCtrl.value = 2;
struct v4l2_ext_controls extCtrls;
extCtrls.controls = &extCtrl;
extCtrls.count = 1;
extCtrls.ctrl_class = V4L2_CTRL_CLASS_MPEG;
ret = ioctl(fd, VIDIOC_S_EXT_CTRLS, &extCtrls);
if (0 < ret)
{
printf("VIDIOC_S_EXT_CTRLS setting (%s)\n", strerror(errno));
return -3;
}/*if*/
ret = ioctl(fd, VIDIOC_G_EXT_CTRLS, &extCtrls);
if (0 < ret)
{
printf("VIDIOC_G_EXT_CTRLS setting (%s)\n", strerror(errno));
return -4;
}/*if*/
printf("extCtrl.value = %d\n", extCtrl.value );
That seems well, the key frame period be 2 (extCtrl.value).
But when I used
ffplay -skip_frame nokey -i saved_raw_h264
The key frame period is obviously much greater than 2.
May anyone help me?
By the way: The Logitech C920, is the only one camera I know, supports h264 output in consumer market.
Does any know other camera supporting h264?

Answer 1

Assuming you are setting the parameters correctly, it's very possible that the Logitech C920 Linux driver is ignoring some, if not many, of the control parameters you are passing in via V4L2. Do you have the driver source for the C920? Or is it using a generic Linux USB camera driver? You could at least see which V4L2 controls are supported by the driver.
edit:
Have you seen these threads which talk about adding C920 support to gstreamer?
http://sourceforge.net/p/linux-uvc/mailman/linux-uvc-devel/thread/505D0DAE.7020907#collabora.co.uk/
http://kakaroto.homelinux.net/2012/09/uvc-h264-encoding-cameras-support-in-gstreamer/