Re: [RFD] frame-size switching: preview / single-shot use-case

[Guennadi Liakhovetski <g.liakhovetski@xxxxxx>]

Hi all

Let me try to further elaborate on this topic. So far, I think, the 
following provides the cleanest solution to our quick format-switching / 
multiple video-queue problem:

On Wed, 16 Feb 2011, Guennadi Liakhovetski wrote:

> (2) cleanly separate setting video data format (S_FMT) from specifying the 
> allocated buffer size.

The control flow then would look like:

	struct v4l2_requestbuffers req;

	fd = open();

	/* initialise and configure the first "still-shot" buffer-queue */
	bufq.id = -EINVAL;
	ret = ioctl(fd, VIDIOC_BUFQ_SELECT, &bufq);

	/*
	 * bufq.id now contains a handle to a newly allocated videobuf 
	 * queue, it also becomes the current queue.
	 */

	still_qid = buf.id;

	/*
	 * specify explicitly required buffer size - use one of reserved 
	 * fields
	 */
	req.size = still_x * still_y * bpp;
	ret = ioctl(fd, VIDIOC_REQBUFS, &req);

	for (i = 0; i < req.count; i++) {
		buf.index = i;
		ret = ioctl(fd, VIDIOC_QUERYBUF, &buf);
		start = mmap();
		ret = ioctl(fd, VIDIOC_QBUF, &buf);
	}

	/* Allocate a second "preview" queue */

	bufq.id = -EINVAL;
	ret = ioctl(fd, VIDIOC_BUFQ_SELECT, &bufq);

	preview_qid = bufq.id;

	req.size = preview_x * preview_y * bpp;
	ret = ioctl(fd, VIDIOC_REQBUFS, &req);

	for (i = 0; i < req.count; i++) {
		buf.index = i;
		ret = ioctl(fd, VIDIOC_QUERYBUF, &buf);
		start = mmap();
		ret = ioctl(fd, VIDIOC_QBUF, &buf);
	}

	/* Stay on the preview queue */
	ret = ioctl(fd, VIDIOC_STREAMON, &type);

	for (i = 0; i < n; i++) {
		ret = ioctl(fd, VIDIOC_DQBUF, &buf);
		ret = ioctl(fd, VIDIOC_QBUF, &buf);
	}

	ret = ioctl(fd, VIDIOC_STREAMOFF, &type);

	/* Switch to the still-shot queue */
	bufq.id = still_qid;
	ret = ioctl(fd, VIDIOC_BUFQ_SELECT, &bufq);

	ret = ioctl(fd, VIDIOC_STREAMON, &type);

	for (i = 0; i < n; i++) {
		ret = ioctl(fd, VIDIOC_DQBUF, &buf);
		ret = ioctl(fd, VIDIOC_QBUF, &buf);
	}

	ret = ioctl(fd, VIDIOC_STREAMOFF, &type);

And so on. If the above is _conceptually_ acceptable, we need 1 to 2 new 
ioctl()s and an extension to the VIDIOC_REQBUFS ioctl() to specify the 
buffer size. I wrote "1 or 2 new ioctl()s," because above the new 
VIDIOC_BUFQ_SELECT ioctl() performs two functions: if the handle is 
negative, it allocates a new queue, if it containes a valid (per-device or 
per file-handle?) queue ID, it switches to it. We might also decide to use 
two ioctl()s for these two functions.

Comments?

Thanks
Guennadi
---
Guennadi Liakhovetski, Ph.D.
Freelance Open-Source Software Developer
http://www.open-technology.de/
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