Hi Stefan, On Monday 01 June 2009 09:26:10 Stefan Kost wrote: > hi, > > I have implemented support for V4L2_MEMORY_USERPTR buffers in gstreamers > v4l2src [1]. This allows to request shared memory buffers from xvideo, > capture into those and therefore save a memcpy. This works great with > the v4l2 driver on our embedded device. > > When I was testing this on my desktop, I noticed that almost no driver > seems to support it. > I tested zc0301 and uvcvideo, but also grepped the kernel driver > sources. It seems that gspca might support it, but I ave not confirmed > it. Is there a technical reason for it, or is it simply not implemented? For the uvcvideo driver it's simply not implemented. I was about to give it a try when I found out a mismatch between the V4L2 specification and the videobuf implementation (which I wanted to use as the reference implementation). The V4L2 specification states, in section 3.3, that "The driver must be switched into user pointer I/O mode by calling the VIDIOC_REQBUFS with the desired buffer type. No buffers are allocated beforehands, consequently they are not indexed and cannot be queried like mapped buffers with the VIDIOC_QUERYBUF ioctl." Example 3-2 shows that v4l2_requestbuffers::count is not used when using USERPTR. However, videobuf pre-allocates v4l2_requestbuffers::count kernel-side buffer descriptors when VIDIOC_REQBUFS is called with USERPTR. If someone could clarify which of the V4L2 specification or the videobuf implementation is right I could give USERPTR a try in the uvcvideo driver. Best regards, Laurent Pinchart -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
