Laurent Pinchart schrieb: > 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. > I actually noticed the same with our the omap3 camera driver. In userspace I now call VIDIOC_REQBUFS with V4L2_MEMORY_USERPTR and count=0, if I get EINVAL, try again with count!=0 and if EINVAL again try V4L2_MEMORY_MMAP (always with count). I also got feedback from the driver developers, that the buffers are still indexed and mlocked, so when I DQBUF and QBUF I should enqueue the same buffers for the same index again.
Stefan > 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 majord...@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
