Hi Mauro, On Tuesday 02 June 2009 13:51:03 Mauro Carvalho Chehab wrote: > Em Tue, 2 Jun 2009 01:12:37 +0200 > > Laurent Pinchart <laurent.pinchart@xxxxxxxxx> escreveu: > > 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. > > It is better to assume that videobuf is right and fix the API spec. > Videobuf is there since kernel 2.4 and not much changed on it, in terms of > its operational mode. > > So, the existing applications assume videobuf behaviour. Any change would > break existing apps. > > So, it is better to fix the API spec to reflect the practical > implementation. Ok, fine with me. Cheers, 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
