On Monday, December 20, 2010 13:35:28 Laurent Pinchart wrote: > Hi Hans, > > On Monday 20 December 2010 13:28:06 Hans Verkuil wrote: > > On Monday, December 20, 2010 13:10:32 Mauro Carvalho Chehab wrote: > > > Em 18-12-2010 08:45, Hans Verkuil escreveu: > > > > On Saturday, December 18, 2010 01:54:41 Laurent Pinchart wrote: > > > >> On Friday 17 December 2010 18:09:39 Mauro Carvalho Chehab wrote: > > > >>> > > > >>> I didn't find any regressions at the BKL removal patches, but I > > > >>> noticed a few issues with qv4l2, not all related to uvcvideo. The > > > >>> remaining of this email is an attempt to document them for later > > > >>> fixes. > > > >>> > > > >>> They don't seem to be regressions caused by BKL removal, but the > > > >>> better would be to fix them later. > > > >>> > > > >>> - with uvcvideo and two video apps, if qv4l2 is started first, the > > > >>> second application doesn't start/capture. I suspect that REQBUFS > > > >>> (used by qv4l2 to probe mmap/userptr capabilities) create some > > > >>> resource locking at uvcvideo. The proper way is to lock the > > > >>> resources only if the driver is streaming, as other drivers and > > > >>> videobuf do. > > > >> > > > >> I don't agree with that. The uvcvideo driver has one buffer queue per > > > >> device, so if an application requests buffers on one file handle it > > > >> will lock other applications out. If the driver didn't it would be > > > >> subject to race conditions. > > > > > > > > I agree with Laurent. Once an application calls REQBUFS with non-zero > > > > count, then it should lock the resources needed for streaming. The > > > > reason behind that is that REQBUFS also locks the current selected > > > > format in place, since the format determines the amount of memory > > > > needed for the buffers. > > > > > > qv4l2 calls REQBUFS(1), then REQBUFS(0). Well, this is currently wrong, > > > as most drivers will only release buffers at VIDIOC_STREAMOFF. > > > > qv4l2 first calls STREAMOFF, then REQBUFS(1), then REQBUFS(0). In the hope > > that one of these will actually free any buffers. It's random at the > > moment when drivers release buffers, one of the reasons for using vb2. > > > > > Anyway, even replacing > > > REQBUFS(0) with VIDIOC_STREAMOFF at qv4l2 won't help with uvcvideo. It > > > seems that, once buffers are requested at uvcvideo, they will release > > > only at close(). > > That's not correct. Buffers are released when calling REQBUFS(0). However, the > file handle is still marked as owning the device for streaming purpose, so > other applications can't change the format or request buffers. Why? After REQBUFS(0) any filehandle ownership should be dropped. Regards, Hans -- Hans Verkuil - video4linux developer - sponsored by Cisco -- 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
