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
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