On Thu, Nov 5, 2020 at 9:36 PM Hans Verkuil <hverkuil-cisco@xxxxxxxxx> wrote: > > On 05/11/2020 13:21, Alexandre Courbot wrote: > > On Tue, Nov 3, 2020 at 6:48 PM Hans Verkuil <hverkuil-cisco@xxxxxxxxx> wrote: > >> > >> On 03/11/2020 09:51, Alexandre Courbot wrote: > >>> Hi Hans, > >>> > >>> On Sat, Oct 31, 2020 at 12:09 AM Hans Verkuil <hverkuil-cisco@xxxxxxxxx> wrote: > >>>> > >>>> On 22/10/2020 14:24, Alexandre Courbot wrote: > >>>>> do_poll()/do_select() seem to set the _qproc member of poll_table to > >>>>> NULL the first time they are called on a given table, making subsequent > >>>>> calls of poll_wait() on that table no-ops. This is a problem for mem2mem > >>>>> which calls poll_wait() on the V4L2 queues' waitqueues only when a > >>>>> queue-related event is requested, which may not necessarily be the case > >>>>> during the first poll. > >>>>> > >>>>> For instance, a stateful decoder is typically only interested in > >>>>> EPOLLPRI events when it starts, and will switch to listening to both > >>>>> EPOLLPRI and EPOLLIN after receiving the initial resolution change event > >>>>> and configuring the CAPTURE queue. However by the time that switch > >>>>> happens and v4l2_m2m_poll_for_data() is called for the first time, > >>>>> poll_wait() has become a no-op and the V4L2 queues waitqueues thus > >>>>> cannot be registered. > >>>>> > >>>>> Fix this by moving the registration to v4l2_m2m_poll() and do it whether > >>>>> or not one of the queue-related events are requested. > >>>> > >>>> This looks good, but would it be possible to add a test for this to > >>>> v4l2-compliance? (Look for POLL_MODE_EPOLL in v4l2-test-buffers.cpp) > >>>> > >>>> If I understand this right, calling EPOLL_CTL_ADD for EPOLLPRI, then > >>>> calling EPOLL_CTL_ADD for EPOLLIN/OUT would trigger this? Or does there > >>>> have to be an epoll_wait call in between? > >>> > >>> Even without an epoll_wait() in between the behavior is visible. > >>> v4l2_m2m_poll() will be called once during the initial EPOLL_CTL_ADD > >>> and this will trigger the bug. > >>> > >>>> Another reason for adding this test is that I wonder if regular capture > >>>> or output V4L2 devices don't have the same issue. > >>>> > >>>> It's a very subtle bug and so adding a test for this to v4l2-compliance > >>>> would be very useful. > >>> > >>> I fully agree, this is very counter-intuitive since what basically > >>> happens is that the kernel's poll_wait() function becomes a no-op > >>> after the poll() hook of a driver is called for the first time. There > >>> is no way one can expect this behavior just from browsing the code so > >>> this is likely to affect other drivers. > >>> > >>> As for the test itself, we can easily reproduce the conditions for > >>> failure in v4l2-test-buffers.cpp's captureBufs() function, but doing > >>> so will make the streaming tests fail without being specific about the > >>> cause. Or maybe we should add another pollmode to specifically test > >>> epoll in this setup? Can I get your thoughts? > >> > >> No, just keep it as part of the poll test. Just add comments at the place > >> where it fails describing this error. > >> > >> After all, it *is* a poll() bug, so it is only fair that it is tested as > >> part of the epoll test. > >> > >> Can you call EPOLL_CTL_ADD with ev.events set to 0? And then call it again > >> with the actual value that you need? If that triggers this issue as well, > >> then that is a nice test (but perhaps EPOLL_CTL_ADD won't call poll() if > >> ev.events is 0, but perhaps EPOLLERR would work instead of 0). > > > > Yup, actually the following is enough to make v4l2-compliance -s fail > > with vicodec: > > Does it also fail with vivid? I am curious to know whether this issue is > m2m specific or a more general problem. It does fail actually! And that made me notice that vb2_poll() uses the same pattern as v4l2_m2m_poll() (probably because the latter is inspired by the former?) and needs to be fixed similarly. I will send another patch to fix vb2_poll() as well, thanks for pointing it out!
