On Thu, Nov 5, 2020 at 10:12 PM Hans Verkuil <hverkuil-cisco@xxxxxxxxx> wrote: > > On 05/11/2020 13:52, Alexandre Courbot wrote: > > 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! > > I was afraid of that. > > Testing epoll for control events would be interesting as well. The > vivid radio device is an example of a device that has controls, but > does not do streaming (so is not using vb2). > > But from what I can see v4l2_ctrl_poll() does the right thing, so this > should be fine. Indeed, it unconditionally calls poll_wait() with all the wait queues that may wake us up (that is, only one), so there is no problem there.
