On Wed, Nov 11, 2020 at 9:47 PM Hans Verkuil <hverkuil-cisco@xxxxxxxxx> wrote: > > On 11/11/2020 13:41, Alexandre Courbot wrote: > > On Thu, Nov 5, 2020 at 11:05 PM Alexandre Courbot <gnurou@xxxxxxxxx> wrote: > >> > >> 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. > > > > Sorry, I noticed that this patch was marked with "Changes Requested" > > in patchwork, but isn't it valid as-is? We need a similar change to > > VB2, but that should go as a separate patch IMHO. I'm fine with doing > > both in one go if you prefer that though. > > > > In at least one reply you mentioned that you wanted to add a comment (reply > from 23 Oct). That's why I changed it to 'Changes Requested'. Ah, that's correct. Thanks for clarifying. > Also, I prefer to fix both m2m and vb2 at the same time (separate patches, > but part of the same patch series). And together with a separate patch improving > v4l2-compliance. Ack - will submit a series and a separate patch for v4l-utils.
