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. Regards, Hans
