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). The epoll_wait() will fail when this issue hits, so that's a good place to add comments explaining this problem. There is one other place where this needs to be tested: testEvents() in v4l2-test-controls.cpp: currently this only tests select(), but there should be a second epoll test here as well that just tests EPOLLPRI. This would catch drivers that do not stream (i.e. no EPOLLIN/OUT) but that do have controls (so support EPOLLPRI). Congratulation, you found a really nasty corner case! :-) Regards, Hans
