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: diff --git a/utils/v4l2-compliance/v4l2-test-buffers.cpp b/utils/v4l2-compliance/v4l2-test-buffers.cpp index 8000db23..b63326cd 100644 --- a/utils/v4l2-compliance/v4l2-test-buffers.cpp +++ b/utils/v4l2-compliance/v4l2-test-buffers.cpp @@ -903,6 +903,10 @@ static int captureBufs(struct node *node, struct node *node_m2m_cap, const cv4l_ epollfd = epoll_create1(0); fail_on_test(epollfd < 0); + + ev.events = 0; + fail_on_test(epoll_ctl(epollfd, EPOLL_CTL_ADD, node->g_fd(), &ev)); + if (node->is_m2m) ev.events = EPOLLIN | EPOLLOUT | EPOLLPRI; else if (v4l_type_is_output(q.g_type())) @@ -910,7 +914,7 @@ static int captureBufs(struct node *node, struct node *node_m2m_cap, const cv4l_ else ev.events = EPOLLIN; ev.data.fd = node->g_fd(); - fail_on_test(epoll_ctl(epollfd, EPOLL_CTL_ADD, node->g_fd(), &ev)); + fail_on_test(epoll_ctl(epollfd, EPOLL_CTL_MOD, node->g_fd(), &ev)); } if (pollmode) > > 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). I'll take a look there as well, and think about a proper comment before sending a patch towards you. Cheers, Alex.