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. Regards, Hans > > 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. >
