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'. 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. Regards, Hans
