Re: [PATCH] media: v4l2-mem2mem: always call poll_wait() on queues

[Hans Verkuil]

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