Re: [PATCH v6 0/5] Add ZynqMP VCU/Allegro DVT H.264 encoder driver

[Hans Verkuil <hverkuil@xs4all.nl>]

On 5/27/19 3:45 PM, Michael Tretter wrote:
> On Wed, 22 May 2019 15:49:45 +0200, Michael Tretter wrote:
>> On Wed, 22 May 2019 14:04:23 +0200, Hans Verkuil wrote:
>>> On 5/13/19 7:21 PM, Michael Tretter wrote:  
>>>> This is v6 of the Allegro DVT H.264 encoder driver found in the EV
>>>> family of the Xilinx ZynqMP platform.
>>>>
>>>> Only minor changes this time. I dropped the implementation of the
>>>> selection api, removed all references mentioning the decoder, and fixed
>>>> a few issues reported by sparse and smatch.
>>>>
>>>> The v4l2-compliance result using the current vicodec branch is
>>>>
>>>> v4l2-compliance SHA: c2ad13e4b7aef9ae160303189c67a91e1775f025, 64 bits
>>>>
>>>> Compliance test for allegro device /dev/video4:  
>> [...]
>>>> I observed that the "MMAP (select)" test occasionally fails, because the
>>>> test did not receive an V4L2_EVENT_EOS when dequeuing a buffer with
>>>> V4L2_BUF_FLAG_LAST being set. The driver always queues the event before
>>>> returning the last buffer and the "MMAP (epoll)" does not fail. Thus, I
>>>> decided to send the series anyway.    
>>>
>>> Where exactly does v4l2-compliance fail? This is weird, and I believe
>>> this warrants a bit more debugging. I recommend adding a debug
>>> statement in allegro_channel_buf_done() to see when a buffer is marked
>>> LAST.  
>>
>> v4l2-compliance fails in line 1074
>>
>> 	fail: v4l2-test-buffers.cpp(1074): !got_eos && !got_source_change
>>
>> The corresponding code in v4l2-compliance is
>>
>> 	if (buf.g_flags() & V4L2_BUF_FLAG_LAST) {
>> 		fail_on_test(buf.dqbuf(node) != EPIPE);
>>> 		fail_on_test(!got_eos && !got_source_change);  
>> 		if (!count)
>> 			break;
>> 		fail_on_test(node->streamoff(m2m_q.g_type()));
>> 		m2m_q.munmap_bufs(node);
>>
>> When the test fails, the select/epoll_wait returns with readable data,
>> but without readable events on the last buffer. If the test is
>> successful, data and events are available. This looks like a race
>> between the event and the LAST buffer and if the LAST buffer comes
>> first, the test fails.
>>
>> As said, the driver always queues the EOS event before calling
>> v4l2_m2m_buf_done() on the LAST buffer. Right now, I don't understand
>> how this can happen, but I will continue debugging.
> 
> There is a race between v4l2_m2m_poll() and allegro_channel_finish_frame().
> 
> v4l2_m2m_poll() first calls v4l2_event_pending() to check if events are
> available and afterwards checks if there are buffers on src_q and
> dst_q. If allegro_channel_finish_frame() queues the V4L2_EVENT_EOS
> after v4l2_event_pending() but before the checks on the queues,
> v4l2_m2m_poll() sets EPOLLIN | EPOLLRDNORM for the LAST buffer, but does
> not set EPOLLPRI, because it missed V4L2_EVENT_EOS.
> 
> As a fix, the driver must hold the m2m_ctx->q_lock mutex while calling
> v4l2_event_queue_fh() for V4L2_EVENT_EOS to ensure that the event is
> not queued during v4l2_m2m_poll() after the v4l2_event_pending() check.

Nice analysis!

I think this can be fixed fairly simply: just call v4l2_event_pending as
the last thing in v4l2_m2m_poll() and in vb2_poll().

That will ensure that no events are missed by poll.

> 
> I'm not completely sure, but it seems to me that other v4l2 mem2mem
> drivers have the same issue.

Most likely, yes.

The good news is that this is not a driver bug, so I'll make a pull request
for this series.

It would be great if you can make two patches (one for vb2_poll, one for
v4l2_m2m_poll) that changes this behavior.

You can test it with your driver to verify that this indeed fixes the problem.

Regards,

	Hans

> 
> Michael
> 
>>
>>>
>>> These tests really should not fail, and it is a strong indication of a
>>> bug somewhere.
>>>
>>> I don't want to merge a driver that has a FAIL in v4l2-compliance without
>>> at the very least understanding why that happens. Ignoring it defeats the
>>> purpose of v4l2-compliance.  
>>
>> Totally agreed.
>>
>> Michael
>>
>>>
>>> Regards,
>>>
>>> 	Hans
>>>   
>>