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