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From: Michael Tretter <m.tretter@pengutronix.de>
To: Hans Verkuil <hverkuil@xs4all.nl>
Cc: linux-media@vger.kernel.org, devicetree@vger.kernel.org,
	kernel@pengutronix.de, robh+dt@kernel.org, mchehab@kernel.org,
	tfiga@chromium.org, dshah@xilinx.com
Subject: Re: [PATCH v6 0/5] Add ZynqMP VCU/Allegro DVT H.264 encoder driver
Date: Mon, 27 May 2019 15:45:57 +0200
Message-ID: <20190527154557.62278063@litschi.hi.pengutronix.de> (raw)
In-Reply-To: <20190522154945.54ac67d7@litschi.hi.pengutronix.de>

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
> >   
> 

  reply index

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-05-13 17:21 Michael Tretter
2019-05-13 17:21 ` [PATCH v6 1/5] videobuf2-v4l2: set last_buffer_dequeued in dqbuf Michael Tretter
2019-05-13 17:21 ` [PATCH v6 2/5] media: dt-bindings: media: document allegro-dvt bindings Michael Tretter
2019-05-13 17:21 ` [PATCH v6 3/5] media: dt-bindings: media: Add vendor prefix for allegro Michael Tretter
2019-05-17  2:31   ` Rob Herring
2019-05-27 14:35   ` Hans Verkuil
2019-05-27 14:57     ` Michael Tretter
2019-05-27 14:58       ` Hans Verkuil
2019-05-13 17:21 ` [PATCH v6 4/5] [media] allegro: add Allegro DVT video IP core driver Michael Tretter
2019-05-13 17:21 ` [PATCH v6 5/5] [media] allegro: add SPS/PPS nal unit writer Michael Tretter
2019-05-22 12:04 ` [PATCH v6 0/5] Add ZynqMP VCU/Allegro DVT H.264 encoder driver Hans Verkuil
2019-05-22 13:49   ` Michael Tretter
2019-05-27 13:45     ` Michael Tretter [this message]
2019-05-27 14:30       ` Hans Verkuil

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