linux-kernel - Re: [PATCH 0/2] media: meson: vdec: Add compliant H264 support

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Date:   Mon, 21 Oct 2019 15:13:53 +0200
From:   Hans Verkuil <hverkuil@...all.nl>
To:     Maxime Jourdan <mjourdan@...libre.com>
Cc:     Mauro Carvalho Chehab <mchehab@...nel.org>,
        Hans Verkuil <hans.verkuil@...co.com>,
        Kevin Hilman <khilman@...libre.com>,
        Jerome Brunet <jbrunet@...libre.com>,
        Neil Armstrong <narmstrong@...libre.com>,
        Martin Blumenstingl <martin.blumenstingl@...glemail.com>,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arm-kernel@...ts.infradead.org,
        linux-amlogic@...ts.infradead.org
Subject: Re: [PATCH 0/2] media: meson: vdec: Add compliant H264 support

On 10/18/19 9:50 AM, Maxime Jourdan wrote:
> On Wed, Oct 9, 2019 at 2:01 PM Hans Verkuil <hverkuil@...all.nl> wrote:
>>
>> On 10/8/19 3:40 PM, Maxime Jourdan wrote:
>>> On 07/10/2019 18:39, Hans Verkuil wrote:
>>>> On 10/7/19 6:24 PM, Maxime Jourdan wrote:
>>>>> On 07/10/2019 17:12, Hans Verkuil wrote:
>>>>>> On 10/7/19 4:59 PM, Maxime Jourdan wrote:
>>>>>>> Hello,
>>>>>>>
>>>>>>> This patch series aims to bring H.264 support as well as compliance update
>>>>>>> to the amlogic stateful video decoder driver.
>>>>>>>
>>>>>>> There is 1 issue that remains currently:
>>>>>>>
>>>>>>>    - The following codepath had to be commented out from v4l2-compliance as
>>>>>>> it led to stalling:
>>>>>>>
>>>>>>> if (node->codec_mask & STATEFUL_DECODER) {
>>>>>>>      struct v4l2_decoder_cmd cmd;
>>>>>>>      buffer buf_cap(m2m_q);
>>>>>>>
>>>>>>>      memset(&cmd, 0, sizeof(cmd));
>>>>>>>      cmd.cmd = V4L2_DEC_CMD_STOP;
>>>>>>>
>>>>>>>      /* No buffers are queued, call STREAMON, then STOP */
>>>>>>>      fail_on_test(node->streamon(q.g_type()));
>>>>>>>      fail_on_test(node->streamon(m2m_q.g_type()));
>>>>>>>      fail_on_test(doioctl(node, VIDIOC_DECODER_CMD, &cmd));
>>>>>>>
>>>>>>>      fail_on_test(buf_cap.querybuf(node, 0));
>>>>>>>      fail_on_test(buf_cap.qbuf(node));
>>>>>>>      fail_on_test(buf_cap.dqbuf(node));
>>>>>>>      fail_on_test(!(buf_cap.g_flags() & V4L2_BUF_FLAG_LAST));
>>>>>>>      for (unsigned p = 0; p < buf_cap.g_num_planes(); p++)
>>>>>>>          fail_on_test(buf_cap.g_bytesused(p));
>>>>>>>      fail_on_test(node->streamoff(q.g_type()));
>>>>>>>      fail_on_test(node->streamoff(m2m_q.g_type()));
>>>>>>>
>>>>>>>      /* Call STREAMON, queue one CAPTURE buffer, then STOP */
>>>>>>>      fail_on_test(node->streamon(q.g_type()));
>>>>>>>      fail_on_test(node->streamon(m2m_q.g_type()));
>>>>>>>      fail_on_test(buf_cap.querybuf(node, 0));
>>>>>>>      fail_on_test(buf_cap.qbuf(node));
>>>>>>>      fail_on_test(doioctl(node, VIDIOC_DECODER_CMD, &cmd));
>>>>>>>
>>>>>>>      fail_on_test(buf_cap.dqbuf(node));
>>>>>>>      fail_on_test(!(buf_cap.g_flags() & V4L2_BUF_FLAG_LAST));
>>>>>>>      for (unsigned p = 0; p < buf_cap.g_num_planes(); p++)
>>>>>>>          fail_on_test(buf_cap.g_bytesused(p));
>>>>>>>      fail_on_test(node->streamoff(q.g_type()));
>>>>>>>      fail_on_test(node->streamoff(m2m_q.g_type()));
>>>>>>> }
>>>>>>>
>>>>>>> The reason for this is because the driver has a limitation where all
>>>>>>> capturebuffers must be queued to the driver before STREAMON is effective.
>>>>>>> The firmware needs to know in advance what all the buffers are before
>>>>>>> starting to decode.
>>>>>>> This limitation is enforced via q->min_buffers_needed.
>>>>>>> As such, in this compliance codepath, STREAMON is never actually called
>>>>>>> driver-side and there is a stall on fail_on_test(buf_cap.dqbuf(node));
>>>>>>
>>>>>> That's interesting. I will have to look more closely at this.
>>
>> This requires a helper function in videobuf2-v4l2.c.
>>
>> In vdec_decoder_cmd you would need code like this:
>>
>>         if (!vb2_start_streaming_called(&capture_queue)) {
>>                 vb2_dequeue_empty_last_buf(&capture_queue);
>>                 return 0;
>>         }
>>
>> The vb2_dequeue_empty_last_buf (function name can probably be improved upon!)
>> does nothing if no capture buffers were queued, otherwise it takes the first
>> buffer, sets the LAST flag and sets bytesused to 0 and marks it as DONE.
>>
>> The driver cannot do this directly, since the buffers were never queued to the
>> driver and are owned by vb2.
>>
>> This is something that needs to be done for any codec driver that sets
>> min_buffers_needed to a value > 1.
>>
>> The vb2 function would look something like this:
>>
>> void vb2_dqbuf_empty_last_buf(struct vb2_queue *q)
>> {
>>         struct vb2_buffer *vb;
>>         struct vb2_v4l2_buffer *vbuf;
>>         unsigned int i;
>>
>>         if (WARN_ON(q->is_output))
>>                 return;
>>         if (list_empty(&q->queued_list))
>>                 return;
>>         vb = list_first_entry(&q->queued_list, struct vb2_buffer, queued_entry);
>>         list_del(&vb->queued_entry);
>>         for (i = 0; i < vb->num_planes; i++)
>>                 vb2_set_plane_payload(vb, i, 0)
>>         vbuf = to_vb2_v4l2_buffer(vb);
>>         vbuf->flags |= V4L2_BUF_FLAG_LAST;
>>         vb2_buffer_done(vb, VB2_BUF_STATE_DONE);
>> }
>> EXPORT_SYMBOL_GPL(vb2_dqbuf_empty_last_buf);
>>
>> Neither compiled, nor tested, and I think this should be in v4l2-mem2mem.c instead of
>> in videobuf2-v4l2.c since this is very m2m specific.
>>
>> So see this as a suggestion :-)
>>
>> Anyway, the key take-away from this is that userspace does not know if your driver
>> behaves the way it does, so STOP should still produce a sane expected result.
>>
>> Which in this is just a single empty capture buffer marked LAST.
> 
> Thanks, this makes sense. It doesn't quite fit the current usage
> unfortunately as the test in v4l2-compliance goes like this:
> 
> fail_on_test(doioctl(node, VIDIOC_DECODER_CMD, &cmd));
> fail_on_test(buf_cap.querybuf(node, 0));
> fail_on_test(buf_cap.qbuf(node));
> fail_on_test(buf_cap.dqbuf(node));
> fail_on_test(!(buf_cap.g_flags() & V4L2_BUF_FLAG_LAST));
> 
> Since the buffer is queued after issuing the stop cmd, it is not
> possible to flag it as DONE in vdec_decoder_cmd.
> 
> A solution would be to hijack vidioc_qbuf and flag the buffer if a
> stop has been issued previously and the capture queue is not
> streaming. Would that be okay ?

Actually, I am wondering if this shouldn't be integrated into
v4l2-mem2mem.c. The corner case where you need to use an empty
last buffer is really awkward for drivers. So perhaps this should
be integrated into v4l2-mem2mem.c where you can mark that the
next queued buffer shall be immediately returned as an empty buffer
with the LAST flag set.

Since v4l2-mem2mem already has its own vidioc_qbuf function it can
easily be added there, and I think that's a much better place than
having to touch vb2 itself.

Regards,

	Hans

> 
> Maxime
> 
>>
>> Regards,
>>
>>         Hans