Subject: Re: [PATCH 0/2] media: meson: vdec: Add compliant H264 support

From: Maxime Jourdan <[email-masked]>
Date: Fri, 18 Oct 2019 11:23:47 +0200

On 18/10/2019 09:50, Maxime Jourdan wrote:
> On Wed, Oct 9, 2019 at 2:01 PM Hans Verkuil <[email-masked]> 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 ?
> 

I was able to pass the vanilla v4l2-compliance tests by hacking in the 
following 2 things:

1) Adding your function that I modified a bit (buffer had to be tagged 
as VB2_BUF_STATE_ACTIVE, it shouldn't be removed from the list, and 
q->owned_by_drv_count needs to be atomic_inc'd)

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);
         for (i = 0; i < vb->num_planes; i++)
                 vb2_set_plane_payload(vb, i, 0);

	vb->state = VB2_BUF_STATE_ACTIVE;
	atomic_inc(&q->owned_by_drv_count);
         vbuf = to_vb2_v4l2_buffer(vb);
         vbuf->flags |= V4L2_BUF_FLAG_LAST;
         vb2_buffer_done(vb, VB2_BUF_STATE_DONE);
}

2) Hijacking vidioc_qbuf to DONE the buffer if a stop was previously 
asked for, and if the capture queue isn't streaming:

static int vdec_qbuf(struct file *file, void *priv,
		     struct v4l2_buffer *buf)
{
	struct v4l2_fh *fh = file->private_data;
	struct amvdec_session *sess;
	struct vb2_queue *vq;
	int ret;

	ret = v4l2_m2m_ioctl_qbuf(file, priv, buf);
	if (ret)
		return ret;

	sess = container_of(file->private_data, struct amvdec_session, fh);
	vq = v4l2_m2m_get_vq(fh->m2m_ctx, buf->type);
	/*
	 * If the capture queue isn't streaming and we were asked to
	 * stop, DONE the buffer instantly
	 */
	if (!V4L2_TYPE_IS_OUTPUT(vq->type) &&
	    !sess->streamon_cap && sess->should_stop)
		vb2_dqbuf_empty_last_buf(vq);

	return 0;
}

Overall it feels quite messy :( . It doesn't look like a buffer is 
supposed to be dequeued if streaming hasn't started (they are tagged as 
VB2_BUF_STATE_ACTIVE only when streaming starts, and this flag is a 
requirement for vb2_buffer_done).

All this could be built in more properly into v4l2-mem2mem.c, though it 
would require the same hacks around VB2_BUF_STATE_ACTIVE and 
q->owned_by_drv_count. Or it would need a vb2 function specifically for 
this case, which would be very similar to vb2_buffer_done but allowing 
the state being VB2_BUF_STATE_QUEUED and not changing q->owned_by_drv_count.

>> Regards,
>>          Hans

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