Re: [FFmpeg-devel] [PATCH v4 3/3] avcodec/v4l2_m2m_dec: setup capture queue before enqueue the first frame

[Andriy Gelman <andriy.gelman@gmail.com>]

On Thu, 19. Aug 16:55, Ming Qian wrote:
> there are two proper ways to setup capture queue.
> 1. client wait the source change event,
>    then setup the capture queue and streamon
> 2. client setup the capture queue in advance,
>    but to avoid time issues, client should start
>    the capture queue before it enqueue the sequence
>    header to decoder driver through output queue.
>    and the sequence header is always in the first
>    frame, so client should start capture before
>    enqueue the first frame.
> 
> ffmpeg use the method 2 to setup capture queue,
> but currently ffmpeg enqueue the first frame
> before starting the capture queue.
> so in driver side, there are time issues.
> when driver has parsed the resolution from sequence header,
> but the client may not finished setup the capture.
> so driver can't decide whether to notify a source change event to
> client. and the following flow may be chaotic.
> 
> And it's OK that client setup capture queue first, then enqueue the
> first frame.
> 
> Signed-off-by: Ming Qian <ming.qian@nxp.com>
> ---
>  libavcodec/v4l2_m2m_dec.c | 18 ++++++++----------
>  1 file changed, 8 insertions(+), 10 deletions(-)
> 
> diff --git a/libavcodec/v4l2_m2m_dec.c b/libavcodec/v4l2_m2m_dec.c
> index 224eb3d5e7be..6b936b6df2a9 100644
> --- a/libavcodec/v4l2_m2m_dec.c
> +++ b/libavcodec/v4l2_m2m_dec.c
> @@ -149,6 +149,14 @@ static int v4l2_receive_frame(AVCodecContext *avctx, AVFrame *frame)
>      if (s->draining)
>          goto dequeue;
>  
> +    ret = v4l2_try_start(avctx);
> +    if (ret) {
> +        /* can't recover */
> +        if (ret != AVERROR(ENOMEM))
> +            ret = 0;
> +        goto fail;
> +    }
> +
>      ret = ff_v4l2_context_enqueue_packet(output, &s->buf_pkt);
>      if (ret < 0 && ret != AVERROR(EAGAIN))
>          goto fail;
> @@ -157,16 +165,6 @@ static int v4l2_receive_frame(AVCodecContext *avctx, AVFrame *frame)
>      if (ret != AVERROR(EAGAIN))
>          av_packet_unref(&s->buf_pkt);
>  
> -    if (!s->draining) {
> -        ret = v4l2_try_start(avctx);
> -        if (ret) {
> -            /* cant recover */
> -            if (ret != AVERROR(ENOMEM))
> -                ret = 0;
> -            goto fail;
> -        }
> -    }
> -
>  dequeue:
>      return ff_v4l2_context_dequeue_frame(capture, frame, -1);
>  fail:

Unfortunately this does not work on odroid xu4.
It needs to parse the parameter sets before VIDIOC_G_FMT ioctl is called on the
capture queue:
https://github.com/hardkernel/linux/blob/odroid-5.4.y/drivers/media/platform/s5p-mfc/s5p_mfc_dec.c#L317

Thanks,
-- 
Andriy
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