[FFmpeg-devel] [PATCH] avcodec/v4l2_m2m_dec: dequeue frame if input isn't ready

[Andriy Gelman]

On Tue, 14. Dec 02:12, Cameron Gutman wrote:
> The V4L2M2M API operates asynchronously, so multiple packets can
> be enqueued before getting a batch of frames back. Since it was
> only possible to receive a frame by submitting another packet,
> there wasn't a way to drain those excess output frames from when
> avcodec_receive_frame() returned AVERROR(EAGAIN).
> 
> In my testing, this change reduced decode latency of a real-time
> 60 FPS H.264 stream by approximately 10x (200ms -> 20ms) on a
> Raspberry Pi 4.
> 
> Signed-off-by: Cameron Gutman <aicommander at gmail.com>
> ---
>  libavcodec/v4l2_m2m_dec.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/libavcodec/v4l2_m2m_dec.c b/libavcodec/v4l2_m2m_dec.c
> index 224eb3d5e7..b0c3d30ac8 100644
> --- a/libavcodec/v4l2_m2m_dec.c
> +++ b/libavcodec/v4l2_m2m_dec.c
> @@ -142,8 +142,12 @@ static int v4l2_receive_frame(AVCodecContext *avctx, AVFrame *frame)
>  
>      if (!s->buf_pkt.size) {
>          ret = ff_decode_get_packet(avctx, &s->buf_pkt);
> -        if (ret < 0 && ret != AVERROR_EOF)
> -            return ret;
> +        if (ret < 0) {
> +            if (ret == AVERROR(EAGAIN))
> +                return ff_v4l2_context_dequeue_frame(capture, frame, 0);
> +            else if (ret != AVERROR_EOF)
> +                return ret;
> +        }
>      }

The change lgtm. I tested on Rpi4 and odroid xu4.
Will apply this on Sunday if no one else comments.

Thanks,
-- 
Andriy