[FFmpeg-devel] [PATCH 4/4] avcodec/mediacodecdec: restructure mediacodec_receive_frame

[Matthieu Bouron]

On Tue, Apr 24, 2018 at 01:59:26PM -0700, Aman Gupta wrote:
> From: Aman Gupta <aman at tmm1.net>
> 
> This fixes issues reported on some devices where both
> avcodec_send_packet and avcodec_receive_frame can return EAGAIN
> at the same time, instead of one or the other blocking.
> 
> The new logic follows a recommendation by @rcombs to use
> dequeueInputBuffer with a timeout of 0 as a way to detect
> whether the codec wants more data. The dequeued buffer index is
> kept in MediaCodecDecContext until it can be used next.
> 
> A similar technique is also used by the Google's official media
> player Exoplayer: see MediaCodecRenderer.feedInputBuffer().
> ---
>  libavcodec/mediacodecdec.c        | 78 ++++++++++++++++++++-------------------
>  libavcodec/mediacodecdec_common.c | 28 ++++++++------
>  libavcodec/mediacodecdec_common.h |  4 +-
>  3 files changed, 59 insertions(+), 51 deletions(-)
> 
> diff --git a/libavcodec/mediacodecdec.c b/libavcodec/mediacodecdec.c
> index e5d3e6a0af..1a681d771e 100644
> --- a/libavcodec/mediacodecdec.c
> +++ b/libavcodec/mediacodecdec.c
> @@ -391,33 +391,11 @@ done:
>      return ret;
>  }
>  
> -static int mediacodec_send_receive(AVCodecContext *avctx,
> -                                   MediaCodecH264DecContext *s,
> -                                   AVFrame *frame, bool wait)
> -{
> -    int ret;
> -
> -    /* send any pending data from buffered packet */
> -    while (s->buffered_pkt.size) {
> -        ret = ff_mediacodec_dec_send(avctx, s->ctx, &s->buffered_pkt);
> -        if (ret == AVERROR(EAGAIN))
> -            break;
> -        else if (ret < 0)
> -            return ret;
> -        s->buffered_pkt.size -= ret;
> -        s->buffered_pkt.data += ret;
> -        if (s->buffered_pkt.size <= 0)
> -            av_packet_unref(&s->buffered_pkt);
> -    }
> -
> -    /* check for new frame */
> -    return ff_mediacodec_dec_receive(avctx, s->ctx, frame, wait);
> -}
> -
>  static int mediacodec_receive_frame(AVCodecContext *avctx, AVFrame *frame)
>  {
>      MediaCodecH264DecContext *s = avctx->priv_data;
>      int ret;
> +    ssize_t index;
>  
>      /* in delay_flush mode, wait until the user has released or rendered
>         all retained frames. */
> @@ -427,28 +405,52 @@ static int mediacodec_receive_frame(AVCodecContext *avctx, AVFrame *frame)
>          }
>      }
>  
> -    /* flush buffered packet and check for new frame */
> -    ret = mediacodec_send_receive(avctx, s, frame, false);
> +    /* poll for new frame */
> +    ret = ff_mediacodec_dec_receive(avctx, s->ctx, frame, false);
>      if (ret != AVERROR(EAGAIN))
>          return ret;
>  
> -    /* skip fetching new packet if we still have one buffered */
> -    if (s->buffered_pkt.size > 0)
> -        return mediacodec_send_receive(avctx, s, frame, true);
> +    /* feed decoder */
> +    while (1) {
> +        if (s->ctx->current_input_buffer < 0) {
> +            /* poll for input space */
> +            index = ff_AMediaCodec_dequeueInputBuffer(s->ctx->codec, 0);
> +            if (index < 0) {
> +                /* no space, wait a while for an output frame to appear */
> +                return ff_mediacodec_dec_receive(avctx, s->ctx, frame, true);
> +            }
> +            s->ctx->current_input_buffer = index;
> +        }
>  
> -    /* fetch new packet or eof */
> -    ret = ff_decode_get_packet(avctx, &s->buffered_pkt);
> -    if (ret == AVERROR_EOF) {
> -        AVPacket null_pkt = { 0 };
> -        ret = ff_mediacodec_dec_send(avctx, s->ctx, &null_pkt);
> -        if (ret < 0)
> +        /* try to flush any buffered packet data */
> +        if (s->buffered_pkt.size > 0) {
> +            ret = ff_mediacodec_dec_send(avctx, s->ctx, &s->buffered_pkt, false);
> +            if (ret > 0) {
> +                s->buffered_pkt.size -= ret;
> +                s->buffered_pkt.data += ret;
> +                if (s->buffered_pkt.size <= 0)
> +                    av_packet_unref(&s->buffered_pkt);
> +            } else if (ret < 0 && ret == AVERROR(EAGAIN)) {
> +                return ret;
> +            }
> +            continue;
> +        }
> +
> +        /* fetch new packet or eof */
> +        ret = ff_decode_get_packet(avctx, &s->buffered_pkt);
> +        if (ret == AVERROR_EOF) {
> +            AVPacket null_pkt = { 0 };
> +            ret = ff_mediacodec_dec_send(avctx, s->ctx, &null_pkt, true);
> +            if (ret < 0)
> +                return ret;
> +        }
> +        else if (ret == AVERROR(EAGAIN) && s->ctx->current_input_buffer < 0)
> +            return ff_mediacodec_dec_receive(avctx, s->ctx, frame, true);

If the input is starved on the FFmpeg side, we shouldn't block on trying
to receive a new frame from MediaCodec as it causes a performance
regression while the decoder is still not buffered enough to output its
first frame (after init or after a flush) as the function will end up
waiting 8ms for each new packet (for each call to avcodec_send_packet()).

Returning directly EAGAIN from here is fine as we will end up calling
ff_mediacodec_dec_receive() from the next call to avcodec_send_packet() or
avcodec_receive_frame().

With this modification the patch works fine on my devices without any
performance regression.

[...]

-- 
Matthieu B.