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Adjust buffer count for hwaccel and v4l2m2m decoders

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- For hwaccel decoders, don't depend on FFmpeg's 4 working surfaces.
  The decoders might need one or more of them internally.

- For V4L2M2M decoders, adjust the buffer counts to match our needs
  and requirements regarding Pacer-held frames.
This commit is contained in:
Cameron Gutman
2026-01-11 14:18:22 -06:00
parent d3957b3cbb
commit c14ebb1ad7
1 changed files with 18 additions and 4 deletions
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22
app/streaming/video/ffmpeg.cpp
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@@ -545,10 +545,7 @@ bool FFmpegVideoDecoder::completeInitialization(const AVCodec* decoder, enum AVP
m_VideoDecoderCtx->pkt_timebase.den = 90000;
// Allocate enough extra frames for Pacer to avoid stalling the decoder
//
// NB: Subtract 4 because FFmpeg always allocates 4 working surfaces when
// constructing a hwframes context (see ff_decode_get_hw_frames_ctx()).
m_VideoDecoderCtx->extra_hw_frames = std::max<int>(0, PACER_MAX_OUTSTANDING_FRAMES - 4);
m_VideoDecoderCtx->extra_hw_frames = PACER_MAX_OUTSTANDING_FRAMES;
// For non-hwaccel decoders, set the pix_fmt to hint to the decoder which
// format should be used. This is necessary for certain decoders like the
@@ -568,6 +565,23 @@ bool FFmpegVideoDecoder::completeInitialization(const AVCodec* decoder, enum AVP
return false;
}
// The V4L2M2M decoders are one of the rare non-hwaccel decoders that allocates
// hardware frames under the hood. We need to adjust the number of buffers it
// allocates to ensure it allocates enough for our use case, but not too many.
// Normal hwaccel decoders will examine the codec/bitstream and the value we
// set in extra_hw_frames to determine how many buffers are required.
if (QString::fromUtf8(decoder->name).endsWith("_v4l2m2m", Qt::CaseInsensitive)) {
// 2 buffers for incoming compressed video data
av_dict_set_int(&options, "num_output_buffers", 2, 0);
// 4 ref frames + what pacer holds + a working surface for each output
//
// NB: The reason we allocate 4 ref frames and not 16 (H.264 maximum)
// is because V4L2M2M decoders have reference frame invalidation disabled
// for compatibility, so we will never actually need 16 reference frames.
av_dict_set_int(&options, "num_capture_buffers", 4 + PACER_MAX_OUTSTANDING_FRAMES + 2, 0);
}
QString optionVarName = QString("%1_AVOPTIONS").arg(decoder->name).toUpper();
QByteArray optionVarValue = qgetenv(optionVarName.toUtf8());
if (!optionVarValue.isNull()) {