lavc: allow asynchronous decoders to return correct pkt_dts values

The generic code in utils.c sets the AVFrame.pkt_dts field from the
packet it was supposedly decoded. This does not have to be true for a
fully asynchronous decoder like mmaldec. It could be overwritten with an
incorrect value. Even if the decoder doesn't determine the DTS (but sets
it to AV_NOPTS_VALUE), it's impossible to determine a correct value in
utils.c.

Decoders can now be marked with FF_CODEC_CAP_SETS_PKT_DTS, in which case
utils.c won't overwrite the field. The decoders are expected to set this
field (even if they only set it to AV_NOPTS_VALUE).
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

libavcodec/internal.h

@@ -46,7 +46,13 @@
  * all.
  */
 #define FF_CODEC_CAP_INIT_CLEANUP           (1 << 1)
-
+/**
+ * Decoders marked with FF_CODEC_CAP_SETS_PKT_DTS want to set
+ * AVFrame.pkt_dts manually. If the flag is set, utils.c won't overwrite
+ * this field. If it's unset, utils.c tries to guess the pkt_dts field
+ * from the input AVPacket.
+ */
+#define FF_CODEC_CAP_SETS_PKT_DTS           (1 << 2)
 
 #ifdef DEBUG
 #   define ff_dlog(ctx, ...) av_log(ctx, AV_LOG_DEBUG, __VA_ARGS__)

libavcodec/mmaldec.c

@@ -792,6 +792,7 @@ AVCodec ff_h264_mmal_decoder = {
     .flush          = ffmmal_flush,
     .priv_class     = &ffmmaldec_class,
     .capabilities   = AV_CODEC_CAP_DELAY,
+    .caps_internal  = FF_CODEC_CAP_SETS_PKT_DTS,
     .pix_fmts       = (const enum AVPixelFormat[]) { AV_PIX_FMT_MMAL,
                                                      AV_PIX_FMT_YUV420P,
                                                      AV_PIX_FMT_NONE},

libavcodec/utils.c

@@ -1482,7 +1482,8 @@ int attribute_align_arg avcodec_decode_video2(AVCodecContext *avctx, AVFrame *pi
         else {
             ret = avctx->codec->decode(avctx, picture, got_picture_ptr,
                                        avpkt);
-            picture->pkt_dts = avpkt->dts;
+            if (!(avctx->codec->caps_internal & FF_CODEC_CAP_SETS_PKT_DTS))
+                picture->pkt_dts = avpkt->dts;
             /* get_buffer is supposed to set frame parameters */
             if (!(avctx->codec->capabilities & AV_CODEC_CAP_DR1)) {
                 picture->sample_aspect_ratio = avctx->sample_aspect_ratio;