vd_lavc: delay images before reading them back

Facilitates hardware pipelining in particular with nvidia/dxva.

video/decode/dxva2.c

@@ -39,6 +39,8 @@
 #include "video/hwdec.h"
 #include "video/d3d.h"
 
+#define ADDTIONAL_SURFACES HWDEC_DELAY_QUEUE_COUNT
+
 // A minor evil.
 #ifndef FF_DXVA2_WORKAROUND_INTEL_CLEARVIDEO
 #define FF_DXVA2_WORKAROUND_INTEL_CLEARVIDEO    2
@@ -574,7 +576,7 @@ static int dxva2_create_decoder(struct lavc_ctx *s, int w, int h,
         surface_alignment = 16;
 
     /* 4 base work surfaces */
-    ctx->num_surfaces = 4;
+    ctx->num_surfaces = 4 + ADDTIONAL_SURFACES;
 
     /* add surfaces based on number of possible refs */
     if (codec_id == AV_CODEC_ID_H264 || codec_id == AV_CODEC_ID_HEVC)

video/decode/lavc.h

@@ -9,6 +9,8 @@
 #include "video/mp_image.h"
 #include "video/hwdec.h"
 
+#define HWDEC_DELAY_QUEUE_COUNT 2
+
 typedef struct lavc_ctx {
     struct mp_log *log;
     struct MPOpts *opts;
@@ -23,6 +25,10 @@ typedef struct lavc_ctx {
     bool hwdec_failed;
     bool hwdec_notified;
 
+    struct mp_image **delay_queue;
+    int num_delay_queue;
+    int max_delay_queue;
+
     // From VO
     struct mp_hwdec_info *hwdec_info;
 

video/decode/vaapi.c

@@ -44,7 +44,7 @@
  * Note that redundant additional surfaces also might allow for some
  * buffering (i.e. not trying to reuse a surface while it's busy).
  */
-#define ADDTIONAL_SURFACES 6
+#define ADDTIONAL_SURFACES (6 + HWDEC_DELAY_QUEUE_COUNT)
 
 // Some upper bound.
 #define MAX_SURFACES 25

video/decode/vd_lavc.c

@@ -391,6 +391,10 @@ static void init_avctx(struct dec_video *vd, const char *decoder,
             avctx->get_buffer2 = get_buffer2_hwdec;
         if (ctx->hwdec->init(ctx) < 0)
             goto error;
+        // This can increase efficiency by not blocking on the hardware
+        // pipeline by reading back immediately after decoding.
+        if (ctx->hwdec->process_image)
+            ctx->max_delay_queue = HWDEC_DELAY_QUEUE_COUNT;
     } else {
         mp_set_avcodec_threads(vd->log, avctx, lavc_param->threads);
     }
@@ -454,6 +458,17 @@ static void reset_avctx(struct dec_video *vd)
     ctx->flushing = false;
 }
 
+static void flush_all(struct dec_video *vd)
+{
+    vd_ffmpeg_ctx *ctx = vd->priv;
+
+    for (int n = 0; n < ctx->num_delay_queue; n++)
+        talloc_free(ctx->delay_queue[n]);
+    ctx->num_delay_queue = 0;
+
+    reset_avctx(vd);
+}
+
 static void uninit_avctx(struct dec_video *vd)
 {
     vd_ffmpeg_ctx *ctx = vd->priv;
@@ -474,9 +489,11 @@ static void uninit_avctx(struct dec_video *vd)
 
     av_frame_free(&ctx->pic);
 
-    ctx->flushing = false;
+    flush_all(vd);
+
     ctx->hwdec_failed = false;
     ctx->hwdec_fail_count = 0;
+    ctx->max_delay_queue = 0;
 }
 
 static void update_image_params(struct dec_video *vd, AVFrame *frame,
@@ -618,6 +635,22 @@ static int get_buffer2_hwdec(AVCodecContext *avctx, AVFrame *pic, int flags)
     return 0;
 }
 
+static struct mp_image *read_output(struct dec_video *vd)
+{
+    vd_ffmpeg_ctx *ctx = vd->priv;
+
+    if (!ctx->num_delay_queue)
+        return NULL;
+
+    struct mp_image *res = ctx->delay_queue[0];
+    MP_TARRAY_REMOVE_AT(ctx->delay_queue, ctx->num_delay_queue, 0);
+
+    if (ctx->hwdec && ctx->hwdec->process_image)
+        res = ctx->hwdec->process_image(ctx, res);
+
+    return mp_img_swap_to_native(res);
+}
+
 static void decode(struct dec_video *vd, struct demux_packet *packet,
                    int flags, struct mp_image **out_image)
 {
@@ -671,8 +704,11 @@ static void decode(struct dec_video *vd, struct demux_packet *packet,
     }
 
     // Skipped frame, or delayed output due to multithreaded decoding.
-    if (!got_picture)
+    if (!got_picture) {
+        if (!packet)
+            *out_image = read_output(vd);
         return;
+    }
 
     ctx->hwdec_fail_count = 0;
 
@@ -701,10 +737,9 @@ static void decode(struct dec_video *vd, struct demux_packet *packet,
 
     av_frame_unref(ctx->pic);
 
-    if (ctx->hwdec && ctx->hwdec->process_image)
-        mpi = ctx->hwdec->process_image(ctx, mpi);
-
-    *out_image = mp_img_swap_to_native(mpi);
+    MP_TARRAY_APPEND(ctx, ctx->delay_queue, ctx->num_delay_queue, mpi);
+    if (ctx->num_delay_queue > ctx->max_delay_queue)
+        *out_image = read_output(vd);
 }
 
 static struct mp_image *decode_with_fallback(struct dec_video *vd,
@@ -740,7 +775,7 @@ static int control(struct dec_video *vd, int cmd, void *arg)
     vd_ffmpeg_ctx *ctx = vd->priv;
     switch (cmd) {
     case VDCTRL_RESET:
-        reset_avctx(vd);
+        flush_all(vd);
         return CONTROL_TRUE;
     case VDCTRL_QUERY_UNSEEN_FRAMES: {
         AVCodecContext *avctx = ctx->avctx;