[WIP] avcodec/nvdec: avoid needless copy of output frame

Replaces the data pointers with the mapped cuvid ones.
Adds buffer_refs to the frame to ensure the needed contexts stay alive
and the cuvid idx stays allocated.
Adds another buffer_ref to unmap the frame when it's unreferenced itself.

libavcodec/nvdec.c

@@ -308,7 +308,7 @@ int ff_nvdec_decode_init(AVCodecContext *avctx)
     params.CodecType           = cuvid_codec_type;
     params.ChromaFormat        = cuvid_chroma_format;
     params.ulNumDecodeSurfaces = frames_ctx->initial_pool_size;
-    params.ulNumOutputSurfaces = 1;
+    params.ulNumOutputSurfaces = frames_ctx->initial_pool_size;
 
     ret = nvdec_decoder_create(&ctx->decoder_ref, frames_ctx->device_ref, &params, avctx);
     if (ret < 0) {
@@ -354,6 +354,28 @@ static void nvdec_fdd_priv_free(void *priv)
     av_freep(&priv);
 }
 
+static void nvdec_unmap_mapped_frame(void *opaque, uint8_t *data)
+{
+    AVFrame *frame        = (AVFrame*)opaque;
+    FrameDecodeData  *fdd = (FrameDecodeData*)frame->private_ref->data;
+    NVDECFrame        *cf = (NVDECFrame*)fdd->hwaccel_priv;
+    NVDECDecoder *decoder = (NVDECDecoder*)cf->decoder_ref->data;
+    CUresult err;
+    CUcontext dummy;
+
+    err = decoder->cudl->cuCtxPushCurrent(decoder->cuda_ctx);
+    if (err != CUDA_SUCCESS) {
+        av_log(NULL, AV_LOG_ERROR, "cuCtxPushCurrent failed\n");
+        return;
+    }
+
+    err = decoder->cvdl->cuvidUnmapVideoFrame(decoder->decoder, (CUdeviceptr)frame->data[0]);
+    if (err != CUDA_SUCCESS)
+        av_log(NULL, AV_LOG_ERROR, "cuvidUnmapVideoFrame failed\n");
+
+    decoder->cudl->cuCtxPopCurrent(&dummy);
+}
+
 static int nvdec_retrieve_data(void *logctx, AVFrame *frame)
 {
     FrameDecodeData  *fdd = (FrameDecodeData*)frame->private_ref->data;
@@ -383,32 +405,31 @@ static int nvdec_retrieve_data(void *logctx, AVFrame *frame)
         goto finish;
     }
 
-    for (i = 0; frame->data[i]; i++) {
-        CUDA_MEMCPY2D cpy = {
-            .srcMemoryType = CU_MEMORYTYPE_DEVICE,
-            .dstMemoryType = CU_MEMORYTYPE_DEVICE,
-            .srcDevice     = devptr,
-            .dstDevice     = (CUdeviceptr)frame->data[i],
-            .srcPitch      = pitch,
-            .dstPitch      = frame->linesize[i],
-            .srcY          = offset,
-            .WidthInBytes  = FFMIN(pitch, frame->linesize[i]),
-            .Height        = frame->height >> (i ? 1 : 0),
-        };
-
-        err = decoder->cudl->cuMemcpy2D(&cpy);
-        if (err != CUDA_SUCCESS) {
-            av_log(logctx, AV_LOG_ERROR, "Error copying decoded frame: %d\n",
-                   err);
-            ret = AVERROR_UNKNOWN;
-            goto copy_fail;
-        }
+    frame->buf[1] = av_buffer_create(NULL, 0, nvdec_unmap_mapped_frame, frame, AV_BUFFER_FLAG_READONLY);
+    frame->buf[2] = av_buffer_ref(cf->idx_ref);
+    frame->buf[3] = av_buffer_ref(cf->decoder_ref);
 
-        offset += cpy.Height;
+    if (!frame->buf[1] || !frame->buf[2] || !frame->buf[3]) {
+        ret = AVERROR(ENOMEM);
+        goto copy_fail;
+    }
+
+    for (i = 0; frame->data[i]; i++) {
+        frame->data[i] = (uint8_t*)(devptr + offset);
+        frame->linesize[i] = pitch;
+        offset += pitch * (frame->height >> (i ? 1 : 0));
     }
 
+    goto finish;
+
 copy_fail:
-    decoder->cvdl->cuvidUnmapVideoFrame(decoder->decoder, devptr);
+    if (!frame->buf[1])
+        decoder->cvdl->cuvidUnmapVideoFrame(decoder->decoder, devptr);
+    else
+        av_buffer_unref(&frame->buf[1]);
+    av_buffer_unref(&frame->buf[2]);
+    av_buffer_unref(&frame->buf[3]);
+    return ret;
 
 finish:
     decoder->cudl->cuCtxPopCurrent(&dummy);
@@ -546,9 +567,12 @@ int ff_nvdec_frame_params(AVCodecContext *avctx,
     }
 
     frames_ctx->format            = AV_PIX_FMT_CUDA;
-    frames_ctx->width             = (avctx->coded_width + 1) & ~1;
-    frames_ctx->height            = (avctx->coded_height + 1) & ~1;
     frames_ctx->initial_pool_size = dpb_size;
+    // We are not actually using this hw_frames_ctx to allocate frames
+    // It only exists because hwaccel infra mandates it to exist. (maybe?)
+    // avcodec_default_get_buffer2 overrides width/height, so we can do this here:
+    frames_ctx->width             = 0;
+    frames_ctx->height            = 0;
 
     switch (sw_desc->comp[0].depth) {
     case 8: