[PATCH 15/18] avcodec/h264dec: refuse multiview with hardware acceleration

No h264 hwaccel knows about views. They all build the accelerator's
picture list from short_ref and long_ref and skip pictures whose
H264Picture.reference is 0 -- see fill_vaapi_ReferenceFrames() and its
equivalents in the vdpau, dxva2, nvdec and vulkan code.

An inter-view reference is in neither list, and reference == 0 is normal
for it, because inter_view_flag makes a picture available for inter-view
prediction independently of whether it is a temporal reference. The
per-slice reference list is filled from sl->ref_list[] regardless, so a
dependent view slice would name a surface the accelerator was never
given.

Decode the base view only and log a warning, rather than submit that.

Signed-off-by: Dom Cobley <[email protected]>
---
 libavcodec/h264_slice.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/libavcodec/h264_slice.c b/libavcodec/h264_slice.c
index 3217e43205..81c275f9c8 100644
--- a/libavcodec/h264_slice.c
+++ b/libavcodec/h264_slice.c
@@ -994,6 +994,19 @@ static int setup_multiview(H264Context *h, const SPS *mvc_sps)
         return AVERROR_BUG;
     }
 
+    /* No hwaccel knows about views. They all build the accelerator's picture
+     * list from short_ref/long_ref, skipping anything with reference == 0, while
+     * an inter-view reference lives in sl->ref_list[] alone and need not be a
+     * temporal reference at all. A dependent view slice would therefore point at
+     * a surface the accelerator was never told about. Decode the base view only
+     * rather than submit that. */
+    if (views_active_output != 1 && h->avctx->hwaccel) {
+        av_log(h->avctx, AV_LOG_WARNING,
+               "Multiview decoding is not supported with hardware acceleration; "
+               "decoding the base view only.\n");
+        return 0;
+    }
+
     highest_view = av_log2(views_active_output);
     if (highest_view >= H264_MAX_MVC_VIEWS) {
         av_log(h->avctx, AV_LOG_ERROR, "Too many views requested: %x\n",
-- 
2.53.0

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