fix(sycl): correct libvmaf_sycl zero-copy VMAF on QSV-decoded 10-bit input

[Tualua]

Summary

Fixes the libvmaf_sycl FFmpeg zero-copy filter returning VMAF score: nan (with integer_motion ~130× too high) on QSV-decoded 10-bit pairs on Intel Arc. A fresh investigation found two independent root causes — neither in the orchestration loop blamed by the prior FIX-01 attempt (which was verified incomplete): a shared QSV/VA surface pool (cross-decoder contamination) and raw P010 MSB-aligned pixels imported without the LSB normalization the CPU path gets for free. See ADR-1121.

Type

• fix — bug fix
• sycl / cuda / simd — backend-specific

Root cause & fix

• BUG-1 (contamination). With one shared -hwaccel_device, FFmpeg gives both decoders one mfxSession/VA surface pool; their distinct mfxFrameSurface1 wrappers map to the same VASurfaceIDs, so HEVC ref[N±1] overwrites the surface AV1 wrote dis[N] into (sycl_dis[N]==sycl_ref[N±1]). Fix (FIX-03): require a separate -init_hw_device qsv=… per decoder — a documented usage contract (libvmaf can't see FFmpeg session topology from inside the filter).
• BUG-2 (NaN + 64× motion). VA-API stores P010/P012 MSB-aligned (V_MSB = V_LSB << (16−bpc)); the CPU path gets >>6 via FFmpeg's P010LE→YUV420P10LE, the SYCL import did not. Fix (FIX-04): apply the >>(16−bpc) shift in the import — fused into the Tile4/Y-tiled de-tile store on the QSV/DG2 hot path (each sample shifted as written; no extra kernel, no second pass) and a standalone launch_p010_normalize() kernel only on the rare LINEAR/readback fallbacks (no-op for 8-bit).
• No DMA-BUF coherency sync. A DMA_BUF_IOCTL_SYNC(READ) flush was tried and removed — proven insufficient for the contamination, and its SYNC_START is a blocking decoder-fence wait that serialised decode→compute and cut 4K throughput with zero correctness benefit. (vaSyncSurface() + the in-order queue already order decode→de-tile.)

Checklist

• Commits follow Conventional Commits.
• make format && make lint green locally (clang-tidy on touched SYCL TUs in-container).
• If I touched any SIMD/GPU code path, I verified parity: SYCL↔CPU to 3 sig figs (table below).
• If I touched a feature extractor with twins — N/A; this is VA-import plumbing, not a feature kernel; the kernels are unchanged.
• New .c/.cpp/.h — none added; existing fork files edited (headers intact).
• If this PR adds an ADR, the row lives in docs/adr/_index_fragments/1121-*.md + appended to _order.txt (README regenerated via concat-adr-index.sh).

Bug-status hygiene (ADR-0165)

• docs/state.md updated — T-SYCL-ZEROCOPY-P010-NAN-2026-06-30 added to Recently closed.

Netflix golden-data gate

• Did not modify any assertAlmostEqual(...). The bug is in the fork-added SYCL VA-import path; the CPU metric engine and golden pairs are untouched. (The fork D-03 oracle baseline was rebased to the de-contaminated value 97.2350 in .planning verification artifacts only — not Netflix golden data.)

Cross-backend numerical results

feature           cpu-oracle (sep-session)   sycl zero-copy (FIX-03+04)   delta
vmaf (500f)       97.2337                    97.2350                      0.0013
integer_motion2   26.6934 (max)              26.6934 (max)                identical
vmaf (30f)        97.5111                    97.5085                      0.0026
integer_motion2   1.15262 (max)              1.15262 (max)                identical
NaN count         0                          0                            —
determinism       —                          byte-identical across runs   (sha256 match)

Performance: the zero-copy path now defaults to direct SYCL dispatch — the
combined graph (ADR-0483) is a net throughput loss on VA-import (byte-identical
output, but the per-frame de-tile import + graph compute-barrier serialise
decode→compute, ~15–25% slower at 4K; ~33→42 fps on Arc A380). select_strategy
gains a va_import_path flag (after the env checks, so VMAF_SYCL_USE_GRAPH=1
still forces graph); users no longer need the deprecated VMAF_SYCL_NO_GRAPH=1.
Secondary: the MSB→LSB shift is fused into the de-tile kernel (no extra GPU pass)
and the DMA_BUF_IOCTL_SYNC blocking flush was removed. Default-dispatch output
is byte-identical to graph mode (sha256 match) — zero numeric change.

Deep-dive deliverables (ADR-0108)

• Research digest — docs/research/2026-06-30-sycl-qsv-zerocopy-two-bug-investigation.md.
• Decision matrix — ADR-1121 ## Alternatives considered (normalize-in-import vs in-kernel vs force-readback vs auto-detect-session vs ioctl-as-fix).
• AGENTS.md invariant note — core/src/sycl/AGENTS.md: normalize on all import paths; FIX-02 is hardening not the fix; D-03 de-contaminated targets.
• Reproducer / smoke-test command — below.
• CHANGELOG fragment — changelog.d/fixed/1121-sycl-qsv-zerocopy-p010-normalization.md.
• Rebase note — docs/rebase-notes.md → fix/sycl-qsv-zerocopy-p010-normalize (no ffmpeg-patch impact; invariants listed).

Reproducer

# Intel Arc A380, container localhost/vmafx-fix04 (or any -Denable_sycl=true build).
# Separate per-decoder QSV sessions (FIX-03) are mandatory.
ffmpeg -y -hide_banner \
  -init_hw_device drm=drm0:/dev/dri/renderD128 \
  -init_hw_device vaapi=va0@drm0 \
  -init_hw_device qsv=qsv_ref@va0 \
  -init_hw_device qsv=qsv_dis@va0 \
  -hwaccel qsv -hwaccel_output_format qsv -hwaccel_device qsv_ref -c:v hevc_qsv -i ref.mkv \
  -hwaccel qsv -hwaccel_output_format qsv -hwaccel_device qsv_dis -c:v av1_qsv  -i dis.mkv \
  -lavfi '[0:v][1:v]libvmaf_sycl=log_fmt=csv:log_path=out.csv' \
  -frames:v 500 -f null -
# Expect: VMAF ~97, integer_motion < 30, 0 NaN. (Old bug: VMAF ~34/nan, integer_motion ~3000+.)

Known follow-ups

• The separate-session requirement is a documented contract, not enforced by the binary (libvmaf cannot observe FFmpeg's mfxSession topology). The libvmaf_sycl filter could log a one-time hint if it detects a shared VA pool — deferred.
• ffmpeg-patch stack: no change — the fix is internal to the fork-added SYCL VA-import path (no public C-API / CLI / meson_options.txt / LIBVMAFContext delta); the separate-session pattern is an ffmpeg invocation, not a vf_libvmaf.c change.

🤖 Generated with Claude Code

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