Phase 3 Candidate K executed: H-D (slot rotation) ELIMINATED via
instrumented bind+read site logging. Slot v4l2_index matches at
BeginPicture and at vaGetImage for every surface; destination_data[0]
matches slot->map[0]. No rotation mismatch.
H-A/B/C/D all eliminated. H-E (kernel-side hantro VP8 partial-write)
confirmed by elimination. The libva backend submits correct controls,
correct slice bytes, correct slices_size, correct slot indices.
Kernel writes erratic partial content (per-frame Y plane transitions
at row 536, 24, ... — not a clean buffer-size truncation, not slot
rotation).
iter6 close PARTIAL: 5 of 6 Phase 1 criteria PASS; criterion 1
(libva_vp8 == kdirect) PARTIAL — kernel-side fix needed, out of
iter6's locked backend-only scope.
No patches landed. Fresnel substrate unchanged: fork tip 70196f8,
backend SHA 2c6ff82c... (identical to iter5b-β close).
Net deliverable: Phase 3 narrowing reduces Bug-6 hypothesis space
from 5 to 1. Future iter7+ (or kernel-agent campaign) picks up the
kernel-side investigation.
Pattern recognized: iter2 HEVC transitive PASS masked Bug 5;
iter3 VP8 transitive PASS masked Bug 6. Both surfaced under direct
verification post-iter5b-β. Transitive proofs against ONE artifact
(control payload) don't catch bugs in OTHER artifacts.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Empirical Phase 3 narrowing:
- H-A slice data corruption: ELIMINATED. SHA256 of libva-dumped slice 0
(300614 bytes) byte-identical to raw VP8 frame 0 from .webm at
offset 10..300624 (post-VP8-header).
- H-B slices_size wrong: ELIMINATED. slices_size = fp_size +
sum(dct_part_sizes) = 300614 exactly.
- H-C cache coherency: ELIMINATED. msync attempt yielded no output
change; VP9 uses same image.c path and works fine.
- Control payloads: byte-identical between libva and kdirect for VP8
keyframe (pre-Phase-2 finding).
Output pattern: erratic partial-write. Frame 0 Y plane has real
content rows 0-535, then 100% zero rows 536-719. UV plane real
rows 0-133, zero 134-359. Frame 1 Y plane real rows 0-23, zero
24-719. Per-frame transitions differ — not buffer-size truncation,
not slot rotation.
Remaining:
- H-D slot rotation (untested; needs instrumentation)
- H-E kernel-side hantro VP8 partial-write quirk (likely; needs
ftrace / kernel investigation)
iter5b-β did fix Bug 2 for VP8 (pre-β all-zero was format mismatch;
post-β real-but-partial content is a separate kernel-side issue).
Phase 3 hands off 4 candidate directions to user:
- K: continue H-D investigation (1-2h next session)
- L: pivot to H-E kernel-side work (multi-session)
- M: park Bug 6, pick different bug (Bug 4/5 or iter4-B1)
- N: close iter6 PARTIAL, defer Bug 6 to iter7+
Substrate unchanged; no regression. Backend SHA still 2c6ff82c....
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
marfrit