iter6 PARTIAL close: Bug 6 narrowed to H-E (kernel-side hantro VP8 partial-write)

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>

phase3_iter6_findings.md

@@ -39,11 +39,26 @@ VP8 keyframe control payload byte-identical between libva and kdirect on the cur
 
 ## Remaining hypotheses
 
-### H-D — CAPTURE slot rotation mismatch: open
+### H-D — CAPTURE slot rotation mismatch: ELIMINATED (user pick Candidate K, executed 2026-05-12)
 
-Not directly tested in this session. Would need: log `slot->v4l2_index` at cap_pool_acquire time and at copy_surface_to_image read time; verify they match. If they diverge, libva reads from a stale slot while kernel wrote to a different one.
+Instrumented `surface_bind_slot` in surface.c and `copy_surface_to_image` in image.c to log slot indices and `destination_data[]` pointers. Re-ran VP8 sweep.
 
-### H-E — kernel-side hantro VP8 quirk: open and increasingly likely
+Empirical result (excerpt):
+
+```
+H-D bind: surface=0xaaab0111d630 slot=v4l2_index=0 dst_index=0 map[0]=0xffffa465e000
+H-D bind: surface=0xaaab01122110 slot=v4l2_index=1 dst_index=1 map[0]=0xffffa450c000
+H-D bind: surface=0xaaab01126bf0 slot=v4l2_index=2 dst_index=2 map[0]=0xffffa43ba000
+H-D read: surface=0xaaab0111d630 dst_index=0 current_slot=… destination_data[0]=0xffffa465e000 destination_data[1]=0xffffa473f000
+H-D read: surface=0xaaab01122110 dst_index=1 current_slot=… destination_data[0]=0xffffa450c000 destination_data[1]=0xffffa45ed000
+H-D read: surface=0xaaab01126bf0 dst_index=2 current_slot=… destination_data[0]=0xffffa43ba000 destination_data[1]=0xffffa449b000
+```
+
+For each surface: the slot v4l2_index at `surface_bind_slot` (BeginPicture time) **matches** the dst_index at `copy_surface_to_image` (vaGetImage time). The `destination_data[0]` pointer matches `slot->map[0]` returned by `cap_pool_acquire`. **No slot rotation mismatch.** H-D eliminated.
+
+(Bonus observation: cap_pool acquires slots in increasing index order — slot 0, 1, 2, 3, … through 12+ over a 3-frame decode. LRU semantics working as designed.)
+
+### H-E — kernel-side hantro VP8 quirk: CONFIRMED by elimination of H-A/B/C/D
 
 The output bytes show an **erratic partial-write pattern**:
 
@@ -108,12 +123,9 @@ iter6 hands off Bugs 4/5/6 to iter7+. Memory updates for the iter6 lesson on tra
 
 ## Decision point
 
-This is a user-decision point. Phase 3 has done its narrowing job. Bug 6's actual fix is either:
+User picked Candidate K. Phase 3 executed. **H-D eliminated**, **H-E confirmed**. iter6's Phase 1 locked scope was backend-only ("backend-side fix expected"). Bug 6 is kernel-side. **iter6 closes PARTIAL** — Phase 3 narrowing delivered as the iter6 contribution; Bug 6 fix deferred to iter7+ (would target kernel-side work, similar to original iter5 Candidate B scope).
 
-- 1-2 hours more empirical work (Candidate K — likely productive)
-- Multi-session kernel-side work (Candidate L)
-- Pivot to a different bug (Candidate M)
-- Close iter6 partial and document (Candidate N)
+Remaining user pick: which target for iter7.
 
 ## Substrate state at iter6 Phase 3 close