fresnel-fourier/phase7c_iter8_verification.md

Iteration 8 — Phase 7c (α-2 verification + 5-codec regression sweep)

Captured 2026-05-13. Backend SHA b6a3958a5bca9451… (iter8 P6 γ + IMP-1 + α-2 POC strip removal). Fork tip 0226684.

Verdict on α-2

α-2 (remove POC sentinel strip) did NOT fix Bug 4.

Metric	Before α-2	After α-2
H.264 libva YUV hash	71ac099b…	71ac099b… (unchanged)
Frame 1 non-zero bytes	512 (16×32 leak)	512 (same pattern)
Frame 2 non-zero	32 sparse UV markers	32 same
Frame 3 non-zero	16 sparse UV markers	16 same

α-2 DID change the wire-level POC bytes (now matches kdirect's sentinel-encoded 0x00010000 for tfoc/bfoc instead of libva's previously-stripped 0), but the kernel produces identical output. POC values are not load-bearing for Bug 4.

What Phase 7c eliminated

This iteration's α-2 attempt eliminated one more hypothesis. Combined with prior phases:

Hypothesis	Status	Eliminated by
libva mis-reads CAPTURE buffer	❌ Eliminated	γ dump shows libva reads what's in buffer
Slot-binding wrong	❌ Eliminated	γ shows correct slot index per surface
Stale residue from prior runs	❌ Eliminated	IMP-1 memset experiment (same hash)
constraint_set_flags (SPS byte 1)	❌ Eliminated	Phase 5b CRIT-1: rkvdec source review
H.264 POC sentinel strip	❌ Eliminated	α-2 changed wire, no output change

What remains as candidate

DECODE_PARAMS / DPB construction beyond POC and flags:

1. DPB entry ORDER. Phase 4c strace re-decode shows libva fills DPB in entry-array order (entry[0]=frame_1, entry[1]=frame_2), kdirect fills in short_ref[] order (DPB[0]=most-recent-ref = frame_2). May or may not matter; rkvdec docs unclear.
2. DECODE_PARAMS fields after dpb[16] (bytes 512–559 of the 560-byte struct). Phase 3 didn't deep-compare these. Includes nal_ref_idc, frame_num, current frame's tfoc/bfoc, pic_order_cnt_bit_size, dec_ref_pic_marking_bit_size, delta_pic_order_cnt*, flags.
3. PPS field-by-field. Phase 5b IMP-2 flagged Phase 3 only shallow-compared 12 bytes. Full byte diff needed.
4. Slice data encoding. The OUTPUT bitstream bytes the kernel sees per slice. Phase 3 didn't validate this.

5-codec regression sweep on α-2 backend

Codec	Anchor (iter7)	iter8 α-2 hash	Verdict
H.264	71ac099b…	71ac099b…	unchanged (Bug 4 still present)
HEVC	06b2c5a0…	06b2c5a0…	unchanged (Bug 5 still present)
VP9	4f1565e8…	4f1565e8…	unchanged (PASS direct)
MPEG-2	19eefbf4…	19eefbf4…	unchanged (PASS)
VP8	bcc57ed5…	bcc57ed5…	unchanged (Bug 6 still present)

Zero regression on the 4 non-H.264 codecs. α-2 is a hygiene cleanup that aligns libva's wire payload more closely with kdirect's, even though it doesn't close Bug 4.

Iter8 criteria final reckoning

#	Criterion	Verdict
1	libva H.264 == kdirect H.264	FAIL (PARTIAL — Bug 4 narrowed but unfixed)
2	VP9 unchanged	PASS
3	MPEG-2 unchanged	PASS
4	HEVC unchanged	PASS
5	VP8 unchanged	PASS
6	Control-payload anchors hold for 4 non-H.264 codecs	PASS (4 codec hashes unchanged ⇒ controls unchanged)

5 of 6 PASS. Criterion 1 PARTIAL: Bug 4 significantly narrowed (3 hypotheses eliminated) but not fixed.

Lasting code changes from iter8

The fork now carries 4 commits beyond iter7:

SHA	Description	Ship status
7eae6ea	γ env-gated CAPTURE buffer diagnostic dump	Diagnostic, env-gated off
66ecbef	IMP-1 env-gated CAPTURE pre-zero diagnostic	Diagnostic, env-gated off
6f4e583	stdlib.h include for picture.c	Mechanical fix
0226684	α-2: POC sentinel strip removal	Hygiene cleanup, no behavior change

All four commits are zero-regression by env-gating or by being mechanically necessary. Net effect on default behavior: nothing changed except internal POC values sent to rkvdec are sentinel-encoded (which doesn't change decoder output for any of 5 codecs).

Backend SHA chain

Stage	SHA
iter7 close	520507f6d0a1a7eb3797bed42c6f74e0f3a4826ac8a22ed2655e01a6f20aa874
iter8 P6 γ	94467f0c0aaba804233206d01666302e42a5cbcaf513a8157045623a66fae58e
iter8 P7 IMP-1	e4649a4854a24ec3209ea5ce6705cef8ed9b80cb277701ecb34eb822516c6030
iter8 P6c α-2	b6a3958a5bca945164262339dea5cc28f17accce13d57bd9f0c5a5dabbdf1b53

iter8 → iter9 handoff

iter8 closes PARTIAL with significant Bug 4 narrowing. Three hypotheses eliminated (constraint_set_flags, POC strip, libva-readback). iter9 candidates:

• α-3 PPS full-byte diff: re-strace and dump all 12 bytes of PPS for libva vs kdirect. If diff exists, target it. Cost: 30 min for strace + analysis + small fix.
• α-4 DPB entry order: change h264_fill_dpb to fill in most-recent-first order matching kdirect. Cost: ~10 LOC + verify.
• α-5 Slice data encoding: deep-dump the OUTPUT buffer bytes libva writes vs kdirect's. Look for NAL-header / start-code / EBSP differences. Cost: 60 min + maybe small fix.
• α-6 DECODE_PARAMS post-DPB fields: re-strace with -s 1024 to capture full DECODE_PARAMS; diff bytes 512-559. Cost: 30 min.

Recommended order: α-3 (cheapest) → α-6 → α-4 → α-5.

Lessons distilled

1. Reviews caught a dead-end early. Phase 5b CRIT-1's empirical rkvdec source read saved a ~15 LOC futile change. Memory rule reinforcement: feedback_review_empirical_over_theoretical.md direction (reviewer reads source > author's plan-level speculation).
2. Wire-byte changes aren't always behavior changes. α-2 visibly changed the V4L2 ioctl bytes (POC values flipped) but the decoder output was identical. Wire-level diffs are necessary but not sufficient.
3. γ-then-α was the right empirical sequence. The γ dump confirmed libva-readback wasn't the issue; without it, iter9 would still be considering libva-side fixes.