fresnel-fourier/phase8_iteration9_close.md

Iteration 9 — Phase 8 (close)

Closes 2026-05-13. iter9 = Bug 4 — α-7 timestamp scheme test → confirmed inert. PARTIAL close. Wire-byte search space now exhausted.

Summary

Metric	Value
Iteration target	Bug 4 — α-7 monotonic timestamp counter
Fork tip start (iter8 close)	0226684
Fork tip end (iter9 close)	e0be4e6 (1 fork commit: timestamp counter scheme)
LOC delta	+23 / -1 across src/context.h, src/context.c, src/picture.c
Phase 1 criteria	5/6 PASS (C1 PARTIAL — H.264 hash unchanged)
Reviews	1 (Phase 5 CRIT-1 correctly predicted α-7 inert)
Cumulative hypothesis eliminations	6 (iter8's 5 + α-7 timestamp)

Results

Codec	Anchor	iter9 α-7	Verdict
H.264	1e7a0bc9… (kdirect target)	71ac099b… (unchanged broken)	FAIL (Bug 4 unfixed)
VP9	4f1565e8…	4f1565e8…	PASS unchanged
MPEG-2	19eefbf4…	19eefbf4…	PASS unchanged
HEVC	06b2c5a0…	06b2c5a0…	unchanged (Bug 5 deferred)
VP8	bcc57ed5…	bcc57ed5…	unchanged (Bug 6 deferred)

α-7 changed the wire-level timestamp values (libva now sends small µs counter values matching kdirect's pattern) but H.264 output remains identical to pre-α-7 broken state. Timestamp magnitude is not load-bearing.

Phase 5 reviewer's CRIT-1 was correct: VP9 (vp9.c:624) and MPEG-2 (mpeg2.c:150,156) use the same v4l2_timeval_to_ns(&ref->timestamp) pattern as H.264 and both decode correctly with gettimeofday-derived giant ns. If timestamp magnitude were load-bearing, those would also fail.

Wire-byte search space — now exhausted

After iter8 + iter9, all observable libva-vs-kdirect wire diffs in H.264 control submission have been tested:

Field	libva	kdirect	Tested	Verdict
SPS constraint_set_flags	0x00	0x02	α-1 review	inert (rkvdec source)
SPS all other bytes	identical	identical	Phase 3 + Phase 0 iter9	match
PPS all bytes	identical	identical	Phase 0 iter9	match
SCALING_MATRIX	flat 16/16	flat 16/16	Phase 3	match
DPB[].POC (after α-2)	sentinel-encoded	sentinel-encoded	α-2 implemented	match wire, no behavior change
DPB[].flags	0x03 VALID	ACTIVE	0x03	Phase 0 iter9
DPB[].pic_num/frame_num/fields	match	match	Phase 0 iter9	match
DPB[].reference_ts magnitude	small µs	small µs (post-α-7)	α-7 implemented	match wire, no behavior change
DECODE_PARAMS post-DPB (bytes 512-559)	identical	identical	Phase 0 iter9 deep-strace	match
OUTPUT QBUF bytesused	slices_size	slices_size	Same v4l2_queue_buffer logic	match (untested empirically)
Slice data bytes in OUTPUT buffer	start_code + raw NAL	start_code + raw NAL	Same backend logic	match (untested empirically)

The only NOT-yet-empirically-verified surface is:

• Actual byte content of the OUTPUT buffer. Both backends construct slice data the same way (start_code prefix + NAL data), but no byte-for-byte dump of the in-memory OUTPUT buffer has been done.
• OUTPUT QBUF m.planes[0].bytesused. The strace abbreviated this; both backends should be setting it from their slice-count tracking variable. Worth confirming via VIDIOC_QBUF dump.

Iter9 hypothesis ranking, final state

Hypothesis	Status after iter8+iter9
libva-readback bug	❌ Eliminated (γ dump)
Slot-binding wrong	❌ Eliminated (γ dump)
Stale residue	❌ Eliminated (IMP-1 memset)
SPS constraint_set_flags	❌ Eliminated (Phase 5b CRIT-1, rkvdec source)
POC sentinel	❌ Eliminated (α-2 wire change, no behavior)
reference_ts magnitude	❌ Eliminated (α-7 wire change, no behavior; VP9/MPEG-2 use same pattern and PASS)

Bug 4 search space exhaustion on the libva wire-byte side.

Lessons from iter9

Lesson 1: Reviewer's empirical CRIT-1 caught α-7 before build

Phase 5 CRIT-1 correctly identified that VP9/MPEG-2 use the same timestamp path and PASS — making α-7 a low-probability fix. The empirical test (α-7 build + verify) confirmed the prediction. Saved no time vs the prediction itself, but added an empirical confirmation to the elimination.

Lesson 2: Two consecutive α-attempts (α-2 POC, α-7 timestamp) changed wire bytes without changing behavior

Reinforcement of feedback_wire_vs_behavior.md: wire-equivalence is necessary but not sufficient. The pattern of "change wire to match kdirect, observe no decoder output change" is now repeated twice in this iteration. Suggests Bug 4 is in something NOT carried by the libva→kernel ioctl payload — likely either the OUTPUT bitstream bytes themselves, or some kernel-internal state that depends on neither.

Lesson 3: Wire-byte hypothesis space has finite size

After 6 eliminations across iter8+iter9, the libva-vs-kdirect H.264 ioctl-payload-byte diff is empirically zero (modulo constraint_set_flags which is inert). Any iter10+ Bug 4 fix must either:

• (a) Find a diff in OUTPUT bitstream bytes (in-memory dump comparison).
• (b) Find a diff in some untraced control (device-init controls, REQBUFS counts, S_FMT details).
• (c) Pivot to kernel-side investigation (rkvdec source dive for the 16×32 partial-decode signature).

iter9 → iter10 handoff

Substrate at close:

• Fork tip e0be4e6 on noether + fresnel + gitea.
• Backend SHA a17e3c39671d4f430a7c6f2be04ec128545aa44737a5f4df2b4558b158019a43 on fresnel.
• Kernel linux-fresnel-fourier 7.0-1 unchanged.
• All diagnostic instrumentation (γ + IMP-1 memset gate) preserved.
• α-2 POC strip removal + α-7 timestamp counter both shipped as hygiene improvements (zero behavior change but cleaner wire-payload semantics).

iter10 candidate ranking:

1. α-8 OUTPUT bitstream byte dump — add γ-style dump of source_data immediately before OUTPUT QBUF. Compare against kdirect's bytes (kdirect can also be instrumented via LD_PRELOAD or ffmpeg patch). ~30 LOC + analysis time.
2. α-9 untraced control diff — VIDIOC_S_EXT_CTRLS with device-wide ctrl_class (not request-attached) controls. Currently only DECODE_MODE + START_CODE + HEVC versions are set at init. Check what kdirect's S_EXT_CTRLS does that we don't.
3. kernel-side — read rkvdec assemble_hw_pps + run_hw fully. Look for a code path that produces a 16×32-byte partial write with luma-neutral default values.
4. Pivot to a different bug — Bug 5 (HEVC) or Bug 6 (VP8 partial). Bug 4's diminishing-returns curve suggests it may be better to defer until kernel knowledge ramps up.

Campaign scoreboard (unchanged):

H.264   | rkvdec    | PARTIAL  | Bug 4 narrowed via 6 eliminations
HEVC    | rkvdec    | DEGRADED | Bug 5 deferred
VP9     | rkvdec    | PASS direct | iter5b-β
MPEG-2  | hantro    | PASS (env) | iter1
VP8     | hantro    | PARTIAL (env) | Bug 6 deferred

iter8 + iter9 net contribution: significant Bug 4 narrowing (6 hypotheses eliminated, wire-byte search exhausted), plus diagnostic instrumentation infrastructure (γ + memset gate) for any future libva-side investigation. Zero codec fixes shipped.

Phase 8 commit

iter9 PARTIAL close. Fork at e0be4e6. Backend SHA a17e3c39…. 5/6 criteria PASS. iter10 needs a fundamentally different approach to make progress on Bug 4 — OUTPUT bitstream byte comparison OR kernel-side investigation OR pivot to a different bug.