fresnel-fourier/phase0_findings_iter11.md

# Iteration 11 — Phase 0 (substrate / motivation / inventory) → Phase 1 lock

Opens 2026-05-14 after iter10 negative close ([`phase8_iteration10_close.md`](phase8_iteration10_close.md)). Bommarito's patch confirmed unreachable on RK3399. Bug 5 (HEVC libva all-zero) remains open; not kernel-OOB.

## Pivot: replay the iter8/iter9 methodology for HEVC

The same approach that surfaced Bug 4's structure (kernel decodes wrong/incomplete, libva wire-payload search-space exhausted with 6 eliminations, real cause still TBD) can be applied to Bug 5. Phase 3 in iter8 already showed libva HEVC produces all-zero output (`06b2c5a0…`) while kdirect HEVC works. Same kernel, same readback path. So the diff is in the libva submission.

## Locked research question (iter11, 2026-05-14)

> *"What specific byte(s) in libva's HEVC control submission cause rkvdec to produce all-zero CAPTURE output while kdirect's submission produces correct decode? After fix: `libva_hevc.yuv == kdirect_hevc.yuv` byte-identical for `bbb_720p10s_hevc.mp4` 3-frame test."*

### Pass/fail (boolean)

1. **HEVC libva == kdirect**: `cmp -s libva_hevc.yuv kdirect_hevc.yuv` returns 0.
2. **H.264 unchanged**: `71ac099b…` (Bug 4 still open, no regression).
3. **VP9 unchanged**: `4f1565e8…` PASS.
4. **MPEG-2 unchanged**: `19eefbf4…` PASS.
5. **VP8 unchanged**: `bcc57ed5…` (Bug 6 still open, no regression).
6. **Control-payload anchors hold** for 4 non-HEVC codecs.

Clean iter11 close = all 6 PASS. Partial close acceptable if Bug 5 narrowed via wire-byte elimination.

## Substrate state at iter11 open

| Property | Value |
|---|---|
| Kernel | `linux-fresnel-fourier 7.0-1` (unchanged) |
| Fork tip | `e0be4e6` (iter9 close) |
| Backend SHA | `a17e3c39…` on fresnel |
| Diagnostic | γ + IMP-1 + α-2 + α-7 instrumentation preserved (env-gated off by default) |
| HEVC anchor | `06b2c5a0c01e515d009c0bfbe0e61fafb105a54da5ec621104915cd5949849e8` (libva, all-zero) |
| HEVC kdirect | TBD (need to capture this iter; iter8 didn't do kdirect HEVC) |

## Mechanism the question targets

HEVC kernel UAPI controls (linux/v4l2-controls.h):
- `V4L2_CID_STATELESS_HEVC_SPS` (0xa40a95)
- `V4L2_CID_STATELESS_HEVC_PPS` (0xa40a96)
- `V4L2_CID_STATELESS_HEVC_SLICE_PARAMS` (0xa40a97)
- `V4L2_CID_STATELESS_HEVC_SCALING_MATRIX` (0xa40a98)
- `V4L2_CID_STATELESS_HEVC_DECODE_PARAMS` (0xa40a99)
- `V4L2_CID_STATELESS_HEVC_DECODE_MODE` / `V4L2_CID_STATELESS_HEVC_START_CODE` (0xa40a9a / 0xa40a9b)
- VDPU381/383-specific: `V4L2_CID_STATELESS_HEVC_EXT_SPS_ST_RPS` / `_LT_RPS` (0xa40a... — newer, not on RK3399)

Bug 5 surfaces as: every libva HEVC DQBUF returns without FLAG_ERROR (kernel accepts), CAPTURE bytes all-zero. kdirect submits structurally same controls and gets correct decode.

## Scope locks

**In scope**:
- `src/h265.c` (libva backend HEVC handler).
- `src/picture.c` HEVC dispatch.
- Phase 3 strace + byte-decode of HEVC controls.
- Comparison against kdirect's strace.

**Out of scope**:
- Bug 4 / Bug 6 / iter4-B1b / other backlog.
- Kernel patches (until empirical evidence of a specific kernel-side gap, in which case route via kernel-agent).
- ext_sps_st/lt_rps (VDPU381/383-only).

## Phase 2 source-read targets

- `src/h265.c` — full file. Map per-frame HEVC pipeline (BeginPicture → controls → QBUF).
- `src/picture.c::codec_set_controls` — VAProfileHEVCMain dispatch.
- `src/picture.c::codec_store_buffer` — HEVCMain buffer storage cases.
- Kernel UAPI `struct v4l2_ctrl_hevc_sps/pps/decode_params/slice_params/scaling_matrix`.
- Reference: `~/src/aur/ffmpeg-git/src/FFmpeg/libavcodec/v4l2_request_hevc.c` (kdirect HEVC fill_*).

## Phase 3 plan

1. Run libva HEVC with strace -s 8192 → capture full control payloads.
2. Run kdirect HEVC with strace -s 8192 → same.
3. Byte-decode SPS, PPS, DECODE_PARAMS, SCALING_MATRIX, SLICE_PARAMS structures (analogous to what iter9 did for H.264 DPB).
4. Identify byte-level diffs.
5. For each diff, check rkvdec source for whether the field is load-bearing.

## Phase 4 plan shape (predicted)

Contingent on Phase 3 findings. Three likely outcomes:

- **Outcome A**: Single field diff in HEVC SPS / PPS / DECODE_PARAMS. Mechanical libva-side fix (~5-15 LOC in h265.c).
- **Outcome B**: Multi-field divergence. Field-by-field amendment of libva's HEVC control fill.
- **Outcome C**: No meaningful diff — controls are identical, but bytes-in-OUTPUT-buffer differ. Pivot to OUTPUT bitstream byte-level inspection.

## Predicted iter11 cadence

- Phase 0: this doc.
- Phase 2: source-read h265.c. ~30 min.
- Phase 3: empirical strace + decode. ~60 min.
- Phase 4: targeted plan. ~20 min.
- Phase 5: sonnet-architect review. ~30 min.
- Phase 6: implement. ~30 min.
- Phase 7: verify. ~30 min.
- Phase 8: close. ~10 min.

Total: ~3.5 hours wallclock contingent on fresnel uptime.

## What "iter11 PASS" looks like

If Bug 5 fixed:
- iter11 PASS. HEVC row goes from DEGRADED (transitive-only) to PASS direct.
- Codec scoreboard: 3 direct PASS (VP9, MPEG-2, HEVC) + 2 PARTIAL (H.264, VP8).

If iter11 PARTIAL:
- Bug 5 narrowed (some hypotheses eliminated). iter12 candidates derived from narrowing.

## Memory-rule guidance for this iter

- `feedback_review_empirical_over_theoretical.md` — apply Phase 5 review.
- `feedback_wire_vs_behavior.md` — Phase 7 must verify criterion-1 hash test, not just wire-byte match.
- `feedback_unconditional_codec_state.md` — any libva backend change must be profile-gated (HEVC-only).
- `feedback_per_driver_kludge_gating.md` — any driver-specific workaround should gate on driver_kind.
- `feedback_rkvdec_patch_reachability.md` — if iter11 ends up proposing a kernel patch, verify reachability before submission.