fresnel-fourier/phase8_iteration11_close.md

# Iteration 11 — Phase 8 (close)

Closes 2026-05-14. iter11 = Bug 5 (HEVC libva all-zero) narrowing via control-payload deep diff. PARTIAL close.

## Summary

| Metric | Value |
|---|---|
| Iteration target | HEVC libva == kdirect via control-payload fix |
| Fork tip start | `e0be4e6` (iter9 close) |
| Fork tip end | `8e2c04f` (1 commit: α-13 SPS hygiene + α-14 IRAP/IDR flags) |
| LOC delta | +53 / -5 across `src/h265.c` |
| Phase 1 criteria | 5/6 PASS (C1 PARTIAL — HEVC hash unchanged) |
| Reviews | 1 (Phase 5b CRIT-1 killed reorder_pics hypothesis; CRIT-2 redirected) |
| Cumulative HEVC eliminations | 4 (sps_max_num_reorder_pics, sps_max_latency_increase_plus1, IRAP/IDR flags placement, num_entry_point_offsets all kernel-ignored) |

## Results

| Codec | iter11 hash | Anchor | Verdict |
|---|---|---|---|
| H.264 | `71ac099b…` | `71ac099b…` | unchanged (Bug 4 still open) |
| HEVC | `06b2c5a0…` | `06b2c5a0…` | unchanged (Bug 5 unfixed) |
| VP9 | `4f1565e8…` | `4f1565e8…` | PASS unchanged |
| MPEG-2 | `19eefbf4…` | `19eefbf4…` | PASS unchanged |
| VP8 | `bcc57ed5…` | `bcc57ed5…` | unchanged (Bug 6) |

α-13 + α-14 changed two HEVC wire-byte fields to match kdirect (`sps_max_num_reorder_pics` from `sps_max_dec_pic_buffering_minus1`, `decode_params.flags = IRAP_PIC|IDR_PIC` from `slice_params[0].nal_unit_type`). Output unchanged. Reviewer's CRIT-1 confirmed empirically: rkvdec on RK3399 doesn't read these fields.

## Cumulative HEVC wire-byte landscape after iter11

| Field | libva (pre-iter11) | libva (post α-14) | kdirect | Rkvdec uses it? |
|---|---|---|---|---|
| SPS sps_max_num_reorder_pics | 0 | 4 | 2 | NO (Phase 5b CRIT-1) |
| SPS sps_max_latency_increase_plus1 | 0 | 0 | 4 | NO |
| PPS UNIFORM_SPACING flag | unset | unset | set | NO (don't-care non-tiled) |
| DECODE_PARAMS flags IRAP+IDR | 0 | 0x03 | 0x03 | NO (kernel doesn't branch on this either) |
| SLICE_PARAMS bit_size | 774096 | 774096 | 773760 | NO |
| SLICE_PARAMS num_entry_point_offsets | 0 | 0 | 22 | NO (Phase 5b CRIT-2) |
| SLICE_PARAMS slice_qp_delta | 0xff (-1) | 0xff (-1) | 0x00 | reads diff_cu_qp_delta_depth from PPS only |

**Every observable wire-byte diff between libva and kdirect HEVC is in a field rkvdec ignores on RK3399.** The Bug 5 root cause is NOT in the per-frame S_EXT_CTRLS payload.

## Where Bug 5 must be

Three remaining unexamined surfaces:

1. **OUTPUT bitstream bytes**. The actual HEVC slice data libva writes to the OUTPUT buffer's source_data may differ from what kdirect writes for the same input frame. Differences could be in start-code prefixing, NAL header bytes, EBSP encoding, or trailing padding. **This is the iter12 candidate.**
2. **OUTPUT QBUF metadata** (m.planes[0].bytesused, request_fd, timestamp). strace abbreviates per-plane bytesused; need explicit verification.
3. **Some other ioctl** sequence diff (REQBUFS counts, S_FMT shape, request_fd lifecycle) — unlikely since VP9 works through the SAME backend with the same ioctl sequence.

## Lessons

### Lesson 1: Wire-byte search has a strict ceiling

iter11 (HEVC) replicates iter8/iter9 (H.264): when the wire-byte diff is the field that user space and reference backend disagree on, but kernel ignores that field, no amount of wire-byte fix-up changes behavior. The Phase 5b methodology (grep rkvdec source for field name before writing fix) saves wasted cycles.

### Lesson 2: VAAPI gap is real but separate from rkvdec consumption

Multiple HEVC fields are "not exposed by VAAPI" in `VAPictureParameterBufferHEVC`. Libva backends fill them with placeholders. For some (`sps_max_num_reorder_pics`, `sps_max_latency_increase_plus1`, IRAP/IDR flags, UNIFORM_SPACING), the placeholder is wire-incorrect but rkvdec ignores it anyway. The campaign's hardcoded zeros are wire-hygiene gaps but NOT decode-correctness bugs.

### Lesson 3: Bug 4 and Bug 5 share an architectural pattern

Both bugs produce wrong/zero CAPTURE output through libva while kdirect succeeds. Both have control payloads that are or can be made byte-equivalent to kdirect via libva fixes. **Both must have their actual cause in the OUTPUT bitstream bytes the kernel reads** — that's the common unexamined surface. iter12 targets that.

## iter11 → iter12 handoff

Substrate at close:
- Fork tip `8e2c04f` on noether + fresnel + gitea.
- Backend SHA `521c1474c51154373e2d27f99d715a5af73a5fd9f72a83d947f8b552f2755d1f` on fresnel.
- Kernel `linux-fresnel-fourier 7.0-1` unchanged.
- Diagnostic γ + IMP-1 memset preserved; α-13/α-14 in shipping state (no regression).

iter12 candidate: extend the γ infrastructure to dump OUTPUT bitstream bytes (source_data + slices_size) immediately before MEDIA_REQUEST_IOC_QUEUE. Compare byte-by-byte with kdirect's OUTPUT buffer (capture via LD_PRELOAD or analogous kdirect instrumentation). Find the diff in actual bitstream bytes.

If OUTPUT bytes match: bug is in non-ioctl path (kernel-state, hardware-state).
If OUTPUT bytes differ: target the divergence in libva's `picture.c::codec_store_buffer` or upstream VAAPI consumer.

Bug 4 and Bug 5 likely both close via this same instrumentation, given the parallel structure.

## Memory rule candidate (defer to curation)

**rkvdec on RK3399 ignores most HEVC SPS / SLICE_PARAMS / DECODE_PARAMS bookkeeping fields** including: `sps_max_num_reorder_pics`, `sps_max_latency_increase_plus1`, `num_entry_point_offsets`, `bit_size`, `slice_qp_delta`, `pic_struct`. The driver only reads geometry (width, height, bit_depth, CTB / PCM sizing), reference-pic counts, ref_idx tables, and a subset of flags. Future hypothesis-generation should grep rkvdec source first before assuming a particular field is load-bearing.

Worth folding into [[feedback-rkvdec-patch-reachability]] or as a sibling memory.