# Iteration 11 — Phase 5 Review

Reviewed 2026-05-14. Empirical anchor: Phase 3 strace, rkvdec-hevc.c on boltzmann
(`build/marfrit-packages/arch/linux-fresnel-fourier/src/linux-7.0`), h265.c in
`libva-v4l2-request-fourier/src/`.

---

## CRIT-1 — rkvdec does not read `sps_max_num_reorder_pics` at all

**Invalidates the primary mechanism.**

Exhaustive grep of `rkvdec-hevc.c`: the field `sps_max_num_reorder_pics` is never
read. The SPS fields rkvdec actually maps to hardware registers are geometry,
bitdepth, CTB sizes, PCM config, short/long-term ref counts, temporal-MVP, and flags
— not the buffering/reorder hints. The field is present in `v4l2_ctrl_hevc_sps` for
spec completeness but rkvdec ignores it entirely.

**Consequence:** α-13 will change wire bytes 10 and produce no behavioural difference
in rkvdec. Phase 7 YUV hash will remain all-zero.

**Required Phase 6b amendment A:** α-13 must be deprioritised to IMP (harmless
wire-correctness improvement). Bug 5 investigation must shift to the two fields
rkvdec *does* act on that differ between libva and kdirect.

---

## CRIT-2 — `num_entry_point_offsets` is the live suspect and needs a Phase 4b plan

rkvdec-hevc.c at line 272-284 writes `num_tile_columns_minus1` and
`num_tile_rows_minus1` from the PPS to hardware. kdirect submits
`num_entry_point_offsets = 22`; libva submits 0. BBB HEVC is a tiled stream
(confirmed by 22 entry points from kdirect). Libva's hardcoded 0 tells the kernel
the bitstream has no tile boundaries, which is false. rkvdec writes tile geometry
into hardware from PPS fields — if those fields are also wrong, or if the driver
validates entry-point-offsets count against tile geometry internally, the zero
produces either a wrong decode or a silent error.

Additionally, the PPS `UNIFORM_SPACING` flag (bit 20) is missing from libva. For
tiled streams this flag controls whether rkvdec computes uniform column/row widths
or reads them from per-column/row arrays. BBB being tiled makes this non-don't-care.

**Required Phase 6b amendment B:** Add Phase 4b scope covering:
1. `num_entry_point_offsets` — derive from `slice->num_entry_point_offsets` if
   VAAPI exposes it, else from tile geometry (num_columns × num_rows per slice).
2. PPS `UNIFORM_SPACING` — set flag when `pic_fields.bits.uniform_spacing_flag` is
   set (VAAPI does expose this field in `VAPictureParameterBufferHEVC.pic_fields`).

---

## IMP-1 — `slice_qp_delta` byte 22 is NOT a hardcoded parallel bug

Phase 4 plan raises this as a concern. Empirical check: h265.c line 368 reads
`slice_params->slice_qp_delta = slice->slice_qp_delta` directly from the VAAPI
buffer — it is not hardcoded. The -1 value (0xff) observed in Phase 3 strace is
what VAAPI delivered for frame 1 of BBB. kdirect's 0x00 (0) comes from its own SPS
NAL parse. This divergence is real but secondary; slice_qp_delta miscalculation
shifts luma values, it does not suppress entire frame output. Not a parallel
hardcoded-zero bug.

---

## IMP-2 — Phase 7 hash check must not be skipped if wire bytes match kdirect's

Per `feedback_wire_vs_behavior.md` and the Phase 4 plan's own §Phase 7: matching
wire bytes alone is insufficient. Phase 7 must SHA the raw NV12 CAPTURE output
against kdirect's `9340b832…` hash, not just strace-diff the control payloads.
The plan states this correctly; reviewer flags it to ensure it survives any "close
enough" shortcut in execution.

---

## MIN-1 — Comment accuracy in α-13 patch

The proposed α-13 comment says "blocked B-frame decode." If CRIT-1 stands and
rkvdec ignores the field, this causal claim is wrong. The comment should be scoped
to "wire-level conformance fix; not confirmed to unblock rkvdec output." Amend
before commit to avoid misleading future readers.

---

## Summary

| # | Severity | Finding | Blocks Phase 6? |
|---|----------|---------|----------------|
| CRIT-1 | CRIT | rkvdec never reads `sps_max_num_reorder_pics`; α-13 is wire-only | Yes — replan required |
| CRIT-2 | CRIT | `num_entry_point_offsets=0` + missing UNIFORM_SPACING are the live suspects for tiled BBB | Yes — Phase 4b needed |
| IMP-1 | IMP | `slice_qp_delta` is not hardcoded; 0xff is VAAPI-supplied, not a parallel bug | No |
| IMP-2 | IMP | Phase 7 must hash YUV not just strace | No |
| MIN-1 | MIN | α-13 comment overclaims causation | No |

**Recommended Phase 6b scope:** Land α-13 as a wire-correctness commit with
corrected comment (MIN-1). Open Phase 4b for `num_entry_point_offsets` +
`UNIFORM_SPACING` as the primary Bug 5 fix path (CRIT-2). Do not gate Phase 6 on
α-13 unblocking Bug 5 — it won't.