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.