iter11 Phase 3 + 4: HEVC SPS wire-byte diff narrows Bug 5 to α-13
Phase 3 deep strace: only meaningful SPS diff is bytes 10-11.
libva bytes 10-11 = 00 00 (sps_max_num_reorder_pics=0, latency=0)
kdirect bytes 10-11 = 02 04 (reorder=2, latency=4)
Hardcoded at h265.c:110-111 with comment "/* not exposed */". VAAPI's
VAPictureParameterBufferHEVC doesn't forward these; kdirect parses
SPS NAL directly. sps_max_num_reorder_pics = 0 tells rkvdec "no
reordering" -> B-frame decode blocked -> all-zero output (Bug 5 fits).
Secondary diffs (Phase 4b candidates if α-13 doesn't close):
- SLICE_PARAMS num_entry_point_offsets = 0 (hardcoded at h265.c:356
with "iter2 doesn't do tiles" comment); kdirect submits 22.
- PPS UNIFORM_SPACING flag bit 20 (don't-care for non-tiled).
Phase 4 α-13: ~2 LOC fix. Set sps_max_num_reorder_pics =
sps_max_dec_pic_buffering_minus1 (safe upper bound per H.265 §A.4.2).
Leave sps_max_latency_increase_plus1 = 0 (spec "unconstrained").
Phase 5b review required before Phase 6b implementation per
"reviews never skippable".
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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# Iteration 11 — Phase 3 (HEVC libva vs kdirect deep strace)
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Captured 2026-05-14 on fresnel (bare hostname, back on LAN). Backend SHA `a17e3c39…` (iter9 close). Fixture `bbb_720p10s_hevc.mp4` 3-frame sweep.
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## Anchors
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| Path | Hash |
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|---|---|
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| libva HEVC | `06b2c5a0c01e515d…` (all-zero, Bug 5 reproduced) |
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| kdirect HEVC | `9340b832e08c23c4d4b2f732cecffb1d4104f2fc3257e561fac217c2d70d84c2` (correct decoded BBB) |
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## HEVC control payload comparison — frame 1
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Per-request `S_EXT_CTRLS` (ctrl_class=0xf010000):
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| Control | libva size | kdirect size | Diff? |
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|---|---|---|---|
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| HEVC_SPS (0xa40a90) | 40 | 40 | **DIFF at bytes 10-11** |
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| HEVC_PPS (0xa40a91) | 64 | 64 | **DIFF at bytes 56-57** |
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| HEVC_SLICE_PARAMS (0xa40a92) | 280 | 280 | **DIFF at bytes 0-3 + 8-11 + 22** |
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| HEVC_SCALING_MATRIX (0xa40a93) | 1000 | 1000 | identical (all-zero) |
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| HEVC_DECODE_PARAMS (0xa40a94) | 328 | 328 | DIFF at tail (poc / flags) |
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### Diff 1 — SPS bytes 10-11
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```
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libva bytes 0..15: 00 00 00 05 d0 02 00 00 04 04 [00 00] 01 01 00 03
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kdirect bytes 0..15: 00 00 00 05 d0 02 00 00 04 04 [02 04] 01 01 00 03
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```
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Field mapping per `struct v4l2_ctrl_hevc_sps`:
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- byte 10: `sps_max_num_reorder_pics` (u8)
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- libva: 0
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- kdirect: 2
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- byte 11: `sps_max_latency_increase_plus1` (u8)
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- libva: 0
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- kdirect: 4
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**Highly suspect**. `sps_max_num_reorder_pics = 0` tells the decoder "no reordering needed in this stream" — but BBB HEVC has B-frames in IBBP order. The kernel may refuse to set up reorder buffers, producing zero output for B-frames.
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### Diff 2 — PPS bytes 56-63 (flags u64)
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```
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libva PPS flags: 0x0000_0000_0000_7040
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kdirect PPS flags: 0x0000_0000_0010_7040
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```
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Diff = bit 20 = `V4L2_HEVC_PPS_FLAG_UNIFORM_SPACING` (set in kdirect, missing in libva). UNIFORM_SPACING is meaningful only when `num_tile_columns_minus1 > 0 || num_tile_rows_minus1 > 0`. For non-tiled streams it's don't-care. BBB HEVC may or may not use tiles — check `num_entry_point_offsets` in slice (Diff 3).
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### Diff 3 — SLICE_PARAMS bytes 0-31
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```
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libva head32: d0d30b00 2d000000 00000000 14010200 00000000 0000ff00 07000000 00000000
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kdirect head32: 80d20b00 2d000000 16000000 14010200 00000000 00000000 07000000 00000000
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^^^^^^^^ ^^^^^^^^ ^^
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bit_size num_entry_pt byte 22
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```
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- bytes 0-3 `bit_size` u32 LE: libva=0x000bd3d0 (774096), kdirect=0x000bd280 (773760). Diff = 336 bits ≈ 42 bytes. Small but present.
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- bytes 8-11 `num_entry_point_offsets` u32 LE: **libva=0, kdirect=22**. BBB HEVC slice IS tiled / wavefront-parallel — kdirect submits 22 entry points; libva hardcodes 0.
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- byte 22: libva=0xff, kdirect=0x00. Per struct layout this is `slice_qp_delta` (s8): libva=-1, kdirect=0.
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### libva source code — where the zeros come from
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`src/h265.c::h265_fill_sps` lines 110-111:
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```c
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sps->sps_max_num_reorder_pics = 0; /* not exposed */
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sps->sps_max_latency_increase_plus1 = 0; /* not exposed */
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```
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VAAPI's `VAPictureParameterBufferHEVC` does not include these fields. Libva fills 0 by default.
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`src/h265.c::h265_fill_slice_params` line 356:
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```c
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slice_params->num_entry_point_offsets = 0; /* iter2 doesn't do tiles */
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```
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Hardcoded 0 with a comment from iter2 acknowledging the scope-cut.
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## Conclusion
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Bug 5 is caused by **at least one** of:
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- SPS `sps_max_num_reorder_pics` hardcoded 0 (vs kdirect's 2 from SPS-NAL parse). Strongest suspect — directly blocks B-frame reordering.
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- SLICE_PARAMS `num_entry_point_offsets` hardcoded 0 (vs kdirect's 22). Affects tile/WPP parallel decode.
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- PPS UNIFORM_SPACING flag missing. Don't-care for non-tiled but BBB does use tiles per Diff 3.
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The classes are analogous to iter8's H.264 `constraint_set_flags` situation: VAAPI doesn't forward the field, libva fills a placeholder, the kernel cares.
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## Phase 4 plan (preview)
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Single iter11 commit, 5-10 LOC change in h265.c:
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- Set `sps->sps_max_num_reorder_pics` from `picture->sps_max_dec_pic_buffering_minus1` (safe upper bound).
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- Leave `sps->sps_max_latency_increase_plus1 = 0` (spec default: "no constraint").
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Secondary attempts if first doesn't fix:
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- Set PPS UNIFORM_SPACING flag when tiles are used (`num_tile_columns_minus1 > 0 || num_tile_rows_minus1 > 0`).
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- Parse slice header for `num_entry_point_offsets` (analogous to h264_slice_header.c). Costlier; defer to Phase 4b if needed.
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Phase 5 review will scrutinize:
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- Whether `sps_max_num_reorder_pics = sps_max_dec_pic_buffering_minus1` is safe vs producing too many reorder pictures.
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- Whether reaching for the bitstream's true value (SPS NAL parse) is needed for correctness.
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- That the change doesn't regress VP9 / VP8 / H.264 / MPEG-2.
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@@ -0,0 +1,90 @@
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# Iteration 11 — Phase 4 plan
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Drafted 2026-05-14 after Phase 3 narrowed Bug 5 to three concrete wire-byte diffs (SPS bytes 10-11, PPS flags bit 20, SLICE_PARAMS bytes 0-3 + 8-11 + 22).
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## Mechanism
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`src/h265.c::h265_fill_sps` lines 110-111 hardcode:
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```c
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sps->sps_max_num_reorder_pics = 0; /* not exposed */
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sps->sps_max_latency_increase_plus1 = 0; /* not exposed */
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```
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The kernel uses `sps_max_num_reorder_pics` to size reorder buffers / allow B-frame decode reordering. `= 0` blocks reordering for streams that need it (BBB HEVC has B-frames in IBBP). kdirect derives the real value (2 for BBB) by parsing the SPS NAL directly; libva can't see the SPS NAL since VAAPI consumes it client-side.
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Spec constraint: `sps_max_num_reorder_pics[i] ≤ sps_max_dec_pic_buffering_minus1[i]` (allowed reorder ≤ DPB capacity). Using `dec_pic_buffering_minus1` as the value is a safe upper bound — allows full DPB-depth reordering, which is always sufficient and never violates the spec inequality.
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For `sps_max_latency_increase_plus1 = 0`: per spec, 0 means "SpsMaxLatencyPictures is not specified" (i.e., no constraint). Leaving at 0 is correct.
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## Proposed fix (α-13)
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Two-line change in `h265_fill_sps`:
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```c
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/*
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* iter11 α-13 (Bug 5 fix): VAAPI's VAPictureParameterBufferHEVC does
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* not expose sps_max_num_reorder_pics. Hardcoding 0 told rkvdec "no
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* reordering needed" and blocked B-frame decode — observed empirically
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* as all-zero CAPTURE output through libva for BBB HEVC (B-frames in
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* IBBP pattern). kdirect parses the SPS NAL directly and submits the
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* stream's actual value (2 for BBB level 4.x Main).
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*
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* Use sps_max_dec_pic_buffering_minus1 as a safe upper bound: per
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* H.265 §A.4.2, sps_max_num_reorder_pics ≤ sps_max_dec_pic_buffering_minus1
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* always holds, so substituting the latter is conformant and gives the
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* kernel enough budget to reorder up to the DPB depth. May allocate
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* slightly more reorder slots than the bitstream's tight constraint
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* (kdirect's 2 vs our 4 for BBB), but kernel-side allocation cost is
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* negligible for typical content.
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*
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* sps_max_latency_increase_plus1 stays at 0 — spec default meaning
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* "SpsMaxLatencyPictures unconstrained" (§A.4.2). Setting kdirect's
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* value of 4 (latency_increase = 3) would be more restrictive, not
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* more correct.
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*/
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sps->sps_max_num_reorder_pics = picture->sps_max_dec_pic_buffering_minus1;
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sps->sps_max_latency_increase_plus1 = 0;
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```
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LOC: ~2 functional + ~20 comment.
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## Scope
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In scope: `src/h265.c::h265_fill_sps`.
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Out of scope:
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- `num_entry_point_offsets` (slice-header field; needs full slice header parse).
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- PPS `UNIFORM_SPACING` flag (don't-care for typical content).
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- VP9 / VP8 / H.264 / MPEG-2 paths (HEVC-specific change by construction).
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## Phase 5 review concerns
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- Is `sps_max_num_reorder_pics = sps_max_dec_pic_buffering_minus1` safe? Reviewer should verify the spec inequality.
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- Could it produce wrong decode for streams where actual reorder < dec_pic_buf? Reviewer should consider whether over-reporting reorder causes any kernel-side issue.
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- Per `feedback_unconditional_codec_state.md`: HEVC fill is gated by VAProfileHEVCMain dispatch in picture.c, so no risk to non-HEVC codecs.
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- Per `feedback_wire_vs_behavior.md`: Phase 7 must verify criterion-1 hash match, not just wire-byte diff vs kdirect.
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## Phase 7 verification matrix
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1. Build, install on fresnel.
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2. Run libva HEVC sweep. Compare hash against kdirect's `9340b832…`.
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3. Re-strace to confirm SPS byte 10 now = `sps_max_dec_pic_buffering_minus1` value.
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4. Run 5-codec regression sweep. H.264 / VP9 / MPEG-2 / VP8 anchors must hold.
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Iter11 PASS = libva_hevc.yuv == kdirect_hevc.yuv + zero regression.
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## Risks
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- **R-1**: Setting sps_max_num_reorder_pics to dec_pic_buf may over-report and confuse the kernel. Probability: low (spec inequality always holds; kernel uses field as upper bound). Mitigation: rollback is 1 line.
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- **R-2**: The `num_entry_point_offsets = 0` issue is the actual cause. α-13 then doesn't fix Bug 5. Probability: medium. Mitigation: Phase 7 hash mismatch → Phase 4b parses slice header for entry points.
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- **R-3**: PPS UNIFORM_SPACING / other byte diff is the actual cause. Probability: low (UNIFORM_SPACING is don't-care for non-tiled; BBB use of tiles surfaced via num_entry_point_offsets — see R-2).
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- **R-4**: Regression on VP9/MPEG-2/H.264/VP8. Zero by construction (HEVC-only change path).
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## Predicted iter11 cadence
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- Phase 5b review: 15-20 min.
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- Phase 6: 10 min.
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- Phase 7: 15 min.
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- Phase 8: 10 min.
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Total: ~1 hour wallclock if α-13 lands fix.
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