libva-v4l2-request-fourier/phase7_pi5_hevc_close.md

Phase 7 close — iter40 Pi 5 HEVC partial

Closed 2026-05-17 evening. Backend tip 3ffa9d0 on master. Higgs (Pi CM5, Debian 13 trixie, kernel 6.12.75+rpt-rpi-2712) is the test target.

Verification matrix

Criterion	Result	Notes
C1 — vainfo enumeration	PASS ✓	VAProfileHEVCMain : VAEntrypointVLD listed under v4l2-request driver
C2 — bit-exact libva vs kdirect	FAIL ✗	All 3 fixtures (640 / 1280 / 1920) produce correct-sized output (10 frames × bytes/frame) but content differs from kdirect. Real decode failure — see C5.
C3 — HW engagement	PASS ✓	lsof shows /dev/video19 open by ffmpeg-vaapi during libva decode. iter40: also opened rpi-hevc-dec at video_fd=5 media_fd=6 log line fires every session.
C4 — Stability under N=3	n/a	Output deterministic but wrong; N=3 would reproduce same wrong SHA.
C5 — Sibling baseline preserved	expected PASS	Not yet re-verified post-iter40. All new fd / video_format / per-driver gates are no-op when rpi-hevc-dec absent (fresnel / ampere).
C6 — Decode succeeds at kernel level	FAIL ✗	Every CAPTURE DQBUF returns V4L2_BUF_FLAG_ERROR. Decode fails per-frame.

What works

• Build clean on higgs (meson release + Debian 13 toolchain, after nv12_col128.h + nv15.h fallback #defines for headers that omit the mainline fourccs).
• ICD discovery: LIBVA_DRIVER_NAME=v4l2_request opens at /usr/lib/aarch64-linux-gnu/dri/v4l2_request_drv_video.so.
• Multi-device probe (iter38 extended to 3 slots) finds rpi-hevc-dec via find_decoder_device_by_driver. New known_decoder_drivers[] entry + else if (strcmp(info.driver, "rpi-hevc-dec") == 0) branch in the primary-driver detection block (Phase 5 review F3 fix).
• request_device_kind_for_profile → 'p' override for HEVC when rpi-hevc-dec is present.
• request_switch_device_for_profile retargets to the rpi fds.
• Synthetic-SPS pre-seed gated off for rpi-hevc-dec (Phase 5 review F6 fix — rpi doesn't have the iter25 rkvdec EBUSY problem).
• NC12 video_format entry; v4l2_set_format uses driver_data->video_format->v4l2_format (not hardcoded NV12), so S_FMT(CAPTURE) gets NC12 (uppercase, single-plane) instead of Nc12 (multi-plane non-contig). Kernel returns expected sizeimage=1382400 bytesperline=1080 num_planes=1 for 1280×720.
• nv12_col128_detile_y + _uv primitives copy per-column row-by-row via memcpy(128 bytes per row × num_columns rows). Unit test (tests/test_nv12_col128_detile.c) passes 10/10 (Y + UV at 640 / 1280 / 1920 / 1366 widths + UV offset helper).
• nv12_col128_uv_plane_offset returns the correct within-column UV start = 128 * ALIGN(height, 8). Earlier wrong formula (num_columns × 128 × aligned_h = sizeof linear Y plane) was caught by Phase 7 SEGV on 640 + 1920 widths — SAND interleaves Y+UV per column, NOT plane-concatenated.
• image.c #ifdef __arm__ guard extended to #if defined(__arm__) || defined(__aarch64__) (Phase 5 review F1 fix — this was already silently dead-coding the iter39 NV15→P010 detile on fresnel + ampere; iter39 5/5 PASS masked it because no 10-bit path was exercised). The tiled_to_planar (Sunxi) call is kept arm-only since the asm symbol isn't built on aarch64.
• RequestCreateImage NC12 override sets pitches[0] = width (linear NV12 Y stride) instead of the kernel-returned column stride (1080 for 1280×720).

What fails

V4L2_BUF_FLAG_ERROR on every CAPTURE DQBUF. Kernel rpi-hevc-dec rejects each frame's decode submission. Output buffer is left at its initial (all-zero) state — the consumer (ffmpeg's hwdownload) reads that and writes 0x00 to format=nv12 output, producing the wrong SHA.

Root cause identified — SPS field encoding diverges from bitstream

Compared per-frame S_EXT_CTRLS class=0xf010000 payload bytes vs kdirect (ffmpeg -hwaccel drm -c:v hevc):

SPS ctrl (id=0xa40a90, size=40), first 16 bytes:

• ours: 00 00 00 05 d0 02 00 00 04 04 04 00 01 01 00 03
• kdirect: 00 00 00 05 d0 02 00 00 04 04 02 04 01 01 00 03

Differing bytes at offset 10–11:

• offset 10: sps_max_num_reorder_pics — ours=4, kdirect=2
• offset 11: sps_max_latency_increase_plus1 — ours=0, kdirect=4

Per src/h265.c:139-140:

/* iter11 α-13: VAAPI doesn't forward sps_max_num_reorder_pics or
 * sps_max_latency_increase_plus1. ... */
sps->sps_max_num_reorder_pics = picture->sps_max_dec_pic_buffering_minus1;
sps->sps_max_latency_increase_plus1 = 0;

We use sps_max_dec_pic_buffering_minus1 as a safe upper bound fallback because VAAPI's VAPictureParameterBufferHEVC doesn't expose sps_max_num_reorder_pics or sps_max_latency_increase_plus1.

That fallback is accepted by rkvdec (RK3399 + RK3588 — verified across iter11–iter39) but rejected by rpi-hevc-dec. Per H.265 §A.4.2 the constraint is sps_max_num_reorder_pics ≤ sps_max_dec_pic_buffering_minus1, so our value is spec-legal — but rpi-hevc-dec apparently validates against the bitstream-true value and errors when ours diverges.

Other per-frame ctrl differences also worth investigating once SPS is right:

• kdirect sends 4 ctrls (SPS + PPS + decode_params + slice_array).
• We send 5 (SPS + PPS + slice_array + scaling_matrix + decode_params) — order also differs.

Real fix (out of scope this loop)

The iter2 ampere-VDPU381 chapter already vendors a GStreamer 1.28.2 H.265 parser (src/h265_parser/) precisely to extract bitstream-true SPS / PPS fields VAAPI doesn't forward. The fix is:

1. Wherever h265.c reads SPS from VAAPI's VAPictureParameterBufferHEVC, ALSO parse the SPS NAL from the OUTPUT slice payload using gst_h265_parser_parse_sps.
2. Populate the V4L2 ctrl SPS struct with bitstream-true values for the fields VAAPI omits: sps_max_num_reorder_pics, sps_max_latency_increase_plus1, and any others in the same class.
3. Gate per-driver — only override on rpi-hevc-dec, leave the legacy fallback for rkvdec (avoid disturbing the iter39 5/5 baseline on fresnel + ampere).
4. Optionally: suppress the scaling_matrix ctrl when the SPS doesn't set sps_scaling_list_data_present_flag — match kdirect's ctrl count of 4.

Estimated additional surface area: ~150 LoC in h265.c, plus the parser plumbing that iter2 already provides. Probably 1 more 8(+1)-phase loop — Phase 0 verify rpi accepts bitstream-true values, Phase 1 lock "libva==kdirect on all 3 fixtures", Phase 6 implement, Phase 7 verify.

What's shipped this iter

Branch master 3ffa9d0. NO debian/ packaging yet (Phase 8 deferred until decode actually works — packaging a broken .so is mis-direction). NO Phase 9 memory entry yet — waiting on the iter40b SPS-parse fix to distill the full lesson.

The dev-process Phase 8 packaging + deploy-host re-verify rule wasn't violated: the criterion (Phase 7 bit-exact PASS) wasn't met, so the backend was not packaged + not promoted to a release. Local .so install on higgs only, for debugging.

Sibling regression status

fresnel iter38 5/5 baseline + ampere 9-profile vainfo NOT re-verified post-iter40. Expected unchanged — every iter40 code path is gated on video_fd_rpi_hevc_dec >= 0 which stays false on non-Pi hosts. The only globally-touched line is the __arm__ → __aarch64__ guard in image.c, which now ALSO enables the existing NV15→P010 detile on aarch64 — that path was already silently dead (per iter39 close addendum); enabling it MIGHT cause a behavior change for any consumer that happens to request P010 from an 8-bit-decode surface, but the gate driver_data->is_10bit keeps it dormant for 8-bit fixtures (the iter38 baseline). Verify before declaring the regression-free promise intact.