fresnel-fourier/phase6_iter4_implementation.md

Iteration 4 — Phase 6 (implementation)

Commits Z + A + B + C landed against fork tip e1aca9c (iter3 close). All built clean on fresnel linux-fresnel-fourier 7.0-1 (libva 1.23, ninja-1.13.2, gcc 15.2.1). No fix-forward Commit D needed — buffer.c allow-list was already permissive for the 3 VAAPI buffer types VP9 uses.

Commits

SHA	Files	LOC	Summary
7f8fa93	src/request.c	+152 / -4	Commit Z — device-path auto-detect via media controller topology. Walk /dev/media*, MEDIA_IOC_DEVICE_INFO → driver match, MEDIA_IOC_G_TOPOLOGY → MEDIA_INTF_T_V4L_VIDEO interface → resolve major:minor via /sys/dev/char/<M>:<N> symlink. Canonical v4l2-request discovery; not enumeration-order dependent. LIBVA_V4L2_REQUEST_NO_AUTODETECT=1 escape hatch for legacy callers.
16b3973	src/config.c	+15	Commit A — VP9 enumeration + dispatch + entrypoints (3 sites).
406d08e	src/vp9.c (new), src/vp9.h (new), src/meson.build, src/context.h, src/surface.h	+810 / -2	Commit B — VP9 backend: 12 contract clauses, VPX range coder, inv_map_table, uncompressed-header partial parser, compressed-header parser, persistent vp9_lf state in object_context. Phase 5 C1+C2+C3 amendments + S4 baked in. Compile-time _Static_assert on struct sizes 168/2040.
beaa914	src/picture.c	+19	Commit C — VP9 dispatcher + 2 buffer-type cases (Picture, Slice). No RequestBeginPicture reset (VP9 has no iqmatrix_set-style flag).

Total: 4 commits, 996 LOC delta, 7 files (2 new + 5 modified). No Commit D.

Verification on fresnel

Substrate

Linux fresnel 7.0.0-fresnel-fourier #1 SMP PREEMPT
/usr/lib/dri/v4l2_request_drv_video.so SHA256
  f2ff65982a777b64f3392fa6be13b934d969068a1309d24364d99708462c04ee

Criterion 1 — vainfo enumerates VAProfileVP9Profile0

$ LIBVA_DRIVER_NAME=v4l2_request vainfo
v4l2-request: auto-selected codec device: /dev/video1 + /dev/media0
vainfo: VA-API version: 1.23 (libva 2.22.0)
vainfo: Driver version: v4l2-request
vainfo: Supported profile and entrypoints
      VAProfileH264Main               :	VAEntrypointVLD
      VAProfileH264High               :	VAEntrypointVLD
      VAProfileH264ConstrainedBaseline:	VAEntrypointVLD
      VAProfileH264MultiviewHigh      :	VAEntrypointVLD
      VAProfileH264StereoHigh         :	VAEntrypointVLD
      VAProfileHEVCMain               :	VAEntrypointVLD
      VAProfileVP9Profile0            :	VAEntrypointVLD       ← NEW

Auto-detect (no env override) successfully picked rkvdec via media topology graph. PASS.

Criterion 2 — vaCreateConfig SUCCESS

Implicit in successful ffmpeg-vaapi run below. PASS.

Criterion 3 — ffmpeg-vaapi VP9 decode exit 0

$ LIBVA_DRIVER_NAME=v4l2_request ffmpeg -hwaccel vaapi -hwaccel_device /dev/dri/renderD128 \
    -i ~/fourier-test/bbb_720p10s_vp9.webm -frames:v 5 -f null -
v4l2-request: cap_pool_init: 24 slots ready (v4l2_index=0..23, 1 plane(s) per slot)
[...]
frame=    5 fps=0.0 q=-0.0 Lsize=N/A time=00:00:00.20 bitrate=N/A speed=0.307x

5 frames decoded at 0.307x speed. Zero kernel ioctl errors. cap_pool_init successful. PASS.

Criterion 4 — HW=SW byte-identical (deferred to Phase 7)

Phase 7 will run mpv VP9 capture vs the Phase 3 SW reference PNGs and compare hashes. mpv's vaapi-init path needs investigation (iter4-B2 — does not currently engage HW even with LIBVA_DRIVER_NAME set, while ffmpeg-vaapi does). Workaround for criterion 4: use ffmpeg-vaapi-hwdownload path to PNG instead of mpv-vaapi-copy, OR use the iter3-style transitive proof (kernel-direct == backend-payload).

Criterion 5 — 4-codec regression block

H.264 + HEVC (rkvdec via auto-detect): Commit Z restored device-path baseline. Profiles enumerate cleanly without env override.

MPEG-2 + VP8 (hantro): require LIBVA_V4L2_REQUEST_VIDEO_PATH=/dev/video3 LIBVA_V4L2_REQUEST_MEDIA_PATH=/dev/media1 env override (Phase 5 C4 known scope limitation; iter4-B1 backlog).

Engagement-status verification deferred to Phase 7.

Phase 5 amendments status (post-Phase 6)

ID	Amendment	Implementation status
C1	frame.interpolation_filter = picture->mcomp_filter_type (no XOR)	vp9.c::vp9_set_controls line near "Phase 5 C1: NO XOR" comment
C2	Persistent vp9_lf state in object_context, init {1,0,-1,-1,0,0} on key/intra/error_res, parse-and-update only on lf_delta.update=1, always copy	context.h struct vp9_lf; vp9.c::vp9_set_controls Clause 6 block
C3	vp9_fill_compressed_hdr takes uint8_t *out_reference_mode; allowcompinter derived from sign biases	vp9.c::vp9_fill_compressed_hdr signature + caller derivation
C4	LIBVA_V4L2_REQUEST_NO_AUTODETECT=1 escape hatch	request.c::VA_DRIVER_INIT_FUNC
S4	uv_mode memcpy omitted (rkvdec reads from kernel persistent table)	vp9.c::vp9_fill_compressed_hdr ends without memcpy

Cross-cutting backlog updates

• iter4-B1 (open) Walk-and-pick-first selects rkvdec on RK3399 (its media controller enumerates first); hantro codecs still need explicit env override. Multi-decoder dispatch refactor required.
• iter4-B2 (open) mpv-vaapi Could not create device failure — appears to be in mpv's libva-DRM device-init path, not in our backend. ffmpeg-vaapi works fine through the same backend; mpv does not. Investigate in Phase 7 if criterion 4 verification needs a mpv path.
• iter4-Q6 (open) Per-segment seg_param[s].luma_ac_quant_scale → kernel feature_data[s][ALT_Q] mapping is fundamentally lossy for non-BBB segmentation-enabled streams. BBB has segmentation disabled so dead code there.
• iter4-COLOR_RANGE (open) VAAPI doesn't expose color_range; backend leaves V4L2_VP9_FRAME_FLAG_COLOR_RANGE_FULL_SWING clear (BT.709 limited). Wrong for full-range JPEG-encoded VP9.

Substrate state at Phase 6 close

• Fork at iter4 Commit C tip beaa914 on noether + fresnel ~/src/libva-v4l2-request-fourier/ + /usr/lib/dri/v4l2_request_drv_video.so.
• vainfo (auto-detect rkvdec) lists VAProfileVP9Profile0.
• ffmpeg-vaapi VP9 decode: criterion 3 PASS empirically.
• mpv-vaapi VP9 still SW-fallback (iter4-B2; not blocking criterion 3).
• Kernel: linux-fresnel-fourier 7.0-1. Same as Phase 3.
• Fork ready for Phase 7 verification (HW=SW + regression).