ampere-av1-enablement/phase3_close.md

ampere-av1-enablement Phase 3 close — AV1 PASS on all-intra; film_grain+show_existing edge case localized

Closed 2026-05-17 evening. Phase 2.1 + Phase 3 substantial iteration. Substrate state on ampere: backend tip c839b94 on av1-iter1 branch (matches av1-iter1-imported on noether). Module installed at /usr/lib/dri/v4l2_request_drv_video.so. Kernel 7.0.0-rc3-devices+ unchanged.

Test matrix

Fixture	Source	Frames	Resolution	Profile	Result
test_av1.ivf	AOM av1-1-b8-01-size-208x208 (also /tmp/test_av1.ivf)	2	208×208	Main, 8-bit, no grain	bit-exact PASS 2/2 (sha 029ee72c214b37c1)
av1-1-b8-02-allintra.ivf	AOM	39	352×288	Main, 8-bit, all-intra	bit-exact PASS 10/10 (first 10 frames sampled)
av1_larger.ivf / av1-1-b8-23-film_grain-50.ivf	AOM	10	352×288	Main, 8-bit, film_grain, show_existing_frame	3/10 PASS (frames 0, 2, 4 — apply_grain=1 IDR-derived)
av1-1-b10-23-film_grain-50.ivf	AOM	10	352×288	Main, 10-bit, film_grain	both libva AND kdirect produce 0 bytes — vpu981 may not support 10-bit AV1

What works (validated on hardware)

• AV1 dispatch + control submission: SEQUENCE, FRAME, TILE_GROUP_ENTRY, FILM_GRAIN all submitted correctly; strace shows kernel accepts every batch.
• All four V4L2 controls byte-identical to kdirect for the first 7 EndPicture calls (verified via patched libavcodec.so LD_LIBRARY_PATH override that adds an fwrite diag to ff_v4l2_request_append_output).
• DPB / reference frame timestamp plumbing: VAAPI ref_frame_map[i] (surface IDs) → SURFACE() lookup → v4l2_timeval_to_ns(&ref_surface->timestamp).
• Film grain link infrastructure: when apply_grain=1, current_display_picture != current_frame; we link the display surface to the decode surface so vaGetImage on the display surface follows back to the decode surface's CAPTURE slot.
• refresh_frame_flags = 0xff: VAAPI doesn't expose; default 0xff matches AV1 spec for KEY/SWITCH frames and kdirect's submission.
• ENABLE_SUPERRES gated on picture->pic_info_fields.bits.use_superres: matches kdirect; was previously unconditional set-true.
• Per-surface AV1 order_hint tracking: surfaces carry av1_order_hint set at decode time; referenced surfaces' values populate the V4L2 ctrl's order_hints[].
• F1/F2/F3 risk mitigations from the Janet plan v2 review: mi_col/row_starts sentinel, superres_denom correct, loop_restoration_size[] gated on USES_LR — all applied.

What's open (Phase 4 territory)

Remaining 7/10 divergence on av1_larger.ivf localized to:

• ffmpeg-vaapi's AV1 hwaccel issues 2 EXTRA vaEndPicture calls on REUSED surfaces (0x4000008 repeated at t8, 0x4000006 repeated at t9) compared to ffmpeg-v4l2request's 7 calls for the same input.
• The reused-surface pattern correlates with the IVF having show_existing_frame OBUs at frame positions 2, 4, 6 (each just 5 bytes — "redisplay frame X").
• Our iter2 Fix 3 (release-on-rebind) invariant: 1 surface ↔ 1 cap_pool slot at a time. When ffmpeg-vaapi rebinds, prior CAPTURE data is gone.
• Falsified: LIBVA_SKIP_REBIND=1 experiment (do not unbind in BeginPicture, leak the old slot) produced identical 3/10 PASS count as default behavior. So iter2 Fix 3 is NOT the cause; the issue is deeper in how the surface→slot accounting interacts with ffmpeg-vaapi's surface reuse.

Hypothesized fix paths (Phase 4)

1. Multi-surface-per-slot cap_pool refactor: track {slot → set of surfaces} so when a surface is re-bound, the slot can still serve vaGetImage for the surface IDs that previously bound it. Bigger refactor than this iteration.
2. Surface-ID identity tracking via picture parameters: snoop AV1's current_frame / current_display_picture across frames to detect when ffmpeg-vaapi means "render this prior frame again" vs "decode a new frame", and dispatch differently. Requires understanding ffmpeg-vaapi's AV1 hwaccel surface allocation logic.
3. ffmpeg-vaapi source fix: modify ffmpeg-vaapi to use distinct surfaces for show_existing_frame display rather than reusing decode surfaces. Cross-package; rejected as default first move.

Commits delivered this iteration

On av1-iter1 branch (both ampere /home/mfritsche/src/libva-v4l2-request-fourier/ + noether /home/mfritsche/src/libva-multiplanar/libva-v4l2-request-fourier/):

c839b94 ampere-av1 Phase 3 finding: iter2 Fix 3 release is NOT the divergence cause
d7ef0f6 ampere-av1 Phase 3: SEQUENCE byte-equal kdirect; 3/10 frames PASS bit-exact
5803cbc ampere-av1 Phase 3 progress: film_grain link + UPDATE_GRAIN; frame 0 bit-exact
ab79ed5 ampere-av1 Phase 3 in-progress notes: UPDATE_GRAIN segfault; 352x288 still 0-byte
5fb7e36 ampere-av1 Phase 3 fix: wire reference_frame_ts[] from VAAPI ref_frame_map[]
85bcddb v4l2: surface error_idx + errno on VIDIOC_S_EXT_CTRLS failure
9c30ecc ampere-av1 Phase 2.1: implement av1_set_controls body (~500 LoC)
78a9978 ampere-av1 Phase 2 step 4: AV1 dispatch scaffolding compiles and wires
61db76e ampere-av1 Phase 2 step 2: advertise VAProfileAV1Profile0 via libva
bed75c0 ampere-av1 Phase 2 step 1: third-device fd scaffolding for vpu981

Branch is NOT yet pushed to gitea (per pre-amnesia me's "ssh sideband disconnect on vp9 branch" note — manual push attempt deferred).

Verifier scripts retained

• /tmp/diff_av1_ctrls.py on ampere: per-CID byte diff between two strace logs. Decodes octal-escaped strings, matches the same ctrl across calls, prints byte-level diffs.
• /tmp/ivf_split.py on ampere: splits an IVF file into per-frame .bin files. Reveals AV1 show_existing_frame OBUs as 5-byte stub frames.
• patched libavcodec.so.62.28.100 shadow build on boltzmann at ~/marfrit-packages/arch/ffmpeg-v4l2-request-git/src/FFmpeg/libavcodec/libavcodec.so.62; the .bak source was restored to clean — re-apply via:

  ssh boltzmann 'sed -i "/memcpy(pic->output->addr + pic->output->bytesused, data, size);/a\\        do { static unsigned int __dump_idx = 0; char __p[256]; snprintf(__p, sizeof(__p), \"/tmp/K_dump_kdirect/append_%04u_size%u.bin\", __dump_idx++, size); FILE *__f = fopen(__p, \"wb\"); if (__f) { fwrite(data, 1, size, __f); fclose(__f); } } while (0);" ~/marfrit-packages/arch/ffmpeg-v4l2-request-git/src/FFmpeg/libavcodec/v4l2_request.c'
  

  then make -j8 libavcodec/libavcodec.so + scp to ampere's /tmp/lib_patched/, run kdirect with LD_LIBRARY_PATH=/tmp/lib_patched:$LD_LIBRARY_PATH.

Stance

Phase 3 closes with AV1 hardware decode WORKING for the common cases: simple intra-only (10/10), grain-free inter (test_av1), and grain-IDR frames (3/10 on av1_larger). The remaining edge case (grain + show_existing inter frames) is a real divergence but is now narrowly localized — its fix space is understood (cap_pool refactor or ffmpeg-vaapi surface tracking), and a future iteration can pick it up with the full diagnostic infrastructure already in place.

Tasks #21 carries the resumption details.