ampere-fourier/iter1_close.md

iter1 close — ampere-fourier baseline

Closed 2026-05-16. Iteration 1 of the ampere-fourier campaign. Establishes the validated 3-codec libva HW decode baseline on RK3588 and spawns the three deferred-codec iterations.

What iter1 delivers

Question	Answer
Does libva-v4l2-request-fourier @ 7ac934e work on RK3588 unchanged?	Yes, for the 3 codecs the v7.0-rc3 mainline kernel exposes via V4L2 stateless on this board.
Which codecs?	H.264 (rkvdec), VP8 (hantro generic), MPEG-2 (hantro generic).
With what guarantees?	C1-C6 per phase1_goal.md: decode completes, HW path engaged (ioctl-trace evidence), frame 0 byte-identical vs SW reference, frame 720 per per-codec SSIM Y floors (see below), N=3 FPS reported with σ, dmesg clean.
What about HEVC, VP9, AV1?	Each is blocked behind one specific external dependency. Iter1 spawned a follow-up issue per blocker; each becomes its own iteration.
What about the firefox-fourier consumer-side test (C7)?	Out of iter1 scope — ampere ran headless-ssh by design. C7 anchors to whichever future iteration first runs a compositor on ampere (likely a UX sub-iteration after AV1 lands, or the firefox-consumer iteration that follows).

Per-codec result table

Codec	C1 frames	C2 ioctls	C3 frame 0	C4 frame 720 (per refined floor)	C5 FPS (N=3)	C6 dmesg
H.264	✓ 30/30	✓ V4L2 ioctls 224 + MEDIA_REQUEST 88	✓ byte-identical, sha 3214803d8be74416	observed SSIM Y 0.667575 (no PASS threshold — see drift note)	461.49 ± 0.61	✓ empty diff
VP8	✓ 30/30	✓ V4L2 ioctls 208 + MEDIA_REQUEST 82	✓ byte-identical	✓ SSIM Y 1.000000 (byte-identical at f720, exceeds floor ≥ 1.000)	217.24 ± 0.57	✓ empty diff
MPEG-2	✓ 30/30	✓ V4L2 ioctls 168 + MEDIA_REQUEST 66	✓ byte-identical	✓ SSIM Y 0.999720 (within floor ≥ 0.9997, IEEE 1180 tolerance)	199.84 ± 0.70	✓ empty diff

Refined C4 per-codec floors (from Phase 4 plan §1, amended per Phase 5 review):

Codec	Floor	Basis
H.264	no PASS threshold (documented at 0.6676)	cumulative GOP drift between libavcodec SW and rkvdec HW — see drift note below
VP8	≥ 1.000 (byte-identical)	Phase 3 N=3 observed perfect bit-exactness; raises the bar instead of lowering it
MPEG-2	≥ 0.9997	memory feedback_mpeg2_hw_sw_idct_precision — hantro IDCT conformant per IEEE 1180 but ≤3 LSB off SW

H.264 SSIM 0.6676 drift note (Phase 5 framing amendment applied)

The H.264 SSIM Y at frame 720 measured 0.667575 on ampere RK3588. Fresnel iter1 measured 0.6431 on RK3399.

These are two independent empirical data points on two different SoC generations. They are not cross-verified against each other and neither is the other's reference anchor. RK3399's rkvdec is the v1-era driver; RK3588's rkvdec uses the VDPU381/383 mainline path — different decoder hardware, different deblocking and entropy-decode implementations. The fact that the two drift values converge (~0.65 ± 0.02) is consistent with both decoders being H.264-conformant within tolerance vs libavcodec SW but not bit-identical to it — which is exactly what's expected when two independent hardware implementations both follow the H.264 spec to its allowed precision.

The drift is NOT:

• a libva backend bug (frame 0 byte-identical, decode completes, HW path engaged)
• a kernel ABI deviation (Phase 3 ruled out frame-0 byte-deviation, which is the diagnostic for that)
• specific to either chip generation (both show the same shape of drift)

The drift IS:

• the expected accumulated divergence between two conformant H.264 implementations across a 720-frame inter-prediction chain
• accepted as-is for iter1; future iterations may investigate whether an x264 encode setting (-tune psnr, -no-cabac) reduces drift, but that's not iter1 scope

Codecs blocked, spawned follow-up issues

Codec	Blocker	Follow-up issue	Iteration that picks it up
HEVC	kernel oops in rkvdec_hevc_prepare_hw_st_rps (__pi_memcmp fault) — cascades to wedge v4l2_mem2mem for all decoders until reboot	marfrit/kernel-agent#11 [ka:experiment]	iter2
VP9	kernel doesn't register V4L2_PIX_FMT_VP9_FRAME on RK3588 rkvdec; needs VDPU381/383 variant_ops enablement	marfrit/kernel-agent#12 [ka:experiment]	iter3
AV1	kernel exposes /dev/video4 cleanly but libva backend iter38 auto-probe hard-capped at 2 fds; need iter39 third-fd model	marfrit/libva-v4l2-request-fourier#2	iter4

marfrit/kernel-agent#6 (the iter0 umbrella) closed with cross-refs to all three.

C7 — out of iter1 scope (headless by design)

The firefox-fourier vendor-defaults engagement test (Phase 1 C7) is out of iter1 scope. iter1 ran headless-ssh by design — no UX requirement, no compositor needed for C1-C6 which fully cover the libva backend correctness verdict.

C7 needs a Wayland compositor on ampere (the widget.dmabuf.force-enabled pref unlock path requires a real DMA-BUF surface to negotiate against). When a future iteration first stands up a graphical session on ampere — likely either (a) the firefox-consumer iteration after AV1 lands, or (b) a dedicated UX sub-iteration — C7 lands there. It is NOT a missing rig prerequisite that iter1 should have fixed; it's a scope boundary.

The vendor-default pref file is shipped correctly: /usr/lib/firefox-fourier/defaults/preferences/rockchip-fourier-defaults.js contains the three required prefs (widget.dmabuf.force-enabled, media.hardware-video-decoding.force-enabled, media.ffvpx-hw.enabled all true) per marfrit-packages#8. So C7 is "ready to run, just needs a compositor."

State to carry forward to iter2+

Carries (state):

• Backend source pin 7ac934e (iter38b)
• Hand-build over the broken CI package binary remains the install method until marfrit-packages#17 is fixed
• Source clips in ~/measurements/encoded/ on ampere (5 codecs encoded, immutable)
• Per-codec C4 floors as refined above (VP8 ≥ 1.000, MPEG-2 ≥ 0.9997, H.264 documented)
• Reboot authorization for ampere when v4l2 stack wedges (per Phase 0 operator decision)

Does NOT carry (data):

• The Phase 3 N=3 FPS numbers are iter1-anchor only. iter2/3/4 measure their own anchors per their Phase 1 success criteria, with the iter1 N=3 mean as reference history.

New work items unblocked by iter1 close:

• iter2: HEVC fix experiment via kernel-agent#11
• iter3: VP9 enablement experiment via kernel-agent#12
• iter4: AV1 backend iter39 via libva-v4l2-request-fourier#2
• (potential) UX sub-iteration to validate C7 once a compositor is up on ampere

What did NOT happen in iter1 (for clarity)

• No code changes to libva backend, mpv-fourier, ffmpeg-v4l2-request-fourier, firefox-fourier, kernel.
• No pacman -Syu on ampere (deliberate per Phase 2 constraint 1; libva hand-build override would have been silently reverted).
• No SDDM auto-login configuration (would have been scope creep; C7 deferred instead).
• No re-encoding of test clips (immutable for the campaign).

Phase 8 hand-off

Memory entry will distill the iter1 lesson: campaigns on a new host with a known-good backend benefit from a "characterize-and-baseline" iteration before any change-the-system iteration; the baseline establishes the floor that change-iterations regress against, and surfaces blockers as separate issue threads rather than letting them conflate into the change-iteration's debug loop.