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thejayman77 2123e646c8 docs: Ch441 board validation + Ch437 A/B scanout evidence

Preserve the Ch441 timing/renderer board-validation evidence and the
Codex-directed follow-up:
- FB reconciliation: the Ch437 A/B dump's 8-word first-beat delta was a
  non-reproducible one-time transient; 3x render+dump and 8x pure-probe
  re-reads all return golden d0047677 (words 0-7 correct). Memory integrity
  confirmed deterministic. (fb_reconciliation.md)
- Structural audit + A-D diagnostic proposal for LPDDR_STATUS[5]: bit5 ORs
  sticky CDC-synced underflow with AXI read-errors; dump path (arid=1) is
  separate from scanout/HDMI (arid=3). Instrumentation proposal only, not
  implemented. (scanout_status_bit5_proposal.md)
- Raw board logs, status samples, RBF hashes, A/B FB dump, board PNG.

No RTL edits, no compile, no builds.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

2026-07-21 20:34:20 -04:00

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Ch441 board validation — framebuffer reconciliation (zsrt139f52)

Scope: reconcile the one framebuffer-content discrepancy seen during the Ch441↔Ch437 A/B, and record the Codex-authorized zero-RTL re-dump triage that classifies it.

The A/B FB discrepancy (as first observed)

During the Ch441-vs-Ch437 A/B, the Ch437 --dump-fb readback logged sum32=0xaf7b63f3 xor32=0x33178b8f, differing from the accepted golden sum32=0xaad0b94d xor32=0x33138181. Local word-level diff localized the entire difference to exactly 8 words — the first 256-bit LPDDR beat, row 0, x=0..7, y=0 (307192/307200 = 99.997% byte-identical). In all 8 words the alpha byte matched; only RGB differed (golden ≈ 0x0b dark, dump ≈ 0xb6 bright, a uniform ~+0xaa lift). Those 8 words fully account for the logged sum32/xor32 deltas and nothing else.

Artifacts:

sh3_zsrt139f52_ch437_ab_fb.mem SHA-256 8fd30cdb926ab7a73c39827a2e3352b72c92af78ebaa7b4d69fd9715bf693aec
golden / prior-Ch437 / Ch441 A/B all: SHA-256 d0047677371a0f6e4e319458926f604a8599c92baaf34d09b8e1fd452e31662b

Key architectural fact: --dump-fb reads LPDDR via gs_lpddr_rd_probe (arid=1), a different AXI master from the scanout line-buffer (gs_lpddr_scanout_lb, arid=3). So this discrepancy lives on the probe-readback / write path, NOT the scanout/HDMI path.

Zero-RTL re-dump triage (Codex-authorized; no RTL / Quartus / builds)

Image under test: retroDE_ps2.ch437.candidate.rbf (the anomaly image), CORE_ID 0x50533200. Two independent axes, on the same f52 replay (sh3_zsrt139f52_epochs.txt):

Write-side determinism — three full render+dumps (A/B/C).
Readback determinism — repeated pure-probe re-reads of words 0..15 via the OFF_LPDDR_RDADDR (0x03C) latch, with NO re-render, on the frame left by dump C.

Result — anomaly did NOT reproduce

dump	SHA-256	sum32	words 0..7
A (render+dump)	`d0047677…`	`0xaad0b94d`	golden (dark)
B (render+dump)	`d0047677…`	`0xaad0b94d`	golden (dark)
C (render+dump)	`d0047677…`	`0xaad0b94d`	golden (dark)

First 16 words, identical across A/B/C: 040c0c0b 030c0b0b 030a0909 030b0b0b 030d0b0b 040e0c0c 050f0e0e 050f0e0e 050b0a0a 050a0909 05090808 050a0a0a 050e0d0d 050d0d0c 050d0c0b 050d0b0b

Pure-probe re-reads (8 rounds, no re-render) returned those same values every round — words 0..7 stable and golden, no bright first beat.

Classification

The earlier 8-word bright first-beat was a non-reproducible one-time transient in that single A/B dump — not a stable property of the stored framebuffer and not a deterministic write or readback defect. Three fresh renders + repeated probe re-reads are byte-identical to golden across the full frame, including words 0..7. Framebuffer-memory integrity for this scene is confirmed deterministic (d0047677…). Evidence integrity is resolved.

This triage does NOT diagnose LPDDR_STATUS[5]; that is the scanout/HDMI (arid=3) path, addressed separately (see scanout_status_bit5_proposal.md).

Raw logs

sh3_zsrt139f52_ch437_redump_[ABC].log, redump_triage_console.log.

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