Both original Wiiteks were bricked **permanently** by register exploration — the death survives cold power cycles and reseats.
- The CUX3610's integrated microcontroller (Marvell Alaska-M, PHY ID 0x002B0BF4, firmware 12.1.5.0) boots from onboard non-volatile storage. The high vendor register windows (3.0x8000+, 31.0xF000+) reached over RollBall are **µC-mapped peripheral space, not PHY-core MDIO**. A read of a trapped peripheral crashes the µC mid-operation and corrupts its persistent boot state; it comes up FATAL forever after.
- **The confirmed kill: any register whose low byte is 0x64 in a high window (register high-byte ≥ 0x80), on any devad.** Module 1 died reading 3.0x8064, module 2 reading 31.0xF064. The mailbox writes the address bytes, then the µC dies executing the access — the command never needs to complete. 3.0x8064 is a known QCA808x CDT status register; the ODM firmware appears to emulate/intercept a CDT block there and crash on a cold read. A sloppy `(reg & 0xFF) == 0x64` decode is one hypothesis that fits both deaths — but it is the *confirmed* pattern, not proof the danger stops there. The whole `3.0x8000+` / `31.0xF000+` space is µC-mapped peripheral space; treat all of it as lethal-until-proven, `??64` merely most-proven.
- Recovery needs a vendor firmware image + the boot-download protocol (1.0xC050 boot status, 1.0xD0F0–D0F3 download) — we have no CUX3610 image, so bricks are effectively dead. A ghost I2C device at 7-bit 0x57 (returns zeros) is the ROM bootloader waiting for a download.
The `rollball_ixgbe.py` client hard-guards the blacklist and raises *before* touching hardware — it structurally cannot repeat the kill.
The shared IEEE 802.3an set ([../README.md](../README.md)) plus two safe vendor reads. Verified on the current replacement unit (SNR [8.8, 7.0, 7.3, 8.9] dB, matching the dead originals' A 9.1 / B 7.1 / C 7.7 / D 8.9 — a representative module):
`rb_forensic.py` in phydiag-work performs this baseline and logs every sff_i2c op (PRE line fsync'd before each op, POST after) to `wiitek-ops.log` — a PRE with no POST names whatever killed the µC. `rollball_ixgbe.py` hard-guards the `*.0x??64` blacklist and raises before touching hardware.
This PHY offers no length capability the product needs (the length strategy lives in [../README.md](../README.md)); every known VCT candidate lands in the exact danger zone, and the safe-looking option is a documentation error corrected here:
- **The 88Q2220 TDR recipe is NOT safe.** Its registers (MMD 3, 0xFECA/0xFED9/0xFEDA/0xFEDE/0xFEDD) sit at `3.0xFExx` — inside the `3.0x8000+` µC-mapped window this doc identifies as the space that bricked both modules. Low-byte ≠ 0x64 does **not** make them safe; the whole high window is µC-intercept space, and the brick was a µC crash on access, not specifically the 0x64 decode. (The 0x64 pattern is the one *confirmed* kill; it is not the boundary of the danger.) Treat the entire `3.0x8000+` / `31.0xF000+` space as lethal-until-proven, not the `*.??64` addresses alone.
- **The QCA-style CDT block** (`qcom/qcom.h`: 3.0x8064 status, 3.0x8065–0x8068 per-pair results, 3.0x8074–0x807E thresholds) is the most likely *intended* VCT — the ODM firmware emulates it — but it is centered on `3.0x8064`, the precise register that killed both originals. Hypothesis: the kill was a *cold* status read before running the test, and the real protocol is write-start → wait → read-status/results. Plausible, unproven, and not worth a live module.
- **No usable public reference exists.** The kernel 10G Marvell driver (`marvell10g.c`) implements no cable test; `gtVct.c` in the qd-dsdt GPL dumps is legacy clause-22 1000M paged access (wrong architecture for this clause-45 part); no public datasheet documents the Alaska-M VCT register map.
1.**The real VCT register map** from Marvell/Wiitek under NDA — the only route to a *confirmed*-safe sequence.
2.**Deliberate sacrifice** of a spare to test the QCA-CDT-start hypothesis, `rb_forensic.py` running — only if the capability is judged worth a module.
[10gbase-t-phy-register-operation.md](10gbase-t-phy-register-operation.md) (converted from the vendor `.doc` alongside it) is Wiitek's own spec of the A2 Table 3 I2C→MDIO mailbox — official confirmation of the RollBall protocol this project drives (A2.127=3 table switch, PW2 password at A2.123–126, CMD states at A2.128). It also documents two registers not covered elsewhere: A2.250 (PHY part number, 1 = 88X3310) and A2.120 (copper link rate; vendor-gated to AQR/RTL8221/firmware 377 v4.5+, unverified on the Marvell CUX3610 units).
