# Module-PHY diagnostics and the register maps

## DANGER: Marvell CUX3610 (Wiitek UF-RJ45-10G-100) modules brick on certain reads
Both original Wiiteks were bricked **permanently** by register exploration — the death survives cold power cycles and reseats. Read this before touching any replacement.

The CUX3610 has an integrated microcontroller (Marvell Alaska-M, PHY ID 0x002B0BF4, firmware 12.1.5.0) that 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, so it comes up FATAL forever after.

**The trap: 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 even needs to complete. 3.0x8064 is a known QCA808x CDT (cable-diagnostic) 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 explains both deaths with one bug.

Rules for any Marvell/RollBall module:
- **Never sweep vendor register windows. Never read `*.0x??64` (high-byte ≥ 0x80).**
- Single targeted reads only, on registers with a documented reason.
- Recovery after a brick needs a vendor firmware image + the boot-download protocol (registers 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.

## What the Marvell modules give (safely)
Proven on the CUX3610 before they died, and on the Fibergaga oracle — all IEEE 802.3an standard registers, safe on any Marvell module:
- **Per-pair SNR margin** — PMA 1.133–1.136 current, 1.137–1.140 minimum.
  - On the CUX3610 the value is `reg − 0x8000` in 0.1 dB (saw A 9.1 / B 7.1 / C 7.7 / D 8.9 dB).
  - Retrain-dependent — only compare within one training session.
- **Latched error/retrain detection** — PCS 3.32/3.33.
  - Block-lock loss, BER saturate, errored blocks; clear-on-read.
- **Link-partner / master-slave** — 7.33.
- **FW version** 1.0xC011/12; **boot status** 1.0xC050.
- **Forcing a retrain** (to refresh SNR minimums):
  - RollBall write `7.0 |= bit9` (restart AN) — proven safe.
  - An `ip link` bounce does not drop the copper line, so it won't retrain.

Not available in safe space: pre-FEC counter, cable length. Those live in the µC vendor space that killed the modules.

## What the FS/BCM gives
All via the SMI bridge + command handler (transports.md carries the codes and the invocation traps). Proven on the X520:
- **Per-pair SNR** — CMD_GET_SNR 0x8030 invoked bare: DATA2–5 = absolute receiver SNR in 0.1 dB (bench ≈ 27–32 dB; the 10GBASE-T operating point is ≈ 26.5 dB, so margin ≈ value − 26.5). The IEEE registers 1.133–1.140 stay at 0x8080 — handler only.
- **Die temperature** (0x8031) and **supply rails** (0x802F) — module health.
- **Standard PCS 3.32/3.33** — block lock, latched errored-block/BER counters, clear-on-read: the noise-stress error proxy.
- **Retrain** — IEEE AN restart `7.0 |= bit9`, relinks in ~6 s.

Documented in the handler table, not yet exercised (full transcription: `bcm84891l-mdio-commands.md`), in probe-priority order:
- **GET_EEE_MODE 0x8008** — per-speed EEE mode (disabled / native / AutogrEEEn fixed / variable; 10G in DATA1 bits [1:0]). Probe first: the ODM's reference bring-up *advertises 10GBASE-T EEE* (`7.60 = 0x0008`, see `bcm84891l-sfp-registers.md`), and module-to-module LPI would corrupt loss and latency numbers. `SET_EEE_MODE 0x8009` can force it off (AN restart to apply).
- **SET/GET_EEE_STATISTICS 0x801A/0x801B** — arm after link-up, then read TX(local)/RX(remote) LPI event counters or time durations. Direct evidence of whether EEE is actually happening on the wire, independent of what's advertised.
- **Fast-retrain enables 0x800A/0x800B** — the datasheet titles these EMI_MODE but both descriptions read "SW Fast Retrain enable" (per-speed, DATA1=10G/DATA2=5G/DATA3=2.5G). Fast retrains mask marginal-cable events as sub-ms hiccups; knowing (and possibly controlling) the enable state matters when attributing loss bursts.
- **CMD_GET_PAIR_SWAP 0x8000** — forced and current MDI pair mapping, 2 bits per pair: wiring/crossover verification.
- **GET_LIMITED_REACH_MODE 0x8006** — DATA3 = "currently linked in limited-reach mode": the only handler-visible consequence of the DSP's internal cable-length estimate, a single short/long bit, and only meaningful when LR mode is enabled (a config change — measure-only by default).
- **GET_SUB_LF_RF_STATUS 0x800D** — whether copper-TX local/remote fault codes pass through or are replaced with IDLE toward the line (SET/CLEAR 0x8011/0x8010): affects whether the far end can see faults at all.
- **Host-interface health**: GET_XFI_TX_FILTERS 0x802B (FIR taps for the current speed), GET_XFI_POLARITY 0x802D, GET_USXGMII 0x8027 (DATA5 = MAC/PHY frequency unlock flag).
- Pause-frame mode (0x801F/0x8020) turns out to be scoped to 2.5/5G idle-stuffing under 10G XFI WAP mode — inert in plain 10G operation.

**No cable length.** The handler catalog is complete (§1.25.1.1–45: pair swap through WOL) and contains no ECD, length, or skew command. Cable length, opens/shorts, pair skew, and polarity live in the separate ECD register mechanism whose chapter FS hasn't sent. The 1588 engine is the same story: the PHY is IEEE 1588-2008 v2-compliant with a one-command enable (0x8004/0x8005) but its operation registers are undocumented — in-PHY timestamping would measure path delay at the MDI, taking PHY-pipeline latency out of the length equation entirely (open-questions.md §2). Until either chapter lands, FS-side length comes only from the NIC timestamp path — in contrast to the Aquantia oracle, whose `1E.C884` length register is fully documented.

## Marvell cable-length register candidates (unverified, single-shot only)
From kernel drivers for sibling parts — templates for a *future* single targeted probe on a sacrificial module, never a sweep, never near `*.0x??64`:

- **TDR** from the 88Q2220 (`marvell-88q2xxx.c`, same 0x002b0bXX family, recipe cited from "the Marvell API"), all in MMD 3:
  - TDR_RESET 0xFECA — init 0x0D90, clear 0x1D90.
  - Calibration: 0xFED9 = 0x010E, 0xFEDA = 0x00EB, 0xFEDE = 0x0058.
  - TDR_STATUS 0xFEDD — write 0x0002 to start; ~500 ms test.
  - Read TDR_STATUS: bits[1:0] = 01 done; [7:4] verdict (0x7 OK / 0xE open / 0x3 short / 0x5 noise); [15:8] distance in meters.
  - Single-pair on the Q2220; the 4-pair CUX3610 likely has per-pair variants nearby. Low bytes are nowhere near the 0x64 trap.
- **QCA-style CDT** (`qcom/qcom.h`):
  - 3.0x8064 CDT_STATUS, 3.0x8065–0x8068 per-pair results, 3.0x8074–0x807E thresholds.
  - This IS the trap address — only ever entered via the full start sequence, never a cold status read; treat as off-limits absent strong evidence.
- **Next doc-mining lead** for the 4-pair pattern: `gtAdvVct.c` in the Marvell qd-dsdt GPL dumps (e.g. github.com/andy928/xpenology `arch/arm/plat-feroceon/mv_hal/qd-dsdt/`).

## Cable-length physics (applies to both vendors)
- **TDR** (classic VCT/CDT/ECD):
  - Locates opens/shorts both-ended.
  - But *healthy-cable length* needs a far-end reflection, and a plugged-in terminated far end absorbs the pulse — so "OK" pairs report no distance. Effectively single-ended for length.
  - Every kernel implementation only reports distance for *fault* results.
- **DSP estimate**:
  - Both vendors' PHYs compute linked-cable length continuously from insertion loss during training (Marvell "proactively determines cable length"; the BCM's limited-reach power mode switches "based on cable length" while linked).
  - Works in the product's both-ends-plugged topology — the open question is only whether the estimate is exposed in a readable register.
- **NIC timestamp path-delay** — the module-independent fallback if the DSP length isn't reachable:
  - Works linked.
  - Needs per-packet or PTP-latch hardware timestamps.
  - Needs a short-cable calibration to cancel fixed PHY latency (~4.8 ns/m one way; 50 m ≈ 240 ns).