Add in-repo project docs: goals, state, hardware, module-PHY transports, module diagnostics, and measurement lessons

docs/modules.md

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+# 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.
+
+## 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).