Per-module diagnostics: capabilities, register maps, hazards. Each device directory holds its doc plus its datasheets. NIC-side transports: [../nics/](../nics/README.md).
> ⚠ **Before touching any Wiitek/Marvell module, read the brick trap in [wiitek/](wiitek/README.md).** Two modules died permanently from register exploration.
## The modules
Counts and current placement: [../state.md](../state.md).
| 10Gtek | (claims SFP-10G-SR) | — | Copper RJ45 despite the SR part number; filler, not in the test set |
## The media lies (mostly)
**Every test module is a copper RJ45 10GBASE-T module** — none are fiber, whatever the EEPROM claims; the real media is a 10GBASE-T PHY inside each module. EEPROM honesty varies by vendor, with driver consequences:
| FS | Honest: RJ45 (0x22), 10GBASE-T Short Reach, 100 m copper, no fake DOM | **Rejected** — kills the whole port probe (error -95, no netdev) |
| 10Gtek | `SFP-10G-SR` part number | (not in the test set) |
-`allow_unsupported_sfp=1` is mandatory on the X520 — and it's the *truthful* module that requires it ([../nics/x520/](../nics/x520/README.md)).
- Physical-layer reasoning must use 10GBASE-T: PAM16, LDPC FEC, self-synchronizing scrambler, 4 twisted pairs, distance/temperature sensitive — never an optical model.
- The module PHYs keep the copper link trained on their own: an admin `ip link set down` does NOT drop the wire unless the i40e `link-down-on-close` priv flag is set (peer sees the drop in ~200 ms, relinks in ~0.9 s).
## IEEE 802.3an standard registers (safe on all copper module PHYs)
Proven on the CUX3610 (before the originals died) and the Fibergaga; the BCM notably does **not** populate the SNR set ([fs/](fs/README.md)).
| Registers | What | Notes |
|---|---|---|
| PMA 1.133–1.136 / 1.137–1.140 | Per-pair SNR margin, current / minimum | CUX3610 value = `reg − 0x8000` in 0.1 dB. Retrain-dependent — only compare within one training session |
| AN 7.0 bit 9 | Restart AN — forces retrain, refreshes SNR minimums | Proven safe everywhere. An `ip link` bounce does not drop the copper line, so it won't retrain |
## RollBall protocol (Marvell and Aquantia modules)
Clause-45 access to the internal PHY over I2C A2h, all messages ≤ 4 B, offset-addressed:
Split transactions (separate STOP per byte) are field-proven on both the Fibergaga and the original Wiiteks; the kernel `mdio-i2c` combined form is not required by the modules.
- **TDR** (classic VCT/CDT/ECD): locates opens/shorts both-ended. The generic concern — *healthy-cable length* needs a far-end reflection, and a plugged-in terminated far end absorbs the pulse, so "OK" pairs report no distance (every kernel implementation only reports distance for *fault* results) — is disproven for the BCM: its ECD reports per-pair length for healthy pairs on a plugged, terminated, linked cable, meter-accurate ([fs/](fs/README.md)). **The BCM ECD is the product length path.**
- **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 question is only whether the estimate is exposed in a readable register (Aquantia: yes, `1E.C884`; BCM/Marvell: not in documented space).
