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Add FS/BCM capability inventory; drop SNR/power-backoff length proxies -- length is PHY features or NIC timestamps only

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flamingcow
2026-08-12 18:23:28 -07:00
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@@ -53,10 +53,9 @@ If route 1 validates, the X520 delivers honest buckets after all — its disqual
Decide which is the product path vs. nice-to-have.
- **A fourth path exists in pure standard registers — no vendor docs at all.** The Clause 45 10GBASE-T PMA group we already read (1.133–1.140) also carries the negotiated TX power backoff / short-reach fields (~1.131 — verify the exact address in 802.3 Table 45–3 before use; all low-address space, nowhere near the `??64` trap). Power backoff is derived from measured channel insertion loss in 2 dB steps → a coarse length bucket on any compliant module. And SNR margin itself falls monotonically with insertion loss on a healthy cable: a small calibration table (known-length cables → margin per pair) likely satisfies goals.md's "sanity check" outright, leaving ECD needed only for fault *localization*.
- **BCM fallback, checked:** OpenBCM's `phy8481.c` covers the copper XGPHY family only through BCM8488x — no 84891, and no ECD code in the retrievable portion — and the kernel's Broadcom ECD (`bcm-phy-lib`) is the BCM54xx GbE register model; neither transfers. SDK/patent mining looks low-yield. FS delivering the ECD chapter stays the primary route, with the standard-register proxy above as the hedge.
- **BCM fallback, checked:** OpenBCM's `phy8481.c` covers the copper XGPHY family only through BCM8488x — no 84891, and no ECD code in the retrievable portion — and the kernel's Broadcom ECD (`bcm-phy-lib`) is the BCM54xx GbE register model; neither transfers. SDK/patent mining looks low-yield. FS delivering the ECD chapter stays the primary route.
- **The Aquantia-based oracle gets a fully documented path the Wiiteks can't have.** The Fibergaga's PHY family register reference is now vendored in this folder (`aquantia-aqr107-aqr109-datasheet.pdf`, see item 5), and it delivers the DSP estimate outright: **`1E.C884` "Global Status" bits 7:0 = estimated cable length in meters, ±1 m, RO, "estimated from the cable diagnostic engine."** Plus full TDR results: per-pair fault verdicts (open/short/mismatch/cross-pair) at `1E.C800`, distances of the four worst reflections per pair (±1 m) at `1E.C801–C807`, reflection impedance/magnitude at `1E.C880–C883`. Bench question: whether C884 is continuously valid on a linked cable or only after an explicit diag run. This also proves at least one vendor exposes the DSP length in a plain register — useful precedent for the FS/Broadcom ECD ask. The CUX3610 stays sacrificial-unit-only.
- **Proposed decision:** product path = standard-register proxy (SNR margin + power backoff) for length sanity; BCM ECD if/when FS delivers = adds fault localization; timestamp path-delay = pursue only if the item-2 retrain-stability experiment passes.
- **Proposed decision:** product path = the PHY's own length machinery — BCM ECD if/when FS delivers, the documented `1E.C884` estimate on the Aquantia oracle; NIC timestamp path-delay = the module-independent fallback, pursued only if the item-2 retrain-stability experiment passes.
## 4. Two-master I2C safety + correctness of the untested clients
The `ixgbe` driver polls the SFP EEPROM (DOM/qualification) on its own; our `sff_i2c` transactions share the bus. The swfw semaphore *should* serialize us, but verify the reasoning. Also scrutinize framing assumptions we couldn't test: