cabletest/docs/modules/fs/README.md

FS SFP-10G-T-100 (Broadcom BCM84891L)

Documented, robust — survived the whole poking campaign un-bricked. Honest EEPROM (RJ45/10GBASE-T codes), which is what trips stock ixgbe qualification (../README.md).

References in this directory: bcm84891l-mdio-commands.md (command handler, transcribed), bcm84891l-sfp-registers.md (transport reference, transcribed), and their source PDFs.

SMI transport (I2C→MDIO bridge)

PHY at I2C 8-bit 0xAC write / 0xAD read.

Op	Frames
Clause-45 write	one I2C write to 0xAC: [000+DevAD, RegH, RegL, DataH, DataL]
Clause-45 read	I2C write to 0xAC: [001+DevAD, RegH, RegL] → delay >1 ms (3 ms proven) → I2C read 2 B from 0xAD

• Single-byte reads see the bridge as inert (returns 0) — the DevAD-prefixed frame + delay is mandatory. This is why early probing wrongly declared 0x56 dead. Treat 0x0000 reads as retry-with-longer-delay: 0 is also the bridge's not-ready signature.
• Recorded for a future multi-byte firmware transport (mlx5 MCIA shape): read = write [001+devad,RegH,RegL] offset_size=1, STOP, >1 ms, read 2 B offset_size=0; write = offset_size=2, addr = devad<<8|RegH, data = [RegL,DataH,DataL].

MDIO command handler

Protocol and full verified catalog: bcm84891l-mdio-commands.md. Traps:

• SET commands execute stale DATA1 — the handler never clears DATA registers; write every parameter register explicitly before any SET.
• Poll STATUS ~100 ms; frozen up to 2 s during 10GBASE-T training — only run after link-up.

Proven diagnostics

Capability	How	Observed
Per-pair SNR	CMD_GET_SNR 0x8030, invoked bare (writing the documented DATA1 display flag returns zeros; IEEE 1.133–1.140 never populate — constant 0x8080)	DATA2–5 = SNR A–D ×0.1 dB; ≈ 27–32 dB absolute on bench, 0.1–0.4 dB jitter. 10GBASE-T operating point ≈ 26.5 dB, so margin ≈ value − 26.5. cabletest shows the margin classified green ≥ 3 dB / amber ≥ 1 dB / red below — provisional thresholds until the graded-noise correlation run
Die temperature	CMD_GET_CURRENT_TEMP 0x8031	~68–70 °C on bench
Supply rails	GET_CURRENT_VOLTAGE 0x802F	0.8 V and 1.88 V rails, tenths of mV
Error counters	IEEE PCS 3.32/3.33 — block lock, latched errored-block/BER, clear-on-read	The noise-stress error proxy
Retrain on demand	IEEE AN restart 7.0 |= bit9	Relinks in ~5–6 s

Probed configuration states

Item	Command	State
EEE / AutogrEEEn	0x8008/0x8009	Once read 0x0047 (10G AutogrEEEn variable latency + 5G/1G native — local-only, invisible in IEEE 7.60/7.61, which read 0); later reads 0, treated as the truth. Forced all-off via SET with explicit params (0, 0, 0x7A12, 0x480, 0) + AN restart, verified. cabletest checks 0x8008 and forces only on mismatch, verifying 7.60 reads 0 either way (phy.go)
EEE wire-truth	arm 0x801A after link-up, read 0x801B	Zero LPI events/duration on idle link; repeat under traffic
Fast retrain	0x800A (datasheet titles it EMI_MODE; description is fast retrain)	Enabled 10G/5G/2.5G; IEEE 1.147 = 0x0019, count bits zero. Keep enabled; read the 1.147 count per run — a marginal cable that fast-retrains still gets counted
Pair map	0x8000	DATA2 = 0x00E4 = identity (A/B/C/D straight through) — MDI wiring verification works
Fault pass-through	0x800D	0 = LF/RF pass to the line (0x8011/0x8010 could mask them as IDLE — leave off)
Host interface	0x802B/0x802D/0x8027	XFI polarity normal; TX FIR = main tap 0x3C only; MAC/PHY frequency locked; KR/SyncE/USXGMII/WOL disabled; temp warnings off
Jumbo	0x801C/0x801D	cabletest checks 0x801D and forces enable on mismatch (phy.go). The ODM firmware reads back the 9K size option regardless of the requested 10K — and 9018-byte wire frames pass at line rate with zero loss/corrupt, so its "9K" covers ≥ 9018 in practice
1588	0x8004	Disabled (engine registers undocumented — see asks below)
Limited reach	0x8006	Disabled — its DATA3 "linked in LR mode" bit, the only handler-visible trace of the DSP length estimate, is unavailable without a config change

Firmware-reliability notes

GET returns are trusted: early exploration saw one anomalous DATA1 (0x0047 from the EEE GET, never reproduced) and die-temperature-like values in other commands' DATA1, treated as one-off noise. cabletest reads config before forcing it and corroborates EEE through 7.60 after any relink. The hard rule that stands: write every DATA register explicitly before any SET — the handler executes stale DATA.

ECD — recovered from the OpenBCM SDK, proven on hardware

The ECD register mechanism is absent from the handler catalog and the datasheet excerpts here, but the OpenBCM SDK's copper-XGPHY driver (sdk-6.5.27/src/soc/phy/phy8481.c phy_8481_cable_diag + phy8481.h) carries it for the 8483x/8485x/8488x family — and the same SDK drives the identical command-handler registers (1E.0x4005/0x4037/0x4038–3C) as the BCM84891L datasheet, confirming the shared map. Validated on the FS:

Register	Role
1E.0x4006	Control/status. Write under mask {15,14,13,12,10}: bit 15 = run now, bit 14 = run at AN, bit 12 = break link, bit 10 = length in meters (SDK writes value 0x8400 = run now + meters). Bit 11 = busy — poll until clear (SDK allows up to 50 s; observed < 0.5 s)
1.0xA896	Verdicts, 4 bits per pair: 1 = OK, 2 = open, 3 = short, 4 = inter-pair short
1.0xA897–0xA89A	Per-pair lengths (pairs 1/2, 3/4, 5/6, 7/8), meters

Observed on a plugged, linked, healthy cable: verdicts OK×4 and per-pair lengths of [45, 45, 41, 46] on a ~45 m cable — meter-accurate with no calibration, and this ECD reports length for healthy pairs, not just faults, resolving the terminated-far-end concern in ../README.md. Caveats:

• The run blips the link (PMA 1.1 latch-low catches a drop even with the break-link bit clear) — do not run mid-measurement until the disturbance is characterized.
• Fault verdicts (open/short/inter-pair) are unexercised — deliberately: the product is a closed-loop tester, both ends always plugged.
• Family constraints from the SDK: port must be enabled; unsupported at forced 100M.
• bcm_ecd_probe.py in phydiag-work implements the recipe.

Missing datasheet chapters

Chapters the datasheet TOC lists but the excerpts here omit — to source elsewhere:

1. The ECD chapter — now for confirming bit meanings rather than unblocking.
2. The 1588 operation chapter.
3. Datasheet §1.20 loopback (copper line loopback) and §1.17 EEE/fast-retrain monitoring.
4. Chapter 2 register summary. The excerpt's TOC names them all.