BCM module diagnostics replace timestamp probes: bringup forces EEE off and runs ECD (re-run on every reset, re-baselining past its blip), 1 Hz SNR margin + corrected counters on panel and console; X520 bench divergences bypassed and tracked in open-questions

docs/open-questions.md

@@ -21,3 +21,13 @@ The register question is answered (post-FEC vs corrected-by-iteration histogram
 ## 4. Wiitek VCT — pursue or leave dead?
 
 No confirmed-safe path exists (every candidate lands in the µC danger window). The open decision is whether the capability is worth the NDA route or a sacrificial unit — the product doesn't need it for length ([modules/wiitek/](modules/wiitek/README.md), [modules/README.md](modules/README.md)).
+
+## 5. X520 bench divergences — features to restore on the product NIC
+
+Running on the X520 (BCM development) required parking product-NIC capabilities the 82599 lacks. Each stays parked only until the ConnectX-5 is in; none is a settled design change:
+
+- **Hardware timestamp hard check bypassed** (`ts.go`): `rx_filter=ALL` failure now reports yellow and continues instead of stopping the run. On the X520 that means the per-frame-stamp rate buckets never fill and the panel/console rates read zero — the measurement doctrine (exact per-frame RX stamps as a hard host requirement) is intact in the docs and must return to fatal on the product NIC.
+- **RX error counter set is per-driver** (`counters.go`, `nicRxStatsByDriver`): the ice set is the reference — jabber, fragments, `illegal_bytes` (64b/66b decode errors) and MAC local/remote faults have **no ixgbe ethtool equivalent**, so those signals are simply absent on the bench. The mlx5 name set needs deriving on CX-5 arrival; the "as close to BER as the link reports" counters (illegal bytes, faults) are the ones to insist on finding there.
+- **TX interrupt moderation** (`system.go`): ixgbe's mixed rx/tx vectors reject a tx-specific value, so `checkCoalesce` falls back to rx-shared-with-tx on EINVAL. Generic and self-reporting, but verify the product NIC takes the full rx+tx pair (the fallback must never fire there).
+- **`testDriver` still names "ice"** (`main.go`): the default pair discovery has no working target — bench runs pass `-a`/`-b` explicitly. Point it at the product driver (mlx5_core) when the CX-5 lands.
+- **Module I2C transport is ixgbe-only** (`phy.go`, `openBCM`): the sff_i2c debugfs path. The CX-5 needs the MCIA answer (question 1) and a second transport arm.