cabletest/docs/open-questions.md

Open questions

The genuinely open, thinking-worthy problems — nothing here is resolved. Analyses and settled facts live in the device docs; each entry points at the doc that holds its detail. Read state.md first for placement context.

1. Can ConnectX-5 MCIA reach the BCM? (decide on arrival)

The decisive unknown for product diagnostics coverage (nics/connectx-5/):

• Does firmware MCIA accept arbitrary I2C device addresses? The register format allows it (8-bit field, per-access status — verified in mlx5_ifc.h); firmware acceptance is the unknown. The module side is settled: the BCM bridge tolerates MCIA's offset-write-then-read framing, proven on hardware (nics/connectx-5/).
• Does MCIA emit one offset byte per write (which would let SMI command frames be synthesized exactly), and STOP-separated or repeated-START reads?
• If firmware says no: product diagnostics via the Marvell/Aquantia modules only.

2. ECD run disturbance

The BCM ECD run blips the link (PMA 1.1 latch-low catches a drop even with the break-link bit clear). Until the disturbance is characterized, length measurement is a between-runs operation, not a during-run one (modules/fs/).

3. Pre-FEC correlation run

The register question is answered (post-FEC vs corrected-by-iteration histogram located); the graded-noise correlation run that verifies the expected ordering has not happened. Design and instrument: modules/fibergaga/.

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/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.