cabletest/docs/nics/x520/README.md

Intel X520-DA2 (82599, ixgbe)

The only certain arbitrary-framing module-I2C transport (bit-banged, no firmware in the path) — the diagnostics workhorse. Cannot run the committed measurement path — PTP-only RX timestamping (../../open-questions.md §2) — unless the bucket-timestamping exploration validates an equivalent.

• PCIe Gen2 ×8 (5 GT/s, 32 Gb/s) — enough for 2×10G full duplex at the default mix.
• allow_unsupported_sfp=1 is mandatory (ixgbe_main.c:165): the FS module's honest 10GBASE-T EEPROM fails Intel qualification and kills the whole port probe (error -95, no netdev). load-ixgbe passes it.

sff_i2c diagnostics transport (patched driver, validated)

Patched ixgbe (~/work/ixgbe-sff/) adds ixgbe_i2c_raw_write/ixgbe_i2c_raw_read (ixgbe_phy.c) — arbitrary START…STOP transactions from the driver's bit-bang primitives, swfw-semaphore bracketed — behind /sys/kernel/debug/ixgbe/<pci>/sff_i2c (w <addr8> <bytes…> / r <addr8> <n>).

• SDA-release fix (required): multi-byte reads returned only byte 0 — the master's ACK left SDA driven low and nothing released it (stock 82599 paths never clock in more than one byte, so the missing release was invisible). raw_read releases SDA after each ACK, mirroring stock's release block after clocking out a byte.
• Two-master windows: each debugfs op takes the semaphore separately; serialization against the driver's own I2C holds per-op, but multi-op protocol sequences have unlocked gaps, and driver I2C is event-driven (SFP identify, link setup, ethtool -m) — firing exactly around cable swaps. Hardening item: a compound op under one semaphore hold (../../open-questions.md §4).

Timestamping facts

• RX latches PTP frames only (TYPE_ALL/TSIP are X550+; HWTSTAMP_FILTER_ALL → -ERANGE, ixgbe_ptp.c:1043): one latch pair (RXSTMPL/H, locked until RXSTMPH read), attribute capture (RXSATRL/H), no RX-descriptor timestamp field.
• Per-port free-running SYSTIME (~6.4 ns at 10G), one shared crystal — two-way exchange cancels the inter-port offset.
• Per-queue hardware counters QPRC/QBRC (16 slots via RQSMR, read-clear, ixgbe_main.c:7777,7787) count at queue write-back, upstream of ring residence — the basis of bucket-exploration route 1.
• Full analysis and bench pokes: ../../open-questions.md §2.

RX steering facts

• ntuple ETHER_FLOW is rejected (ixgbe_flowspec_to_flow_type: TCP/UDP/SCTP-v4 and IPv4 only) — the tool fails at startup as committed.
• ETQF/ETQS L2 EtherType filters are the answer on paper: dedicated ethertype→queue stage ahead of RSS/fdir, 8 slots, 7 free (index 3 reserved for 1588). Flow Director cannot match non-IP frames at all. Analysis and patch shape: ../../open-questions.md §1.