26 lines
2.5 KiB
Markdown
26 lines
2.5 KiB
Markdown
# NICs
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The NIC decides two things: whether the host can reach module-PHY diagnostics at all, and whether the measurement path gets the hardware timestamps it requires. Per-device directories hold the details.
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## The transport rule
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A NIC is usable for module diagnostics only if the **host** can master the module I2C bus with arbitrary multi-byte framing and writes. Firmware-mediated I2C imposes limits that break the module command protocols.
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## Comparison
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Which card is where: [../state.md](../state.md).
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| NIC (driver) | Module I2C | All-packet RX hw stamps | Verdict |
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| [X520 / 82599 (ixgbe)](x520/README.md) | Host bit-bangs — full control, **proven** | No (single serial latch; 1588-event frames only, EtherType configurable) | Diagnostics transport only — no all-packet exact stamps, so disqualified as product NIC |
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| [ConnectX-4/5 (mlx5)](connectx-5/README.md) | Firmware MCIA; writes allowed; framing open | **Yes** (CQE-stamps everything) | **The product candidate** — the only card meeting the full requirement set |
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| [X710 (i40e)](x710/README.md) | 1-byte-offset only | No | RollBall oracle path; PTP length-experiment platform |
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| [E810 (ice)](e810/README.md) | Multi-byte framing, writes EPERM-blocked | Yes | Dead for diagnostics writes |
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Requirement weights behind the verdicts: all-packet hardware RX timestamps are non-negotiable (software stamping was tried, not precise enough), raw-L2 steering is not (encap acceptable). Full analyses live in each device directory.
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## Architecture
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- **Co-location is forced, not chosen.** SNR under load, the pre-FEC correlation, and diagnosing the module that carried the stress all require diagnostics I2C to reach the modules the traffic flows through, and module I2C is only reachable through the hosting NIC. A "diagnostics NIC" would need the modules in *its* cages, off the traffic path — structurally impossible, not merely awkward. The chassis seconds it: one usable slot.
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- **The single card is the ConnectX-5.** The requirement weights flip the selection the X520 was chosen under: the 82599 stamps 1588-event frames only, txgbe is likewise PTP-only (verified in upstream `wx_ptp.c`; evaluated on paper, never owned), and mlx5 is the only driver meeting the full set — CQE-stamps every packet, one shared clock across ports, native ETHER_FLOW steering. If MCIA can't reach the BCM, product diagnostics run via the Marvell/Aquantia modules and the X520 stays an offline BCM rig ([connectx-5](connectx-5/README.md)).
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