信号质量指标 (SQI) 特性并非在所有 PHY 中都实现，它用于检查信号质量（0 表示无链路，7 表示最佳）。用于调试错误源头，例如：链路稳定但系统发现丢包。SQI 值较低，表示问题可能出在 MDI 接口（连接器侧）内部。如果器件的驱动程序中实现了 SQI，可以在 ethtool 命令中找到 SQI。如果只想显示 SQI，可以使用 ethtool ethx | grep SQI。

ethtool eth3
Settings for eth3:
        Supported ports: [ TP    MII ]
        Supported link modes:   100baseT/Full
        Supported pause frame use: Symmetric
        Supports auto-negotiation: No
        Supported FEC modes: Not reported
        Advertised link modes:  Not reported
        Advertised pause frame use: Symmetric
        Advertised auto-negotiation: No
        Advertised FEC modes: Not reported
        Speed: 100Mb/s
        Duplex: Full
        Auto-negotiation: off
        master-slave cfg: forced master
        master-slave status: master
        Port: Twisted Pair
        PHYAD: 5
        Transceiver: external
        MDI-X: Unknown
        Supports Wake-on: d
        Wake-on: d
        Current message level: 0x000020f7 (8439)
                               drv probe link ifdown ifup rx_err tx_err hw
        Link detected: yes
        SQI: 4/7
ethtool eth3 | grep SQI
        SQI: 4/7