1.问题描述
有一块板卡，外接SPI小网卡用做调试网口，如下图所示，某宝有售。对于嵌入式系统，为节约板卡面积和降低成本，经常这样搭配，将产品用不着的调试网卡外接，虽然可能带来一些调试问题（比如连接不可靠等）。这不，问题来了，老外换了一台新电脑，发现有时候电脑连不通调试网口，于是提了个FR LEMI sometimes not actived(LEMI 是Local Ethernet Management Interface的简称，即调试网卡)给上海的platform来解决。很奇怪，上海板卡却没有这一问题。
2. 问题分析
2.1 dump网卡寄存器
遇到这种问题，第一想到的是dump出网卡的寄存器。问题来了，怎么dump出寄存器呢？”Talk is cheap, show me the code.”
首先看看网卡驱动/drivers/net/ethernet/microchip/encx24j600.c是否提供这一功能，还真有：
static const struct ethtool_ops encx24j600_ethtool_ops = {
.get_settings = encx24j600_get_settings,
.set_settings = encx24j600_set_settings,
.get_drvinfo = encx24j600_get_drvinfo,
.get_msglevel = encx24j600_get_msglevel,
.set_msglevel = encx24j600_set_msglevel,
.get_regs_len = encx24j600_get_regs_len,
.get_regs = encx24j600_get_regs,
};
驱动ethtool_ops的encx24j600_get_regs函数可以dump寄存器，即用户空间命令ethtool就可以dump网卡寄存器。
/sys/kernel/debug # ethtool -d eth-mgnt
[ 1835.500841] encx24j600: register dump
[ 1835.503916] encx24j600 PHCON1: 1200
[ 1835.508033] encx24j600 PHCON2: 0002
[ 1835.511000] encx24j600 PHANA: 05E1
[ 1835.515027] encx24j600 PHANLPA: 05E1
[ 1835.518454] encx24j600 PHANE: 0003
[ 1835.522003] encx24j600 PHSTAT1: 782D
[ 1835.525644] encx24j600 PHSTAT2: 001A
[ 1835.529095] encx24j600 PHSTAT3: 1058
[ 1835.532273] encx24j600 ECON1: 0001
[ 1835.535846] encx24j600 ECON2: CB00
[ 1835.539334] encx24j600 ERXFCON: 004B
[ 1835.542961] encx24j600 ESTAT: 5F00
[ 1835.546443] encx24j600 EIR: 0780
[ 1835.549950] encx24j600 EIDLED: CB21
[ 1835.553527] encx24j600 EIE: 884C
[ 1835.557035] encx24j600 ERXST: 0600
[ 1835.560582] encx24j600 ERXTAIL: 1CAC
[ 1835.564254] encx24j600 ERXHEAD: 1CAE
[ 1835.567686] encx24j600 ERXRDPT: 1BF2
[ 1835.571214] encx24j600 EUDAST: 6000
[ 1835.574798] encx24j600 EUDAND: 6001
[ 1835.578347] encx24j600 EDMACS: 0000
[ 1835.581835] encx24j600 MACON1: 0009
[ 1835.585409] encx24j600 MACON2: 40B3
[ 1835.588922] encx24j600 MAIPG: 0C12
[ 1835.592469] encx24j600 MACLCON: 370F
[ 1835.596034] encx24j600 MABBIPG: 0015
Offset Values
------ ------
0x0000: 00 00 00 00 00 00 00 00 00 06 00 00 ac 1c 00 00
0x0010: ae 1c 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x0020: 00 00 00 00 00 00 00 00 00 00 00 00 00 60 00 00
0x0030: 01 60 00 00 00 5f 00 00 80 07 00 00 01 00 00 00
0x0040: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x0050: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x0060: 00 00 00 00 00 00 00 00 4b 00 00 00 00 00 00 00
0x0070: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x0080: 09 00 00 00 b3 40 00 00 15 00 00 00 12 0c 00 00
0x0090: 0f 37 00 00 ee 05 00 00 00 00 00 00 00 00 00 00
2.2 寄存器对比
远程访问老外的平台，dump故障时网卡寄存器，和正常时的寄存器对比，发现PHCON2寄存器不同 name normal abnormal differnece PHCON2 2002 2000 bit 1 EDSTAT: Energy Detect Status bit 1 = Energy detect circuit has detected energy on the TPIN+/- pins within the last 256 ms = No energy has been detected on the TPIN+/- pins within the last 256 ms
2.3 root cause
查看网卡data sheet，找到原因，网卡使能了energy_detect_powerdown模式，需要等待远端设备唤醒；如果不幸远端设备也使能了这个模式，那样双方可能都陷入千年的等待。。。老外正是遇到了这种情况。
/isam/slot_default/run # ethtool --show-priv-flags eth-mgnt
Private flags for eth-mgnt:
use_irq : on
test_spi : off
energy_detect_powerdown: on
The lemi chip works in a mode called “Energy Detect Power-Down”, In this mode, the PHY remains powered down until a signal is detected on the Ethernet interface. When a signal is detected on the Ethernet medium, the EDSTAT flag (PHCON2<1>) is set. The problem is that the EDSTAT flag (PHCON2<1>) is not set when plug-in.
We can solve this issue by disable this mode by command “ethtool –set-priv-flags eth-mgnt energy_detect_powerdown off”.
3. 解决方案
使用ethtool命令关闭网卡energy_detect_powerdown模式，你一个调试网卡，就不用凑energy_detect_powerdown的热闹了。
/isam/slot_default/run # ethtool --show-priv-flags eth-mgnt
Private flags for eth-mgnt:
use_irq : on
test_spi : off
energy_detect_powerdown: on
/isam/slot_default/run # ethtool --set-priv-flags eth-mgnt energy_detect_powerdown off