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1.cmdline中传入initcall_debug
对于android开发环境来说，可以在device中的BoardConfig.mk中添加如下：
BOARD_KERNEL_CMDLINE += initcal_debug
对于linux系统，可以在defconfig中添加：
CONFIG_CMDLINE="initcall_debug"
当然这种方式同样适用于android设备。
当我们使用上面的配置重新编译了boot.img之后烧入系统，会发现在dmesg打印的启动阶段的log中有如下风格的打印：
[ 4.746509] initcall phy_init+0x0/0x6c returned 0 after 186 usecs
[ 4.763443] initcall usb_init+0x0/0x17c returned 0 after 16525 usecs
[ 4.763483] initcall dual_role_class_init+0x0/0x8c returned 0 after 27 usecs
[ 4.774618] initcall usb_phy_generic_init+0x0/0x24 returned 0 after 10861 usecs
[ 4.774662] initcall usb_udc_init+0x0/0x70 returned 0 after 31 usecs
[ 4.774741] initcall serio_init+0x0/0x44 returned 0 after 65 usecs
[ 4.774800] initcall input_init+0x0/0x110 returned 0 after 45 usecs
[ 4.775528] initcall qpnp_pon_init+0x0/0x24 returned 0 after 698 usecs
[ 4.775575] initcall rtc_init+0x0/0x6c returned 0 after 34 usecs
[ 4.776189] initcall i2c_gpio_init+0x0/0x48 returned 0 after 587 usecs
[ 4.780986] initcall media_devnode_init+0x0/0x98 returned 0 after 4672 usecs
[ 4.785784] initcall videodev_init+0x0/0x9c returned 0 after 4673 usecs
[ 4.785842] initcall init_dvbdev+0x0/0xf4 returned 0 after 43 usecs
[ 4.785906] initcall rc_core_init+0x0/0x6c returned 0 after 51 usecs
[ 4.785953] initcall power_supply_class_init+0x0/0x58 returned 0 after 33 usecs
[ 4.785993] initcall hwmon_init+0x0/0x48 returned 0 after 27 usecs
[ 4.817853] initcall thermal_init+0x0/0x140 returned 0 after 31094 usecs
[ 4.844966] initcall tsens_tm_init_driver+0x0/0x24 returned 0 after 26456 usecs
2.使能loglevel
当我们使能initcall_debug功能之后发现，这个log并没有默认输出到串口中，对于那些系统启动过程就卡死的问题，我们很难看到对应的log来定位问题，所以我们可以考虑把loglevel默认打印级别调高，比如在cmdline中设置loglevel=8，这样就会默认把所有log输出到串口了。方法如下：
对于android开发环境来说，可以在device中的BoardConfig.mk中添加如下：
BOARD_KERNEL_CMDLINE += initcal_debug loglevel=8
对于linux系统，可以在defconfig中添加：
CONFIG_CMDLINE="initcall_debug loglevel=8"
3.解析initcall debug log
cat log.txt | grep "initcall" | sed "s/\(.*\)after\(.*\)/\2 \1/g" | sort -n
这个命令可以解析log中的initcall数据，并根据时间长短进行排序，结果如下：
27 usecs [ 4.763483] initcall dual_role_class_init+0x0/0x8c returned 0
27 usecs [ 4.785993] initcall hwmon_init+0x0/0x48 returned 0
31 usecs [ 4.774662] initcall usb_udc_init+0x0/0x70 returned 0
33 usecs [ 4.785953] initcall power_supply_class_init+0x0/0x58 returned 0
34 usecs [ 4.775575] initcall rtc_init+0x0/0x6c returned 0
43 usecs [ 4.785842] initcall init_dvbdev+0x0/0xf4 returned 0
45 usecs [ 4.774800] initcall input_init+0x0/0x110 returned 0
51 usecs [ 4.785906] initcall rc_core_init+0x0/0x6c returned 0
65 usecs [ 4.774741] initcall serio_init+0x0/0x44 returned 0
186 usecs [ 4.746509] initcall phy_init+0x0/0x6c returned 0
587 usecs [ 4.776189] initcall i2c_gpio_init+0x0/0x48 returned 0
698 usecs [ 4.775528] initcall qpnp_pon_init+0x0/0x24 returned 0
4672 usecs [ 4.780986] initcall media_devnode_init+0x0/0x98 returned 0
4673 usecs [ 4.785784] initcall videodev_init+0x0/0x9c returned 0
10861 usecs [ 4.774618] initcall usb_phy_generic_init+0x0/0x24 returned 0
16525 usecs [ 4.763443] initcall usb_init+0x0/0x17c returned 0
26456 usecs [ 4.844966] initcall tsens_tm_init_driver+0x0/0x24 returned 0
31094 usecs [ 4.817853] initcall thermal_init+0x0/0x140 returned 0