无法挂载根目录排查

linux启动后，发现无法自动挂载 /dev/system 下的根目录。于是开始研究

一、根目录在哪个设备上，是由uboot传递过来的内核参数决定的。通过内核启动日志，或者 /proc/cmdline 可以看到 root=/dev/system rootfstype=ext4 init=/sbin/init，说明uboot传递过来的参数没有问题

二、启动过程中有一个报错 Failed to switch root, dropping to a shell
通过搜索，发现包含此字符串的地方位于
output/sm1_ac200_a6432_release/build/buildroot-fs/cpio/target_recovery/init 和
output/sm1_ac200_a6432_release/build//buildroot-fs/cpio/target_ota/init

在切换根目录失败的板子上，看到init内容为

#!/bin/sh
#Mount things needed by this script
/bin/mount -t sysfs sysfs /sys
/bin/mount -t devtmpfs devtmpfs /dev
/bin/mount -t proc proc /proc

#Create device nodes
mdev -s

#This will only be run if the exec above failed
echo "Failed to switch root, dropping to a shell"
exec /sbin/init

内容很简短，正是 target_recovery/init 的内容。由于脚本之中压根就没有读取内核参数root的路径，也没有挂载，因此必然走到 "Failed to switch root"。说明系统是进入recovery了，而不是正常的启动。正常启动时 target_ota/init 的内容节选

#Process command line options
for i in $(cat /proc/cmdline); do
    case $i in
        root\=*)
            root=$(get_opt $i)
            ;;
        firmware\=*)
            firmware=$(get_opt $i)
            ;;
        rootfstype\=*)
            rootfstype=$(get_opt $i)
            ;;
        init\=*)
            init=$(get_opt $i)
            ;;
        slot_suffix\=*)
            slot_suffix=$(get_opt $i)
            ;;
        system_partition_name\=*)
            system_partition_name=$(get_opt $i)
            ;;
        kernel_version\=*)
            kernel_version=$(get_opt $i)
            ;;
    esac
done

if [ "${rootfstype}" = "ubifs" ]; then
    echo "#[ubiattach /dev/ubi_ctrl -m ${system_mtd_number}]"
    ubiattach /dev/ubi_ctrl -m ${system_mtd_number}
    echo "#[ mount -t ubifs /dev/ubi0_0 /mnt ]"
    mount -t ${rootfstype} /dev/ubi0_0  /mnt
elif [ "${rootfstype}" != "" ]; then
    mount -t ${rootfstype} "${root}" /mnt
else
    mount "${root}" /mnt
fi

#Check if $init exists and is executable
if [[ -x "/mnt/${init}" ]] ; then
    #Unmount all other mounts so that the ram used by
    #the initramfs can be cleared after switch_root
    umount /sys /proc /dev

    #Switch to the new root and execute init
    /bin/mount -t devtmpfs devtmpfs /mnt/dev
    exec 0</mnt/dev/console
    exec 1>/mnt/dev/console
    exec 2>/mnt/dev/console
    exec switch_root -c /dev/console /mnt "${init}"
fi

#This will only be run if the exec above failed
echo "Failed to switch root, dropping to a shell"

在正常启动时，init会解析内核参数，找到root挂载路径，然后挂载到 /mnt 下。最后通过 exec switch_root 切换到新根目录下

三、分析为何会进recovery
查看uboot的日志，发现有以下一行
gpio: pin GPIOAO_3 (gpio 3) value is 0
detect upgrade key

通过搜索代码，发现detect upgrade key是uboot中打印的 uboot-next-2015-dev/bl33/v2015/board/amlogic/configs/g12a_u200_v1.h

#define CONFIG_PREBOOT  \
            "run bcb_cmd; "\
            "run factory_reset_poweroff_protect;"\
            "run upgrade_check;"\
            "run init_display;"\
            "run storeargs;"\
            "run upgrade_key;" \
            "forceupdate;" \
            "bcb uboot-command;"\
            "run switch_bootmode;"

        "upgrade_key="\
            "if gpio input GPIOAO_3; then "\
                "echo detect upgrade key; run update;"\
            "fi;"\
            "\0"\

         "update="\
            /*first usb burning, second sdc_burn, third ext-sd autoscr/recovery, last udisk autoscr/recovery*/\
            "run usb_burning; "\
            "run sdc_burning; "\
            "if mmcinfo; then "\
                "run recovery_from_sdcard;"\
            "fi;"\
            "if usb start 0; then "\
                "run recovery_from_udisk;"\
            "fi;"\
            "run recovery_from_flash;"\
            "\0"\

因为检测到GPIOAO_3有输入，因此执行了update，最后执行了recovery_from_flash