adb 开关
启动是在rc文件中
./out/target/product/xxxx/root/init.usb.rc

# adb only USB configuration
# This is the fallback configuration if the
# USB manager fails to set a standard configuration
on property:sys.usb.config=adb && property:sys.usb.configfs=0
    write /sys/class/android_usb/android0/enable 0
    write /sys/class/android_usb/android0/idVendor 18d1
    write /sys/class/android_usb/android0/idProduct 4EE7
    write /sys/class/android_usb/android0/functions ${sys.usb.config}
    write /sys/class/android_usb/android0/enable 1
    start adbd
    setprop sys.usb.state ${sys.usb.config}

则sys.usb.config=adb
属性默认值在系统编译时生成的default.prop

build.prop

./device/mediatek/mt6580/device.mk

ifeq ($(TARGET_BUILD_VARIANT),user)
  PRODUCT_DEFAULT_PROPERTY_OVERRIDES += persist.sys.usb.config=mtp
else
  PRODUCT_DEFAULT_PROPERTY_OVERRIDES += persist.sys.usb.config=adb
endif

则user版本为mtp不打开adb,其他版本为adb,打开 adb