user版本串口不能输入

os：android7.1

描述：编译为userdebug版本时调试时，调试串口是可以正常输入输出的，编译为user版本时，串口就只能输出不能输入了。

原因：

　out/target/product/rk3288/recovery/root/init.rc　　

　　# Always start adbd on userdebug and eng builds
    on property:ro.debuggable=1
    write /sys/class/android_usb/android0/enable 1
    start adbd
    start console

可见，console的启用受ro.debuggable的控制，即是ro.debuggable=1时，才启动console。

可以通过指令getprop ro.debuggable查看该属性的值：

改属性在build/core/main.mk中定义：

ifeq (true,$(strip $(enable_target_debugging)))
  # Target is more debuggable and adbd is on by default
  ADDITIONAL_DEFAULT_PROPERTIES += ro.debuggable=1
  # Enable Dalvik lock contention logging.
  ADDITIONAL_BUILD_PROPERTIES += dalvik.vm.lockprof.threshold=500
  # Include the debugging/testing OTA keys in this build.
  INCLUDE_TEST_OTA_KEYS := true
else # !enable_target_debugging
  # Target is less debuggable and adbd is off by default
  ADDITIONAL_DEFAULT_PROPERTIES += ro.debuggable=0
endif # !enable_target_debugging

解决：在main.mk中修改ro.debuggable=1