MTK Android serial port permission configuration

question

Adding a serial port application to an Android 11 device encounters an error when opening the serial port:
System.err: Caused by: java.io.IOException: error=13, Permission denied

Solution steps

1. Add permission in APP to apply for
   AndroidManifest.xml

    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
    <uses-permission android:name="android.permission.MOUNT_UNMOUNT_FILESYSTEMS" /

2. System-level application
   AndroidManifest.xml adds android.uid.system

<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    android:sharedUserId="android.uid.system"
    >

Only the system application configuration is introduced here. If it is an ordinary application, the modification method is different.

3. Configure selinux
   to modify according to the prompt of avc
   type=1400 audit(0.0:1182): avc: denied { read write } for name="ttyS2" dev="tmpfs" ino=12037 context=u:r:system_app:s0 tcontext =u:object_r:ttyS_device:s0 tclass=chr_file permissive=0

Modify according to the avc print prompt, different platforms have different modification places, here is the mtk platform
device/mediatek/sepolicy/basic/non_plat/system_app.te

# serial port
allow system_app ttyS_device:chr_file { open read write ioctl };

4. Device permissions
   Since the board corresponds to mt6771, modify
   device/mediatek/mt6771/init.mt6771.rc
   and add
   chmod 0666 /dev/ttyS2
   Different types of CPUs correspond to different directories, and this step needs to be changed, otherwise the following error will appear
   open() fd = -1
   Cannot open port
   native open returns null
   openSerialPort: Open serial port exception: java.io.IOException
   Author: too handsome to go out