如何在AOSP中创建自定义HAL并从Android应用调用其方法？

一、创建HAL的HIDL接口 #

1. 在AOSP源码的hardware/interfaces/目录下创建自定义HAL的目录结构：

   hardware/interfaces/mycustomhal/1.0/
   

2. 在该目录下创建HIDL接口文件IMyCustomHal.hal，定义需要暴露的方法：

   package android.hardware.mycustomhal@1.0;
   
   interface IMyCustomHal {
       getHelloString() generates (string result);
       setAndGetValue(int32_t input) generates (int32_t output);
   };
   

3. 运行HIDL生成工具生成对应代码：

   ./hardware/interfaces/update-makefiles.sh
   

二、实现HAL服务 #

1. 在hardware/interfaces/mycustomhal/1.0/default/目录下创建实现文件MyCustomHal.cpp：

   #include <android/hardware/mycustomhal/1.0/IMyCustomHal.h>
   #include <hidl/LegacySupport.h>
   
   using android::hardware::mycustomhal::V1_0::IMyCustomHal;
   using android::hardware::Return;
   using android::hardware::Void;
   using android::sp;
   
   struct MyCustomHal : public IMyCustomHal {
       Return<void> getHelloString(getHelloString_cb _hidl_cb) override {
           _hidl_cb("Hello from Custom HAL!");
           return Void();
       }
   
       Return<void> setAndGetValue(int32_t input, setAndGetValue_cb _hidl_cb) override {
           int32_t output = input * 2;
           _hidl_cb(output);
           return Void();
       }
   };
   
   extern "C" IMyCustomHal* HIDL_FETCH_IMyCustomHal(const char* /* name */) {
       return new MyCustomHal();
   }
   

2. 创建Android.bp编译脚本：

   cc_library_shared {
       name: "android.hardware.mycustomhal@1.0-impl",
       relative_install_path: "hw",
       defaults: ["hidl_defaults"],
       srcs: ["MyCustomHal.cpp"],
       shared_libs: [
           "libhidlbase",
           "libhidltransport",
           "libutils",
           "android.hardware.mycustomhal@1.0",
       ],
   }
   

3. 编译HAL模块：

   m android.hardware.mycustomhal@1.0-impl
   

三、编写系统中转服务 #

Android应用无法直接访问HAL，需通过系统服务封装HAL方法：

1. 在frameworks/base/services/core/java/com/android/server/目录下创建服务类MyCustomService.java：

   package com.android.server;
   
   import android.hardware.mycustomhal.V1_0.IMyCustomHal;
   import android.os.RemoteException;
   import android.util.Slog;
   import java.util.NoSuchElementException;
   
   public class MyCustomService extends IMyCustomService.Stub {
       private static final String TAG = "MyCustomService";
       private IMyCustomHal mHalService;
   
       public MyCustomService() {
           try {
               mHalService = IMyCustomHal.getService();
               if (mHalService == null) {
                   Slog.e(TAG, "Failed to get HAL service");
               }
           } catch (NoSuchElementException | RemoteException e) {
               Slog.e(TAG, "Error accessing HAL: " + e.getMessage());
           }
       }
   
       @Override
       public String getHelloString() throws RemoteException {
           return mHalService == null ? "HAL service not available" : mHalService.getHelloString();
       }
   
       @Override
       public int setAndGetValue(int input) throws RemoteException {
           return mHalService == null ? -1 : mHalService.setAndGetValue(input);
       }
   }
   

2. 在frameworks/base/core/java/android/os/创建AIDL文件IMyCustomService.aidl：

   package android.os;
   
   interface IMyCustomService {
       String getHelloString();
       int setAndGetValue(int input);
   }
   

3. 在SystemServer.java的startOtherServices()方法中注册服务：

   try {
       ServiceManager.addService("mycustomservice", new MyCustomService());
   } catch (Throwable e) {
       Slog.e(TAG, "Failure starting MyCustomService", e);
   }
   

4. 编译系统服务模块：

   m frameworks/base/services/core
   

四、配置权限与SELinux规则 #

1. 在frameworks/base/core/res/AndroidManifest.xml添加自定义权限：

   <permission android:name="android.permission.ACCESS_MY_CUSTOM_SERVICE"
       android:protectionLevel="normal" />
   

2. 在system/sepolicy/private/service_contexts添加服务上下文：

   mycustomservice                          u:object_r:my_custom_service:s0
   

3. 在system/sepolicy/private/service.te添加SELinux规则：

   type my_custom_service, system_api_service, system_server_service, service_manager_type;
   allow system_server my_custom_service:service_manager add;
   allow appdomain my_custom_service:service_manager find;
   allow appdomain my_custom_service:binder call;
   

4. 重新编译SEPolicy：

   m sepolicy
   

五、编写Android应用访问服务 #

1. 在应用AndroidManifest.xml声明权限：

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

2. 在应用代码中绑定服务并调用方法：

   import android.os.IMyCustomService;
   import android.os.ServiceManager;
   import android.util.Log;
   
   // 获取服务实例
   IMyCustomService service = IMyCustomService.Stub.asInterface(ServiceManager.getService("mycustomservice"));
   if (service != null) {
       try {
           String hello = service.getHelloString();
           Log.d("CustomHalApp", "HAL response: " + hello);
           int result = service.setAndGetValue(10);
           Log.d("CustomHalApp", "Set and get result: " + result);
       } catch (RemoteException e) {
           Log.e("CustomHalApp", "Error calling service: " + e.getMessage());
       }
   }
   

六、验证调试 #

• 刷入编译好的系统镜像到设备
• 运行应用，通过Logcat查看输出确认调用结果
• 若遇权限问题，检查SELinux规则和权限声明是否配置正确

内容的提问来源于stack exchange，提问作者Nishant raj