Android automotive车载开发（5）-----系统架构

系统架构

前面简单介绍了一下Android Automotive 的架构，具体Android Automotive 在系统的每个层，都做了哪些东西，这里总结一下。

系统应用层

Android Automotive 为系统定制了一些专门适用车载系统的应用，以代替传统的手机应用模块。

系统应用层的Application都位于源码目录下packages/apps/Car/

包含的应用如下

系统框架层

系统框架层提供了多个模块，来对Android Automotive 进行支持，最重要的有一个服务CarService (com.android.car) 。

系统框架层的模块都位于源码目录下packages/services/Car/

CarService是一个类似Android系统中SystemSever的服务。它由一个服务启动，而里面又控制着数十个子模块服务。

CarService

CarService中CarService只作为服务的入口，具体的业务逻辑都在内部的子服务中处理，CarService的子服务如下。

CarService启动

CarService本质上是一个特殊的APP，它编译后生成CarService.apk；在系统中，它是在/system/priv-app/CarService/CarService.apk

CarService在启动启动时，由SystemServer拉起

代码：frameworks/base/services/java/com/android/server/SystemServer.java

private static final String CAR_SERVICE_HELPER_SERVICE_CLASS =
    "com.android.internal.car.CarServiceHelperService";
private void startOtherServices() {
    mActivityManagerService.systemReady(() -> {
        if (mPackageManager.hasSystemFeature(PackageManager.FEATURE_AUTOMOTIVE)) {
            traceBeginAndSlog("StartCarServiceHelperService");
            mSystemServiceManager.startService(CAR_SERVICE_HELPER_SERVICE_CLASS);
            traceEnd();
        }
    });
}
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代码：frameworks/opt/car/services/src/com/android/internal/car/CarServiceHelperService.java

private static final String CAR_SERVICE_INTERFACE = "android.car.ICar";
public class CarServiceHelperService extends SystemService {

    @Override
    public void onStart() {
        Intent intent = new Intent();
        intent.setPackage("com.android.car");
        intent.setAction(CAR_SERVICE_INTERFACE);
        if (!getContext().bindServiceAsUser(intent, mCarServiceConnection, Context.BIND_AUTO_CREATE, UserHandle.SYSTEM)) {
            Slog.wtf(TAG, "cannot start car service");
        }
        System.loadLibrary("car-framework-service-jni");
    }
} 
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SystemServer中ActivityManagerService.systemReady后会通过SystemServiceManager启动CarServiceHelperService

CarServiceHelperService中绑定CarService
代码：packages/services/Car/service/AndroidManifest.xml

    
        
    

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通过在CarService的AndroidManifest的配置，CarServiceHelperService可以通过包名和action的名称实现和CarService绑定。

Car API

Car API 是Android系统为汽车应用提供的一套SDK接口（Android Automotive Library）。

源代码位于packages/services/Car/car-lib目录下

Google官方的API文档在Android Q之后，也开始支持Android Automotive库。

链接地址： https://developer.android.google.cn/reference/android/car/package-summary

同CarService一样Car API 也提供的多个模块对应不同的功能调用。

硬件抽象层

硬件抽象层主要是提供了一个native服务android.hardware.automotive.vehicle@2.0-service，负责处理车辆相关的业务。

硬件抽象层的模块位于源码目录下hardware/interfaces/automotive/

VehicleService

VehicleService是一个native服务，代码实现在目录hardware/interfaces/automotive/vehicle/2.0/default/impl/vhal_v2_0/

VehicleServices是Android Automotive在硬件抽象层的入口。它通过initrc启动。

代码：hardware/interfaces/automotive/vehicle/2.0/default/android.hardware.automotive.vehicle@2.0-service.rc

service vendor.vehicle-hal-2.0 /vendor/bin/hw/android.hardware.automotive.vehicle@2.0-service
    class hal
    user vehicle_network
    group system inet
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VehicleHalManager

VehicleHalManager是Android Automotive在硬件抽象层的API接口。代码在目录

hardware/interfaces/automotive/vehicle/2.0/default/common/。

HAL接口语言

从硬件抽象层到系统框架层，也就是从VehicleService到CarService，从Android O版本开始使用了一种新的HIDL接口。

HIDL 是用于指定 HAL 和其用户之间的接口的一种接口描述语言 (IDL)。虽然从 Android 10 开始，HIDL 已废弃，Android 将在所有位置改用 AIDL。 但是Android Automotive的HIDL架构还保留着，直到Android 13才发生变化。

HIDL使用一种以.hal为后缀文件作为接口定义，在Android Automotive中硬件抽象层到系统框架层使用IVehicle.hal（代码路径：hardware/interfaces/automotive/vehicle/2.0/IVehicle.halhardware/interfaces/automotive/vehicle/2.0/IVehicle.hal）来定义。

代码：hardware/interfaces/automotive/vehicle/2.0/IVehicle.hal

package android.hardware.automotive.vehicle@2.0;

import IVehicleCallback;

interface IVehicle {
  /**
   * Returns a list of all property configurations supported by this vehicle
   * HAL.
   */
  //返回这个vehicle hal支持的全部property属性
  getAllPropConfigs() generates (vec propConfigs);

  /**
   * Returns a list of property configurations for given properties.
   *
   * If requested VehicleProperty wasn't found it must return
   * StatusCode::INVALID_ARG, otherwise a list of vehicle property
   * configurations with StatusCode::OK
   */
  getPropConfigs(vec props)
          generates (StatusCode status, vec propConfigs);

  /**
   * Get a vehicle property value.
   *
   * For VehiclePropertyChangeMode::STATIC properties, this method must always
   * return the same value always.
   * For VehiclePropertyChangeMode::ON_CHANGE properties, it must return the
   * latest available value.
   *
   * Some properties like RADIO_PRESET requires to pass additional data in
   * GET request in VehiclePropValue object.
   *
   * If there is no data available yet, which can happen during initial stage,
   * this call must return immediately with an error code of
   * StatusCode::TRY_AGAIN.
   */
  get(VehiclePropValue requestedPropValue)
          generates (StatusCode status, VehiclePropValue propValue);

  /**
   * Set a vehicle property value.
   *
   * Timestamp of data must be ignored for set operation.
   *
   * Setting some properties require having initial state available. If initial
   * data is not available yet this call must return StatusCode::TRY_AGAIN.
   * For a property with separate power control this call must return
   * StatusCode::NOT_AVAILABLE error if property is not powered on.
   */
  set(VehiclePropValue propValue) generates (StatusCode status);

  /**
   * Subscribes to property events.
   *
   * Clients must be able to subscribe to multiple properties at a time
   * depending on data provided in options argument.
   *
   * @param listener This client must be called on appropriate event.
   * @param options List of options to subscribe. SubscribeOption contains
   *                information such as property Id, area Id, sample rate, etc.
   */
  subscribe(IVehicleCallback callback, vec options)
          generates (StatusCode status);

  /**
   * Unsubscribes from property events.
   *
   * If this client wasn't subscribed to the given property, this method
   * must return StatusCode::INVALID_ARG.
   */
  unsubscribe(IVehicleCallback callback, int32_t propId)
          generates (StatusCode status);

  /**
   * Print out debugging state for the vehicle hal.
   *
   * The text must be in ASCII encoding only.
   *
   * Performance requirements:
   *
   * The HAL must return from this call in less than 10ms. This call must avoid
   * deadlocks, as it may be called at any point of operation. Any synchronization
   * primitives used (such as mutex locks or semaphores) must be acquired
   * with a timeout.
   *
   */
  debugDump() generates (string s);
};
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IVehicle接口支持功能如下

update-makefiles.sh

通过update-makefiles.sh 可以创建编译HIDL文件的Android.bp。

代码路径：hardware/interfaces/update-makefiles.sh

假设创建一个helloworld模块，在hardware/interfaces 下创建helloworld/1.0/IHelloWorld.hal：

package android.hardware.helloworld@1.0;                                 
interface IHelloWorld {      
    justTest(string name);   
};
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通过update-makefiles.sh就可以在对应package的目录下创建Android.bp：

// This file is autogenerated by hidl-gen -Landroidbp.
hidl_interface {         
    name: "android.hardware.helloworld@1.0",
    root: "android.hardware",
    vndk: {
        enabled: true,
    },
    srcs: [
        "IHelloWorld.hal",
    ],
    interfaces: [
        "android.hidl.base@1.0",
    ],
    gen_java: true,
}
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name：模块的全名

root：包根目录

interfaces：编译过程中依赖的模块

gen_java：是否编译为Java 使用的接口

当然，还有其他的参数，例如gen_java_constants设为true 的时候会生成为Java 使用的Constants类。

编译产物

Android各层编译产物如下：

系统框架层
代码库：packages/services/Car/service
编译模块：CarService(com.android.car)
编译产物：/system/priv-app/CarService/CarService.apk
代码库：packages/services/Car/car-lib
编译模块：android.car
编译产物：/system/Framework/android.car.jar
HIDL
HIDL接口编译的库会生产JAVA和C++两套代码给系统框架层和硬件抽象层调用。
代码库：hardware/interface/automotive/vehicle/2.0
编译模块：android.hardware.automotive.vehicle@2.0
编译产物：
android.hardware.automotive.vehicle-V2.0-java.jar
android.hardware.automotive.vehicle@2.0.so
硬件抽象层
代码库： hardware/interface/automotive/vehicle/2.0/default
编译模块：
vhal_v2_0_defaults
vhal_v2_0_target_defaults
vhal_v2_0_common_headers
android.hardware.automotive.vehicle@2.0-manager-lib
android.hardware.automotive.vehicle@2.0-default-impl-lib
android.hardware.automotive.vehicle@2.0-emulated-user-hal-lib
android.hardware.automotive.vehicle@2.0-server-common-lib
android.hardware.automotive.vehicle@2.0-server-impl-lib
android.hardware.automotive.vehicle@2.0-service
编译产物：
可执行文件 android.hardware.automotive.vehicle@2.0-service
动态库 android.hardware.automotive.vehicle@2.0-manager-lib.so
静态库 android.hardware.automotive.vehicle@2.0-default-impl-lib.a