AIDL for HALs实战

Stable AIDL HAL 实战

背景:

官方:https://source.android.com/devices/architecture/aidl/aidl-hals.

Google 在Android 11引入了AIDL for HALs,旨在代替HIDL原先的作用。在之后的Android版本推荐使用AIDL 实现Hal层的访问。
这样做的原因,应该有以下几点:

  1. AIDL比HIDL存在的时间更长(仅从Android 8到Android 10),并在许多其他地方使用,如Android框架组件之间或应用程序中。既然AIDL具有稳定性支持,就可以用单一的IPC方式从HAL到框架进程或者应用进程。
  2. AIDL还有一个比HIDL更好的版本控制系统。

再详细的展开说就是:

  1. AIDL 更成熟,使用更广泛,如果HAL层也使用了AIDL的方式,那么就可以直接从应用进程调用到HAL 进程,以前使用HIDL的时候实现应用进程访问HAL的服务,需要在system server进程的中介。来个图:
    AIDL for HALs实战_第1张图片
  2. 以前使用HIDL的方式,如果后续vendor HAL version需要迭代升级,那么就需要再创建一个子目录,过程中实际上做很多的重复工作,冗余而效率不高。

值得注意的是:在HAL 层使用AIDL必须使用Stable AIDL, 和我们在应用层或者框架层稍微不同,因为和vendor的接口设计要兼顾稳定性,system和vendor的更新速率不一样。

HALs using AIDL to communicate between framework components must use Stable AIDL.

使用AIDL for HALs

1. 定义HAL接口

创建对应的模块目录:/hardware/interfaces/hongxi/aidl/
创建aidl文件:/hardware/interfaces/hongxi/aidl/android/hardware/hongxi/IHongxi.aidl

package android.hardware.hongxi;

@VintfStability
interface IHongxi {
    String getName();
    
    void setName(in String msg);
}

每个类型定义都必须使用@VintfStability进行注释。
如果想要定义类型,参考同用的AIDL的定义就行,同时比通用的AIDL多了枚举、结构体、parcelable类型(注意这些类型跟Android版本有关,13以下的版本不一定有全)

2. 配置Android.bp

创建顶层Android.bp:/hardware/interfaces/hongxi/aidl/Android.bp

aidl_interface {
   name: "android.hardware.hongxi",
   vendor: true,
   srcs: ["android/hardware/hongxi/*.aidl"],
   stability: "vintf",
   owner: "hongxi.zhu",
   backend: {
       cpp: {
           enabled: false,
       },
       java: {
       	   enabled: false,
       },
   },
}
  1. backend: 服务的后端,AIDL支持四种后端,分别是C++/JAVA/NDK/RUST, 我们将使用NDK(谷歌推荐),因此将CPP和JAVA后端声明为false(实际上我试了这两个,编译有问题,还没解决,后续解决了更新出来)。

  2. 为了方便测试,设置vendor:true并删除vendor_available,因为这是一个自定义供应商HAL,删除vndk部分,因此这个HAL仅位于vendor分区,不受VNDK限制,真正开发中,如需开启需要自己解决VNDK的限制问题,这里就不单独列出。

3. 编译模块

mmm hardware/interfaces/hongxi/

然后就会报错:

[ 74% 227/303] echo "API dump for the current version of AIDL interface android.hardware.hongxi does not exist." && echo Run "m android.hardware.hongxi-update-api", or add "unstable: true" to the build ru
FAILED: out/soong/.intermediates/hardware/interfaces/hongxi/aidl/android.hardware.hongxi-api/checkapi_current.timestamp
echo "API dump for the current version of AIDL interface android.hardware.hongxi does not exist." && echo Run "m android.hardware.hongxi-update-api", or add "unstable: true" to the build rule for the inte
rface if it does not need to be versioned && false
API dump for the current version of AIDL interface android.hardware.hongxi does not exist.
Run m android.hardware.hongxi-update-api, or add unstable: true to the build rule for the interface if it does not need to be versioned
19:29:13 ninja failed with: exit status 1

原因是当前版本没有这个接口,需要更新下API,按照提示来:

m android.hardware.hongxi-update-api

然后再重新编译模块:

mmm hardware/interfaces/hongxi/

4. 实现HAL 接口

We will use the ndk_platfrom library, therefore, let check the generated code for ndk_platform.我们需要在实现的接口编译脚本中引用模块的ndk_platfrom, 且我们要实现的接口在编译时都生成了对应的源码,我们只需要拷贝出来并实现,所以先看下,刚才的编译都生成了什么:

cd out/soong/.intermediates/hardware/interfaces/hongxi/aidl/android.hardware.hongxi-ndk_platform-source
find .

.
./gen
./gen/android
./gen/android/hardware
./gen/android/hardware/hongxi
./gen/android/hardware/hongxi/IHongxi.cpp.d
./gen/android/hardware/hongxi/IHongxi.cpp
./gen/include
./gen/include/aidl
./gen/include/aidl/android
./gen/include/aidl/android/hardware
./gen/include/aidl/android/hardware/hongxi
./gen/include/aidl/android/hardware/hongxi/BpHongxi.h
./gen/include/aidl/android/hardware/hongxi/BnHongxi.h
./gen/include/aidl/android/hardware/hongxi/IHongxi.h
./gen/timestamp

在IHongxi.h头文件中找到我们要实现的接口:

  virtual ::ndk::ScopedAStatus getName(std::string* _aidl_return) = 0;
  virtual ::ndk::ScopedAStatus setName(const std::string& in_msg) = 0;

接下来就需要创建后端源码文件,来实现这些接口:
/hardware/interfaces/hongxi/aidl/default/Hongxi.h

#pragma once

#include 

namespace aidl {
namespace android {
namespace hardware {
namespace hongxi {

class Hongxi : public BnHongxi {
    public:
        //String getName();
        ndk::ScopedAStatus getName(std::string* _aidl_return);

        //void setName(in String msg);
        ndk::ScopedAStatus setName(const std::string& in_msg);

    private:
        std::string name = "";
};

}  // namespace hongxi
}  // namespace hardware
}  // namespace android
}  // namespace aidl

/hardware/interfaces/hongxi/aidl/default/Hongxi.cpp

#define LOG_TAG "Hongxi"

#include 
#include 
#include "Hongxi.h"

namespace aidl {
namespace android {
namespace hardware {
namespace hongxi {

ndk::ScopedAStatus Hongxi::getName(std::string* _aidl_return) {

    *_aidl_return = name;

    return ndk::ScopedAStatus::ok();
}

ndk::ScopedAStatus Hongxi::setName(const std::string& in_msg) {

    name = in_msg;

    return ndk::ScopedAStatus::ok();
}

}  // namespace hongxi
}  // namespace hardware
}  // namespace android
}  // namespace aidl

5. 实现服务:

/hardware/interfaces/hongxi/aidl/default/main.cpp

#define LOG_TAG "Hongxi"

#include 
#include 
#include 
#include 
#include 
#include "Hongxi.h"

using aidl::android::hardware::hongxi::Hongxi;
using std::string_literals::operator""s;

int main() {
    // Enable vndbinder to allow vendor-to-venfor binder call
    android::ProcessState::initWithDriver("/dev/vndbinder"); //使用vnbinder的配置

    ABinderProcess_setThreadPoolMaxThreadCount(0); // vnbinder的线程池独立,需要单独配置 
    ABinderProcess_startThreadPool();

    std::shared_ptr hongxi = ndk::SharedRefBase::make();
    const std::string desc = Hongxi::descriptor + "/default"s;

    if (hongxi != nullptr) {
        if(AServiceManager_addService(hongxi->asBinder().get(), desc.c_str()) != STATUS_OK) {
            ALOGE("Failed to register IHongxi service");
            return -1;
        }
    } else {
        ALOGE("Failed to get IHongxi instance");
        return -1;
    }

    ALOGD("IHongxi service starts to join service pool");
    ABinderProcess_joinThreadPool();

    return EXIT_FAILURE;  // should not reached
}

8. 编写服务启动的rc脚本

/hardware/interfaces/hongxi/aidl/default/android.hardware.hongxi-service.rc

service android.hardware.hongxi-service /vendor/bin/hw/android.hardware.hongxi-service
        interface aidl android.hardware.hongxi.IHongxi/default
        class hal
        user system
        group system

9. 声明VINTF AIDL 接口

/hardware/interfaces/hongxi/aidl/default/android.hardware.hongxi-service.xml

<manifest version="1.0" type="device">
    <hal format="aidl">
        <name>android.hardware.hongxi</name>
        <fqname>IHongxi/default</fqname>
    </hal>
</manifest>

7. 编写服务构建脚本

/hardware/interfaces/hongxi/aidl/default/Android.bp

cc_binary {
    name: "android.hardware.hongxi-service",
    vendor: true,
    relative_install_path: "hw",
    init_rc: ["android.hardware.hongxi-service.rc"],
    vintf_fragments: ["android.hardware.hongxi-service.xml"],

    srcs: [
        "Hongxi.cpp",
        "main.cpp",
    ],

    cflags: [
        "-Wall",
        "-Werror",
    ],

    shared_libs: [
        "libbase",
        "liblog",
        "libhardware",
        "libbinder_ndk",
        "libbinder",
        "libutils",
        "android.hardware.hongxi-ndk_platform",
    ],
}

将模块加入系统中

/build/target/product/base_vendor.mk

# add for Hongxi
PRODUCT_PACKAGES += \
    android.hardware.hongxi \
    android.hardware.hongxi-service \

将模块添加到兼容性矩阵中

# (选下标最新的那个)
hardware/interfaces/compatibility_matrices/compatibility_matrix.5.xml
#(这个不一定有,如果没有就不加)
hardware/interfaces/compatibility_matrices/compatibility_matrix.current.xml
    <hal format="aidl" optional="true">
        <name>android.hardware.hongxi</name>
        <version>1.0</version>
        <interface>
            <name>IHongxi</name>
            <instance>default</instance>
        </interface>
    </hal>

解决Selinux权限

这个后续补充,测试中,会有宏版本和直接添加的版本

客户端测试

  1. cpp-client(user process)
  2. apk-client(user process)
  3. SystemService-client(system server process)

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