HIDL接口定义
创建hidl目录
在aosp中的hardware/interfaces目录下创建自己的hal目录

mkdir -p hardware/interfaces/qmiller/1.0/default
创建hal文件
在我们刚才创建的hardware/interfaces/qmiller/1.0/目录下新建IQmiller.hal文件,传入string name，返回string类型的result。

package android.hardware.qmiller@1.0;
    
    interface IQmiller {
        helloWorld(string name) generates (string result);
    };

生产HAL文件
使用hidl-gen工具来处理IQmiller.hal文件，这里可以写成一个shell脚本。

PACKAGE=android.hardware.qmiller@1.0
LOC=hardware/interfaces/qmiller/1.0/default/
make hidl-gen -j4
hidl-gen -o $LOC -Lc++-impl -randroid.hardware:hardware/interfaces -randroid.hidl:system/libhidl/transport $PACKAGE
hidl-gen -o $LOC -Landroidbp-impl -randroid.hardware:hardware/interfaces -randroid.hidl:system/libhidl/transport $PACKAGE

此时的文件树：

hardware/interfaces/qmiller/1.0/
├── default
│   ├── Android.bp
│   ├── Qmiller.cpp
│   └── Qmiller.h
└── IQmiller.hal

然后使用脚本来更新Makefile，自动生成Android,mk, Android.bp

./hardware/interfaces/update-makefiles.sh
此时的文件树：

hardware/interfaces/qmiller/1.0/
├── Android.bp
├── default
│   ├── Android.bp
│   ├── Qmiller.cpp
│   └── Qmiller.h
└── IQmiller.hal

新建两个空文件：

touch hardware/interfaces/qmiller/1.0/default/android.hardware.qmiller@1.0-service.rc
touch hardware/interfaces/qmiller/1.0/default/service.cpp
实现HAL的implementation库
自动生成的Qmiller.h文件,这里介绍的是绑定式HAL，而不是直通式。

#pragma once

#include <android/hardware/qmiller/1.0/IQmiller.h>
#include <hidl/MQDescriptor.h>
#include <hidl/Status.h>

namespace android::hardware::qmiller::implementation {

using ::android::hardware::hidl_array;
using ::android::hardware::hidl_memory;
using ::android::hardware::hidl_string;
using ::android::hardware::hidl_vec;
using ::android::hardware::Return;
using ::android::hardware::Void;
using ::android::sp;

struct Qmiller : public V1_0::IQmiller {
    // Methods from ::android::hardware::qmiller::V1_0::IQmiller follow.
    Return<void> helloWorld(const hidl_string& name, helloWorld_cb _hidl_cb) override;

    // Methods from ::android::hidl::base::V1_0::IBase follow.

};

// FIXME: most likely delete, this is only for passthrough implementations
// extern "C" IQmiller* HIDL_FETCH_IQmiller(const char* name);

}  // namespace android::hardware::qmiller::implementation

自动生成的Qmiller.cpp

#include "Qmiller.h"

namespace android::hardware::qmiller::implementation {

// Methods from ::android::hardware::qmiller::V1_0::IQmiller follow.
Return<void> Qmiller::helloWorld(const hidl_string& name, helloWorld_cb _hidl_cb) {
    // TODO implement
    return Void();
}


// Methods from ::android::hidl::base::V1_0::IBase follow.

//IQmiller* HIDL_FETCH_IQmiller(const char* /* name */) {
    //return new Qmiller();
//}
//
}  // namespace android::hardware::qmiller::implementation

修改Android.bp文件，将-impl改成-service，将cc_library_shared改成cc_library，在-service后面加上vendor名称。

cc_library {
    
    name: "android.hardware.qmiller@1.0-service-lzm",
    relative_install_path: "hw",
    vendor:true,
    // FIXME: this should be 'vendor: true' for modules that will eventually be
    // on AOSP.
    proprietary: true,
    srcs: [
        "Qmiller.cpp",
    ],
    shared_libs: [
        "libhidlbase",
        "libutils",
        "android.hardware.qmiller@1.0",
    ],
}

实现service.cpp

#define LOG_TAG   "android.hardware.qmiller@1.0-service-lzm"
#include <hidl/HidlLazyUtils.h>
#include <hidl/HidlSupport.h>
#include <hidl/HidlTransportSupport.h>
#include "Qmiller.h"

using ::android::OK;
using ::android::sp;
using ::android::hardware::configureRpcThreadpool;
using ::android::hardware::joinRpcThreadpool;
using ::android::hardware::LazyServiceRegistrar;
using ::android::hardware::qmiller::V1_0::IQmiller;
using ::android::hardware::qmiller::V1_0::implementation::Qmiller;
  
int main(int argc, char* []) {
    sp<IQmiller> qmiller = new Qmiller;
    configureRpcThreadpool(1, true);
    auto registrar = LazyServiceRegistrar::getInstance();
    if (registrar.registerService(qmiller) != OK) {
        ALOGE("Could not register service.");
        return 1;
    }
    joinRpcThreadpool();
    ALOGE("Service exited!");
    return 1;
}

修改

Android.bpcc_library {
    
    name: "android.hardware.qmiller@1.0-service-lzm",
    relative_install_path: "hw",
    vendor:true,
    // FIXME: this should be 'vendor: true' for modules that will eventually be
    // on AOSP.
    proprietary: true,
    srcs: [
        "Qmiller.cpp",
    ],
    shared_libs: [
        "libhidlbase",
        "libutils",
        "android.hardware.qmiller@1.0",
    ],
}
cc_binary{
    name: "android.hardware.qmiller@1.0-service",
        defaults: ["hidl_defaults"],
        relative_install_path: "hw",
        proprietary: true,
        srcs: ["service.cpp"],
        init_rc: ["android.hardware.qmiller@1.0-service.rc"],
        shared_libs: [
            "libhidlbase",
            "libhidltransport",
            "libutils",
            "liblog",
            "libhardware",
            "android.hardware.qmiller@1.0",
            "android.hardware.qmiller@1.0-service-lzm",
        ],
}

实现Qmiller.cpp文件

#include "Qmiller.h"
namespace android::hardware::qmiller::implementation {
    
Return<void> Qmiller::helloWorld(const hidl_string& name, helloWorld_cb _hidl_cb) {
    char buf[100];
    ::memset(buf, 0x00, 100);
    ::snprintf(buf, 100, "Hello World, %s", name.c_str());
    hidl_string result(buf);
    _hidl_cb(result);
    return Void();
    }

} // namespace android::hardware::qmiller::implementation

编译hal
mmm hardware/interfaces/qmiller/1.0/default/
最终会生成一个android.hardware.qmiller@1.0-service-lzm.so, 生成在/vendor/lib64/hw/下。service.rc
service.cpp会被编译成bin文件，我们需要在系统启动的时候将我们编写的hal服务加进系统。android.hardware.qmiller@1.0-service.rc文件内容如下：

service qmiller_hal_service /vendor/bin/hw/android.hardware.qmiller@1.0-service
        class hal
        user system
        group system

为了让服务器被客户端访问到，还需要在device/xxx/xxx/manifest.xml（不同厂商路径不同）添加如下：

<hal format="hidl">
          <name>android.hardware.qmiller</name>
          <transport>hwbinder</transport>
          <impl level="generic"></impl>
          <version>1.0<version>
          <interface>
              <name>IQmiller</name>
              <instance>default</instance>
          </interface>
</hal>