Android驱动开发的示例分析

小编给大家分享一下Android驱动开发的示例分析，相信大部分人都还不怎么了解，因此分享这篇文章给大家参考一下，希望大家阅读完这篇文章后大有收获，下面让我们一起去了解一下吧！

1. Linux 内核驱动实例

以下均在Android Linux内核目录下操作， 在drivers目录下创建驱动目录hello
这个目下要创建3个文件，hello.c, Makefile and Kconfig
1.1 hello.c
cd drivers
mkdir hello

vim hello.c
code as follows:

/linux kernel driver: hello.c => /dev/hello /
/create the device file: /sys/class/hello/hello/
please find the code in the below. here skip the code to make the page clean and clear.

1.2 Makefile
Create the Makefile and add:
obj-y += hello.o
1.3 Kconfig
Create Kconfig and add:

config HELLO 
    tristate "Eric: First Android Driver"
default n
help
  This is the first Android driver.

this file is used when we make menuconfig.
1.4 Modify drivers/Makefile
Add following in the end
obj-y += hello.o
1.5 Add the driver into system configuration
Before we build the kernal, we need to config the system.
1.5.1 Modify arch/arm64/Kconfig
Add following in the end
source "drivers/hello/Kconfig"
It seems that this config not work, may be skipped.
1.5.2 Modify drivers/Kconfig
Following the menu:
menu "Device Drivers"
Please add
source "drivers/hello/Kconfig"
1.5.3 Modify drivers/Kconfig

make menuconfig
To enble the menu 'Eric: First Android Driver'  in the "Device Drivers" item.
And save , then to build the linux kernel code.

2. 测试驱动

以下是需要操作的目录在AOSP目录下

2.1 create the application on externel
在external下创建hello目录
目录下将有两个文件:hello.c and Android.mk
cd external
mkdir hello
vim hello.c
/ AOSP app : ./external/hello.c =>/system/bin/hello/
Android.mk as follows:

 LOCAL_PATH := $(call my-dir)
include $(CLEAR_VARS)
LOCAL_MODULE_TAGS := optional
LOCAL_MODULE := hello
LOCAL_SRC_FILES := $(call all-subdir-c-files)
include $(BUILD_EXECUTABLE)

2.2 build the application
To build the hello application
mmm external/hello/
Add it into the system.img
make snod
2.3 Modify the authority of the application
vim system/core/rootdir/ueventd.rc
Add ‘/dev/hello 0666 root root’ in the end of the file.
Build the AOSP before flash.

2.4 Test/Debug the driver hello

Before flash, we can build the AOSP again, of cource, don't forget replace the linux kernel file 'Image.gz-dtb'.
After flash into the phone, reboot the phone, and then do follows:

C:\Users\ylgi>adb devices
List of devices attached
01059f9781509a67        device

C:\Users\ylgi>adb -s 01059f9781509a67 shell
bullhead:/ $ ls

cd /system/bin
./hello

运行效果如上图，表示Linux驱动已经成功加载并运行。

3. 驱动的HAL层实例

以下是需要操作的目录在AOSP目录下
3.1 create hello.c file

cd hardware/libhardware/modules
mkdir hello
vim hello.c

/ to implement the HAL/

3.2 create hello.h file
vim hardware/libhardware/include/hardware/hello.h
/add the implement /
#ifndef ANDROID_HELLO_INTERFACE_H

#define ANDROID_HELLO_INTERFACE_H

#include <hardware/hardware.h>

__BEGIN_DECLS

#define HELLO_HARDWARE_MODULE_ID "hello"//ID

struct hello_module_t {

struct hw_module_t common;

};//hw_module_t的继承者

struct hello_device_t {

struct hw_device_t common;

int fd;

int (*set_val)(struct hello_device_t* dev, int val);

int (*get_val)(struct hello_device_t* dev, int* val);

};//hw_device_t的继承者

__END_DECLS

#endif

..............................
3.3 create Android.mk file
vim Android.mk

LOCAL_PATH := $(call my-dir)

include $(CLEAR_VARS)

LOCAL_MODULE := hello.default

LOCAL_MODULE_RELATIVE_PATH := hw

LOCAL_SRC_FILES := hello.c

LOCAL_SHARED_LIBRARIES := liblog

LOCAL_MODULE_TAGS := optional

include $(BUILD_SHARED_LIBRARY)
3.4 build the hello module
mmm hardware/libhardware/modules/hello

then check the so file in the path:
ll out/target/product/bullhead/system/lib/hw/hello.default.so

4. HAL Layer之上Java的Jni硬件接口封装。

4.1 create com_android_server_HelloService.cpp

cd frameworks/base/services/core/jni
vim com_android_server_HelloService.cpp

其中：

• HELLO_HARDWARE_MODULE_ID为hardware/libhardware/include/hardware/hello.h中定义的hello模块名字，

• jniRegisterNativeMethods注册HelloService包路径为com.android.server.HelloService

4.2 Modify onload.cpp
cd frameworks/base/services/core/jni/
vim onload.cpp

namespace android {
........
int register_android_server_HelloService(JNIEnv *env);//added by eric.y
};

extern "C" jint JNI_OnLoad(JavaVM vm, void / reserved /)
{
..............
register_android_server_HelloService(env);//add by eric
return JNI_VERSION_1_4;
}

4.3 Modify Android.mk

cd frameworks/base/services/core/jni/
vim Android.mk

LOCAL_SRC_FILES += \
     ....................
    $(LOCAL_REL_DIR)/com_android_server_HelloService.cpp \
    $(LOCAL_REL_DIR)/onload.cpp

4.4 build the jni module
mmm frameworks/base/services/core/jni
mmm frameworks/base/services/core/jni
make snod

out/target/product/bullhead/system/lib/libandroid_servers.so

5. JNI上层Service实例的实现,Framework接口

5.1 建立aidl通信接口；
AOSP目录下操作
cd frameworks/base/core/java/android/os/
vim IHelloService.aidl

package android.os;

interface IHelloService {
    void setVal(int val);
    int getVal();
}

5.2 在system_server中注册hello_service到servicemanager
cd frameworks/base/services/java/com/android/server/
vim SystemServer.java

        try {

        Slog.i(TAG, "Service: hello");

        ServiceManager.addService("hello", new HelloService());//

        } catch (Throwable e) {

        Slog.e(TAG, "Failure starting Service: hello", e);

        }

5.3 实现hello_service
create the service file

cd frameworks/base/services/core/java/com/android/server/
vim HelloService.java

/------Eric: hello service-----/

package com.android.server;
import android.content.Context;
import android.os.IHelloService;
public class HelloService extends IHelloService.Stub {
    private static final String TAG = "HelloService";
    HelloService() {
        init_native();
    }

    public void setVal(int val) {
        setVal_native(val);
    }

public int getVal() {
        return getVal_native();
    }
//native implement from HAL layer
private static native boolean init_native();
private static native void setVal_native(int val);
private static native int getVal_native();

};

5.4 modify the Android.mk

cd frameworks/base/
vim Android.mk
add follow into LOCAL_SRC_FILES +=
core/java/android/os/IHelloService.aidl \

5.5 Build the service

mmm frameworks/base

5.6 Add into the SELinux
android5.0以后的系统引入了SELinux,要想让JNI可以成功的访问/dev/hello硬件就必须修改SELinux的策略，否则Android系统再启动是就是出现
add_service ('hello',4e) uid=1000 - PERMISSION DENIED 的错误信息。
AOSP中，SELinux相关的策略配置文件保存在 /external/sepolicy/中，为了完成我们这次实验，需要修改5个 .te 文件.

modify: system/sepolicy/service.te
Add following:
type hello_service, system_api_service, system_server_service, service_manager_type;
modify: external\sepolicy\service_contexts
Add following:
hello          u:object_r:hello_service:s0
为了确保修改后的 .te 文件被成功的编译进system.img 建议执行一次 make update-api ，然后重新执行 make 进行编译
make -j4

5.6 Double check the service

To double check if the serivce works:
service list | grep hello

6. Application on the UI

6.1 create the NfcPosTest applicatoin with button and edit box

copy android project NfcPosTest into the packages/experimental/

代码中需要调用到：
import android.os.IHelloService;
//请求hello_service
private IHelloService helloService = null;
helloService = IHelloService.Stub.asInterface(
ServiceManager.getService("hello"));

//调用HAL接口
int val = helloService.getVal();
String text = String.valueOf(val);
valueText.setText(text);

6.2 Add the Android.mk
vim packages/experimental/NfcPosTest/Android.mk
LOCAL_PATH:= $(call my-dir)
include $(CLEAR_VARS)
LOCAL_MODULE_TAGS := optional
LOCAL_SRC_FILES := $(call all-subdir-java-files)
LOCAL_PACKAGE_NAME := NfcPosTest
include $(BUILD_PACKAGE)

6.3 build the application
build the demo:
mmm packages/experimental/NfcPosTest
succeeded check the apk file in the path:
ll out/target/product/bullhead/system/app/NfcPosDemo.apk

6.4 update system.img

make snod

7. Check NfcPosTest application in Android phone after flash succeeded.

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