本人在开发Android Nfc POS之初，探索调试了一番驱动，目前在Nexus 5X 7.1.1上已经调成，之前的步骤可以参考我发的文章，Android驱动开发经验分享如下。
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 (on Windows command line):
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
