在Android开发和移植过程中,有时需要对某设备进行读写,但系统可能并未提供相应的服务。我们就需要自己开发硬件访问服务来控制设备。下面的例子是读写最简单的i2c设备eeprom的流程, i2c的驱动编写有两种方式,一种是利用系统提供的i2c-dev.c来实现一个i2c适配器的设备文件,然后通过在应用层操作I2C适配器来控制I2C设备;另一种是为I2C从设备独立编写一个设备驱动,不需要i2c-dev.c文件。由于前者比较简单通用性强,我们采用前者来展开。
根据android层次划分,我们照例对开发分为如下几步:
1. 添加HAL层接口模块访问设备
2. 使用JNI在应用程序框架层添加服务访问接口
3. 使用服务接口api开发应用程序
一. 添加HAL层接口模块访问设备
首先确认物理设备正常。根据开发板说明书获知设备挂载在/dev/i2c-1上,检测到该设备的存在,则通用设备驱动正常。
eeprom设备为at24c**系列,根据说明书获知设备从地址为0x50,准备工作完毕。
1. 编写hal层接口模块头文件iic.h
进入源码根目录下android\hardware\libhardware\include\hardware目录新建iic.h,代码如下:
- #ifndef ANDROID_IIC_INTERFACE_H
- #define ANDROID_IIC_INTERFACE_H
- #include
-
- __BEGIN_DECLS
-
-
- #define IIC_HARDWARE_MODULE_ID "iic"
-
-
- struct iic_module_t {
- struct hw_module_t common;
- };
-
-
- struct iic_device_t {
- struct hw_device_t common;
- int fd;
- int (*iic_write)(struct iic_device_t* dev, unsigned char* dataBuf, unsigned short slaveAddr, unsigned short subAddr, int len);
- int (*iic_read)(struct iic_device_t* dev, unsigned char* dataBuf, unsigned short slaveAddr, int len);
- };
-
- __END_DECLS
-
- #endif
这里定义了iic_write和iic_read两个接口,头文件按照hal规范编写。
2. 编写hal层接口模块文件
进入源码根目录下android\hardware\libhardware\modules目录新建iic目录,并在iic目录中添加iic.c,代码如下:
- #include
- #include
- #include
- #include
- #include
- #include
- #include
- #include
- #include
- #include
- #include
- #include
- #include
- #include
- #include
- "font-size:14px;">#define DEVICE_NAME "/dev/i2c-1"
- #define MODULE_NAME "iic"
- #define MODULE_AUTHOR "[email protected]"
-
-
- #define I2C_RETRIES 0x0701/* number of times a device address should be polled when not acknowledging */
- #define I2C_TIMEOUT 0x0702/* set timeout in units of 10 ms */
- #define I2C_RDWR 0x0707
-
-
-
- struct i2c_msg
- {
- unsigned short addr;
- unsigned short flags;
- #define I2C_M_TEN 0x0010
- #define I2C_M_RD 0x0001
- unsigned short len;
- unsigned char *buf;
- };
-
-
- struct i2c_rdwr_ioctl_data {
- struct i2c_msg *msgs;
- int nmsgs;
- };
-
-
-
- static int iic_device_open(const struct hw_module_t* module, const char* name, struct hw_device_t** device);
- static int iic_device_close(struct hw_device_t* device);
-
-
- static int iic_write(struct iic_device_t* dev, unsigned char* dataBuf, unsigned short slaveAddr, unsigned short subAddr, int len);
- static int iic_read(struct iic_device_t* dev, unsigned char* dataBuf, unsigned short slaveAddr, int len);
-
-
- static struct hw_module_methods_t iic_module_methods = {
- open: iic_device_open
- };
-
-
- struct i2c_rdwr_ioctl_data iic_data;
- int ret;
-
-
-
- struct iic_module_t HAL_MODULE_INFO_SYM = {
- common: {
- tag: HARDWARE_MODULE_TAG,
- version_major: 1,
- version_minor: 0,
- id: IIC_HARDWARE_MODULE_ID,
- name: MODULE_NAME,
- author: MODULE_AUTHOR,
- methods: &iic_module_methods,
- }
- };
-
-
- static int iic_device_open(const struct hw_module_t* module, const char* name, struct hw_device_t** device){
- struct iic_device_t* dev;
- dev = (struct iic_device_t*)malloc(sizeof(struct iic_device_t));
- if(!dev) {
- LOGE("iic Stub: failed to alloc space");
- return -EFAULT;
- }else{
- LOGE("hal: alloc space succ!");
- }
-
- memset(dev, 0, sizeof(struct iic_device_t));
- dev->common.tag = HARDWARE_DEVICE_TAG;
- dev->common.version = 0;
- dev->common.module = (hw_module_t*)module;
- dev->common.close = iic_device_close;
- dev->iic_write = iic_write;
- dev->iic_read = iic_read;
- *device = &dev->common;
-
-
- if((dev->fd = open(DEVICE_NAME, O_RDWR)) == -1) {
- LOGE("iic Stub hal: failed to open /dev/i2c-1 -- %s.", strerror(errno));
- free(dev);
- return -EFAULT;
- }else{
- LOGI("iic Stub hal: open /dev/i2c-1 successfully.");
- iic_data.nmsgs=2;
- iic_data.msgs=(struct i2c_msg*)malloc(iic_data.nmsgs*sizeof(struct i2c_msg));
-
-
- if(!iic_data.msgs){
- LOGE("malloc error");
- close(dev->fd);
- exit(1);
- }
- ioctl(dev->fd, I2C_TIMEOUT, 2);
- ioctl(dev->fd, I2C_RETRIES, 1);
- }
- return 0;
- }
-
-
- static int iic_device_close(struct hw_device_t* device) {
- struct iic_device_t* iic_device = (struct iic_device_t*)device;
-
- if(iic_device) {
- close(iic_device->fd);
- free(iic_device);
- }
-
- return 0;
- }
-
- static int iic_write(struct iic_device_t* dev, unsigned char* dataBuf, unsigned short slaveAddr, unsigned short subAddr, int len) {
- int count = 0;
- unsigned char data[2];
- unsigned char bytes;
-
-
- LOGI("iic Stub hal: set value %s to device.", dataBuf);
- iic_data.nmsgs=1;
- (iic_data.msgs[0]).len=2;
- (iic_data.msgs[0]).addr=slaveAddr;
- (iic_data.msgs[0]).flags=0;
- (iic_data.msgs[0]).buf=(unsigned char*)malloc(2);
- while(count
- bytes = 0;
- data[bytes++] = subAddr;
- data[bytes] = dataBuf[count];
- LOGI("IIC write HAL: %x,%x", data[0],data[1]);
- (iic_data.msgs[0]).buf=data;
- ret=ioctl(dev->fd,I2C_RDWR,(unsigned long)&iic_data);
- if(ret<0){
- LOGI("IIC HAL ioctl error");
- }
- count++;
- subAddr++;
- usleep(3000);
- }
- LOGI("you have write %s into iic at %x address len: %d",dataBuf, subAddr, len);
-
- return 0;
- }
-
- static int iic_read(struct iic_device_t* dev, unsigned char* dataBuf, unsigned short slaveAddr, int len){
- int count = 0;
-
- iic_data.nmsgs=1;
-
- (iic_data.msgs[0]).len=1;
- (iic_data.msgs[0]).addr=slaveAddr;
- (iic_data.msgs[0]).flags=I2C_M_RD;
- (iic_data.msgs[0]).buf=(unsigned char*)malloc(1);
- while(count
- (iic_data.msgs[0]).buf= dataBuf++;
- if(ioctl(dev->fd,I2C_RDWR,(unsigned long)&iic_data)<0){
- LOGE("ioctl read error");
- }
- LOGI("IIC read HAL: %x", dataBuf[count]);
- count++;
- }
-
- return 0;
- }
注意:需打开设备/dev/i2c-1权限,否则会碰到Pemission Denied错误。从源码根目录下进入system/core/rootdir目录,打开ueventd.rc 添加一行:/dev/i2c-1 0666 root root (这里设备各开发板可能不同)
3. 在iic目录下编写android.mk进行编译
- LOCAL_PATH := $(call my-dir)
- include $(CLEAR_VARS)
- LOCAL_MODULE_TAGS := optional
- LOCAL_PRELINK_MODULE := false
- LOCAL_MODULE_PATH := $(TARGET_OUT_SHARED_LIBRARIES)/hw
- LOCAL_SRC_FILES := iic.c
- LOCAL_MODULE := iic.default
- include $(BUILD_SHARED_LIBRARY)
编译命令:mmm -B hardware/libhardware/module/iic 编译成功会得到iic.default.so,打包进img默认会被加载。
二、 使用JNI在应用程序框架层添加服务访问接口
APP应用不能直接访问HAL层,需要JNI层访问HAL模块并向上提供API接口。可以直接提供接口,但建议最好使用服务的方式提供访问。
我们先看JNI如何访问刚才的HAL模块。
进入源码根目录下的frameworks/base/service/jni目录,新建com_android_server_IICService.cpp,代码如下:
- #include "jni.h"
- #include "JNIHelp.h"
- #include "android_runtime/AndroidRuntime.h"
- #include
- #include
- #include
- #include
- #include
-
- namespace android
- {
-
- struct iic_device_t* iic_device = NULL;
-
- static void iic_setVal(JNIEnv* env, jobject clazz, jstring val, jint slaveAddr, jint subAddr, jint len) {
- const char *str = env->GetStringUTFChars(val, NULL);
- LOGI("iic JNI: set value %s to device.", str);
- if(!iic_device) {
- LOGI("iic JNI: device is not open.");
- return;
- }
- iic_device->iic_write(iic_device, (unsigned char*)str, slaveAddr, subAddr, len);
- env->ReleaseStringUTFChars(val, str);
- }
-
-
- static jstring iic_getVal(JNIEnv* env, jobject clazz, jint slaveAddr, jint len) {
- unsigned char* data = (unsigned char*)malloc(len);
- iic_device->iic_read(iic_device, data, slaveAddr, len);
- if(!iic_device) {
- LOGI("iic JNI: device is not open.");
- }
- int i = 0;
- for(;iconst char*)data);i++){
- LOGI("data: %c ", data[i]);
- }
-
- jstring tmp = env->NewStringUTF((const char*)data);
- free(data);
- data = NULL;
- return tmp;
- }
-
-
- static inline int iic_device_open(const hw_module_t* module, struct iic_device_t** device) {
- return module->methods->open(module, IIC_HARDWARE_MODULE_ID, (struct hw_device_t**)device);
- }
-
- static jboolean iic_init(JNIEnv* env, jclass clazz) {
- iic_module_t* module;
-
- LOGI("iic JNI: initializing......");
- if(hw_get_module(IIC_HARDWARE_MODULE_ID, (const struct hw_module_t**)&module) == 0) {
- LOGI("iic JNI: iic Stub found.");
- if(iic_device_open(&(module->common), &iic_device) == 0) {
- LOGI("eeprom JNI: iic device is opening...");
- return 0;
- }
- LOGE("eeprom JNI: failed to open iic device.");
- return -1;
- }
- LOGE("eeprom JNI: failed to get iic stub module.");
- return -1;
- }
-
- static const JNINativeMethod method_table[] = {
- {"init_native", "()Z", (void*)iic_init},
- {"setVal_native", "(Ljava/lang/String;III)V", (void*)iic_setVal},
- {"getVal_native", "(III)Ljava/lang/String;", (void*)iic_getVal},
- };
-
- int register_android_server_IICService(JNIEnv *env) {
- return jniRegisterNativeMethods(env, "com/android/server/IICService", method_table, NELEM(method_table));
- }
- };
然后需要让android启动时加载此jni模块
在同目录下修改onload.cpp:
在namespace android中添加一行 int register_android_server_IICService(JNIEnv *env);
在JNI_onLoad方法中添加一行 register_android_server_IICService(env);
在同目录下修改Android.mk:
LOCAL_SRC_FILES增加一行 com_android_server_IICService \
编译命令:mmm frameworks/base/services/jni
注意:
HAL是根据iic_init中的IIC_HARDWARE_MODULE_ID加载相应模块。
然后,使用AIDL进行进程间通信,使APP能访问自定义的硬件服务。
我们需要在frameworks/base/core/Java/android/os中新建IIICService.aidl(注意是III)
package android.os;
interface IIICService {
void setVal(String val, int slaveAddr, int regAddr, int len);
String getVal(int slaveAddr, int len);
}
它定义了服务的接口,接口在IICService中实现并关联到jni本地方法中。
同时我们需要修改frameworkd/base下的Android.mk编译文件,在LOCAL_SRC_FILES中增加 core/java/android/os/IIICService.aidl
编译命令: mmm frameworks/base
下面是AIDL的实现方法类:com.android.server.IICService 位置为:frameworks/base/services/java/com/android/server 代码如下:
- package com.android.server;
- import android.content.Context;
- import android.os.IIICService;
- import android.util.Slog;
- public class IICService extends IIICService.Stub {
- private static final String TAG = "IICService";
- IICService() {
- init_native();
- }
- public void setVal(String val,int slaveAddr, int regAddr, int len) {
- setVal_native(val, slaveAddr, regAddr, len);
- }
- public String getVal(int slaveAddr,int len) {
- return getVal_native( slaveAddr, len);
- }
-
-
- private static native boolean init_native();
- private static native void setVal_native(String val, int slaveAddr, int regAddr, int len);
- private static native String getVal_native(int slaveAddr, int len);
- };
从代码中我们可以看到它继承了IIICService.Stub,实现两个接口方法。因为硬件访问一般需要放在一个独立的线程中,这里使用了代理的方法来处理app与硬件服务的通信。
最后需要把新增的IICService服务加入到ServiceManager中,这样就可以通过ServiceManager进行调用。
修改frameworks/base/services/java/com/android/server下的SystemServer.java 在run()方法中添加
try{
Slog.i(TAG, "IIC SERVICE");
ServiceManager.addService("iic", new IICService());
}catch(Throwable e){
Slog.e(TAG, "Failure starting IIC Service", e);
}
编译命令:mmm frameworks/base/services/java
或者使用另一种形式来调用服务:如同使用binder机制绑定service一样的方法, 具体就不详细写了。
注意:有可能会编译不通过,因为这里修改了android的官方api, 需要运行make update-api更新frameworks/base/api/current.xml
打包后,app就可以使用IICService接口来访问硬件了。
下一节发上app相关代码
三、app调用服务接口访问硬件
上主要代码EEPROMActivity.java
- package com.zkgd.eeprom;
-
- import android.app.Activity;
- import android.os.Bundle;
- import android.os.ServiceManager;
- import android.os.IIICService;
- import android.os.RemoteException;
- import android.util.Log;
- import android.view.View;
- import android.view.View.OnClickListener;
- import android.widget.Button;
- import android.widget.EditText;
-
- public class EEPROMActivity extends Activity implements OnClickListener{
- private final static String LOG_TAG = "com.zkgd.eeprom";
-
- private IIICService iicService = null;
-
- private EditText valueText = null;
- private Button readButton = null;
- private Button writeButton = null;
- private Button clearButton = null;
- int len = 1;
-
- @Override
- public void onCreate(Bundle savedInstanceState) {
- super.onCreate(savedInstanceState);
- setContentView(R.layout.main);
-
- iicService = IIICService.Stub.asInterface(
- ServiceManager.getService("iic"));
-
- valueText = (EditText)findViewById(R.id.edit_value);
- readButton = (Button)findViewById(R.id.button_read);
- writeButton = (Button)findViewById(R.id.button_write);
- clearButton = (Button)findViewById(R.id.button_clear);
-
- readButton.setOnClickListener(this);
- writeButton.setOnClickListener(this);
- clearButton.setOnClickListener(this);
-
- Log.i(LOG_TAG, "Activity Created");
- }
-
- public void onClick(View v) {
- if(v.equals(readButton)) {
- try {
- len = 1;
-
- String val = iicService.getVal(0x50,len);
- valueText.setText(val);
- } catch (RemoteException e) {
- Log.e(LOG_TAG, "Remote Exception while reading value from device.");
- }
- }
- else if(v.equals(writeButton)) {
- try {
- String val = valueText.getText().toString();
- len = val.length();
-
- iicService.setVal(val,0x50,0x10,len);
- } catch (RemoteException e) {
- Log.e(LOG_TAG, "Remote Exception while writing value to device.");
- }
- }
- else if(v.equals(clearButton)) {
- String text = "";
- valueText.setText(text);
- }
- }
- }
工程eeprom放置在源码目录package/app/下
编译命令:mmm package/app/eeprom
打包,烧写固件至开发板,启动就可以看到该应用的图标了。
小结:
整个调用流程为:app<---AIDL访问服务<---JNI本地方法实现<---HALso文件<---硬件
一个问题,这种方法改动了android原生api,毕竟是访问了硬件。如果想做通用app又想使用c/c++提高效率,直接进行NDK开发,功能编译成库文件打进app应用的工程中。
另一个问题,硬件访问会遭遇到权限问题。如果做通用app,需要设备root了,然后在代码里添加权限修改操作,例如:"chmod 777 "+getPackageCodePath(); "chmod 777 /dev/i2c-1";