Android平台读写i2c设备开发笔记一
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
/*定义模块ID*/
#define IIC_HARDWARE_MODULE_ID "iic"
/*硬件模块结构体*/
struct iic_module_t {
struct hw_module_t common;
};
/*硬件接口结构体*/
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 #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
/*********定义struct i2c_rdwr_ioctl_data和struct i2c_msg，要和内核一致*******/
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;/* pointers to i2c_msgs */
int nmsgs; /* number of i2c_msgs */
};
/*设备打开和关闭接口*/
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);
/*设备访问接口*/
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);
/*模块方法表*/
static struct hw_module_methods_t iic_module_methods = {
open: iic_device_open
};
struct i2c_rdwr_ioctl_data iic_data;
int ret;
/*模块实例变量*/
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, //实现了一个open的方法供jni层调用，从而实例化eeprom_device_t
}
};
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; //将实例化后的iic_device_t地址返回给jni层,这样jni层就可以直接调用方法了。
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);//设置超时时间
ioctl(dev->fd, I2C_RETRIES, 1);//设置重发次数
}
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; //写入地址位和数据长度
(iic_data.msgs[0]).addr=slaveAddr;// 设备地址0x50
(iic_data.msgs[0]).flags=0; //write
(iic_data.msgs[0]).buf=(unsigned char*)malloc(2);
while(countfd,I2C_RDWR,(unsigned long)&iic_data);
if(ret<0){
LOGI("IIC HAL ioctl error");
}
count++;
subAddr++;
usleep(3000);//延迟3毫秒
}
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; // 设备地址
(iic_data.msgs[0]).flags=I2C_M_RD;//read
(iic_data.msgs[0]).buf=(unsigned char*)malloc(1);
while(countfd,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。
（待续）