Android平台读写i2c设备开发笔记一

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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 <hardware/hardware.h>            __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 <hardware/hardware.h>  #include <hardware/iic.h>  #include <fcntl.h>  #include <errno.h>  #include <cutils/log.h>  #include <cutils/atomic.h> #include <stdio.h>#include<linux/i2c.h>#include<linux/i2c-dev.h>#include <stdlib.h>#include <linux/types.h>#include <unistd.h>#include <sys/types.h>#include <sys/ioctl.h>#include <string.h>

#define DEVICE_NAME "/dev/i2c-1"  #define MODULE_NAME "iic"  #define MODULE_AUTHOR "mfayz@sohu.com"  #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(count<len){ bytes = 0; data[bytes++] =  subAddr;//先写子地址  data[bytes]   = dataBuf[count];//再写value          LOGI("IIC write HAL: %x,%x", data[0],data[1]);          (iic_data.msgs[0]).buf=data;//the data to write           ret=ioctl(dev->fd,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(count<len){         (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。

（待续）