一：I2C设备操作方式：
1. 应用程序操作法：i2c的设备的驱动可以直接利用linux内核提供的i2c-dev.c文件提供的ioctl函数接口在应用层实现对i2c设备的读写，但是在应用层使用ioctl函数对应用程序员要求较高，需要自行构建msg结构体，必须了解设备的操作流程，时序之类的。
这方式实现需要用用程序员调用 read， write， ioctl， open， close等linux标准文件接口操作/dev/i2c(X)设备文件。
2. 驱动程序操作法：i2c设备的驱动也可以通过普通的设备驱动实现，像往常的驱动一样实现，然后在应用层就可以像读取普通文件一样操作，无需再考虑读写时序。其实普通的设备驱动也可以用两种方法实现，
1）构建字符设备驱动，在open,read,write等函数中直接操作i2c总线的相关寄存器来读写i2c设备，但是这种方法因平台不同，设备不同都要重新写驱动
2）在设备驱动中调用i2c-core.c提供的i2c_transfer函数来实现和i2c设备的通信，这样只要对不同的设备写不同的驱动就行了。
二：首先我们来看一看应用程序操作法：
标准i2c接口提供的标准的设备/dev/i2c，首先我们看看linux下标准i2c的结构。
在/linux-kernel-3.8/drivers/i2c目录下
----Algos/ 一些i2c总线适配器通信的算法，个人感觉是用I/O口模拟实现i2c通信的算法
----Busses/ I2C总线驱动的方法，对应于君正4775适配器驱动的文件是i2c-jz4775.c
----Chips/ I2C设备驱动，具体到某个设备，比如at24c08等
----I2c-boardinfo.c 定义i2c_register_board_info 注册i2c设备相关信息（其实就是将要注册的i2c设备添加到i2c设备链表）
----I2c-core.c I2C核心文件，用于联系设备驱动和总线驱动，作为一个桥梁，有用的函数i2c_add_addapter
i2c_add_driver，和i2c_transfer函数 （其实就是定义了一些标准的API接口函数）
----I2c-dev.c 通用的i2c设备驱动（其实就是生成的标准的/dev/i2c字符设备文件接口）
----Kconfig
----Makefile
我们要选择编译/linux-kernel-3.8/drivers/i2c/相关的platform driver和arch/mips/xburst/soc-4775/board//trunk/core/core-misc.c相结合最终生成的固件就会有/dev/i2c0标准接口。
以下是笔者做qn8025FM驱动时写下的通过标准i2c设备文件实现的读写函数：通过这两个读写函数就可以直接读写i2c设备的寄存器实现想用的功能
/*
* Version: V1.0.2.
* Date: 2014-11-25.
*/
#include "qn8025_i2c.h"
/*源头即是 IOCTL 呼叫, 通道为 I2C_SMBUS,
而 args 则定义了: 读或写, 位置, 长度, 以及回传值的资讯内容.*/
//ioctl(file, I2C_SMBUS, (i2c_smbus_ioctl_data*)msgset);
static inline __s32 i2c_smbus_access(int file, char read_write, __u8 command,
int size, union i2c_smbus_data *data)
{
struct i2c_smbus_ioctl_data args;
args.read_write = read_write;
args.command = command;
args.size = size;
args.data = data;
int ret= ioctl(file,I2C_SMBUS,&args);//set Smbus transfer
//printf("ioctl-read_i2c: %#x\n", args.data->byte);
return ret;
}
//向设备发送一个比特
static inline __s32 i2c_smbus_write_quick(int file, __u8 value)
{
return i2c_smbus_access(file,value,0,I2C_SMBUS_QUICK,(union i2c_smbus_data *)0);
}
static inline __s32 i2c_smbus_read_byte(int file)
{
union i2c_smbus_data data;
if (i2c_smbus_access(file,I2C_SMBUS_READ,0,I2C_SMBUS_BYTE,&data))
return -1;
else
return 0x0FF & data.byte;
}
static inline __s32 i2c_smbus_write_byte(int file, __u8 value)
{
return i2c_smbus_access(file,I2C_SMBUS_WRITE,value,
I2C_SMBUS_BYTE,(union i2c_smbus_data *)0);
}
static inline __s32 i2c_smbus_read_byte_data(int file, __u8 command)
{
union i2c_smbus_data data;
data.byte = 0;
// printf("before read_byte: %#x\n", data.byte);
if (i2c_smbus_access(file,I2C_SMBUS_READ,command,
I2C_SMBUS_BYTE_DATA,&data))
return -1;
else{
// printf("read_byte: %#x\n", data.byte);
return 0x0FF & data.byte;
}
}
static inline __s32 i2c_smbus_write_byte_data(int file, __u8 command,
__u8 value)
{
union i2c_smbus_data data;
data.byte = value;
// printf("before write_byte: %#x\n", data.byte);
int ret = i2c_smbus_access(file,I2C_SMBUS_WRITE,command,
I2C_SMBUS_BYTE_DATA, &data);
// printf("write_byte: %#x\n", data.byte);
return ret;
}
static inline __s32 i2c_smbus_read_word_data(int file, __u8 command)
{
union i2c_smbus_data data;
if (i2c_smbus_access(file,I2C_SMBUS_READ,command,
I2C_SMBUS_WORD_DATA,&data))
return -1;
else
return 0x0FFFF & data.word;
}
static inline __s32 i2c_smbus_write_word_data(int file, __u8 command,
__u16 value)
{
union i2c_smbus_data data;
data.word = value;
return i2c_smbus_access(file,I2C_SMBUS_WRITE,command,
I2C_SMBUS_WORD_DATA, &data);
}
static inline __s32 i2c_smbus_process_call(int file, __u8 command, __u16 value)
{
union i2c_smbus_data data;
data.word = value;
if (i2c_smbus_access(file,I2C_SMBUS_WRITE,command,
I2C_SMBUS_PROC_CALL,&data))
return -1;
else
return 0x0FFFF & data.word;
}
/* Returns the number of read bytes */
static inline __s32 i2c_smbus_read_block_data(int file, __u8 command,
__u8 *values)
{
union i2c_smbus_data data;
int i;
if (i2c_smbus_access(file,I2C_SMBUS_READ,command,
I2C_SMBUS_BLOCK_DATA,&data))
return -1;
else {
for (i = 1; i <= data.block[0]; i++)
values[i-1] = data.block[i];
return data.block[0];
}
}
static inline __s32 i2c_smbus_write_block_data(int file, __u8 command,
__u8 length, const __u8 *values)
{
union i2c_smbus_data data;
int i;
if (length > 32)
length = 32;
for (i = 1; i <= length; i++)
data.block[i] = values[i-1];
data.block[0] = length;
return i2c_smbus_access(file,I2C_SMBUS_WRITE,command,
I2C_SMBUS_BLOCK_DATA, &data);
}
/* Returns the number of read bytes */
/* Until kernel 2.6.22, the length is hardcoded to 32 bytes. If you
ask for less than 32 bytes, your code will only work with kernels
2.6.23 and later. */
static inline __s32 i2c_smbus_read_i2c_block_data(int file, __u8 command,
__u8 length, __u8 *values)
{
union i2c_smbus_data data;
int i;
if (length > 32)
length = 32;
data.block[0] = length;
if (i2c_smbus_access(file,I2C_SMBUS_READ,command,
length == 32 ? I2C_SMBUS_I2C_BLOCK_BROKEN :
I2C_SMBUS_I2C_BLOCK_DATA,&data))
return -1;
else {
for (i = 1; i <= data.block[0]; i++)
values[i-1] = data.block[i];
return data.block[0];
}
}
static inline __s32 i2c_smbus_write_i2c_block_data(int file, __u8 command,
__u8 length,
const __u8 *values)
{
union i2c_smbus_data data;
int i;
if (length > 32)
length = 32;
for (i = 1; i <= length; i++)
data.block[i] = values[i-1];
data.block[0] = length;
return i2c_smbus_access(file,I2C_SMBUS_WRITE,command,
I2C_SMBUS_I2C_BLOCK_BROKEN, &data);
}
/* Returns the number of read bytes */
static inline __s32 i2c_smbus_block_process_call(int file, __u8 command,
__u8 length, __u8 *values)
{
union i2c_smbus_data data;
int i;
if (length > 32)
length = 32;
for (i = 1; i <= length; i++)
data.block[i] = values[i-1];
data.block[0] = length;
if (i2c_smbus_access(file,I2C_SMBUS_WRITE,command,
I2C_SMBUS_BLOCK_PROC_CALL,&data))
return -1;
else {
for (i = 1; i <= data.block[0]; i++)
values[i-1] = data.block[i];
return data.block[0];
}
}
//检测i2c设备功能
/*static int check_funcs(int file, int size, int daddress, int pec)
{
unsigned long funcs;
//check adapter functionality
if (ioctl(file, I2C_FUNCS, &funcs) < 0) {//获取i2c设备支持的功能
fprintf(stderr, "Error: Could not get the adapter "
"functionality matrix: %s\n", strerror(errno));
return -1;
}
switch (size) {
case I2C_SMBUS_BYTE:
if (!(funcs & I2C_FUNC_SMBUS_READ_BYTE)) {
fprintf(stderr, MISSING_FUNC_FMT, "SMBus receive byte");
return -1;
}
if (daddress >= 0
&& !(funcs & I2C_FUNC_SMBUS_WRITE_BYTE)) {
fprintf(stderr, MISSING_FUNC_FMT, "SMBus send byte");
return -1;
}
break;
case I2C_SMBUS_BYTE_DATA:
if (!(funcs & I2C_FUNC_SMBUS_READ_BYTE_DATA)) {
fprintf(stderr, MISSING_FUNC_FMT, "SMBus read byte");
return -1;
}
break;
case I2C_SMBUS_WORD_DATA:
if (!(funcs & I2C_FUNC_SMBUS_READ_WORD_DATA)) {
fprintf(stderr, MISSING_FUNC_FMT, "SMBus read word");
return -1;
}
break;
}
if (pec
&& !(funcs & (I2C_FUNC_SMBUS_PEC | I2C_FUNC_I2C))) {
fprintf(stderr, "Warning: Adapter does "
"not seem to support PEC\n");
}
return 0;
}
*/
//设置i2c从设备地址
static int set_slave_addr(int file, int address, int force)
{
/* With force, let the user read from/write to the registers
even when a driver is also running */
if (ioctl(file, force ? I2C_SLAVE_FORCE : I2C_SLAVE, address) < 0) {
fprintf(stderr,
"Error: Could not set address to 0x%02x: %s\n",
address, strerror(errno));
return -errno;
}
return 0;
}
//打开i2c设备
static int open_i2c_dev(int i2cbus, char *filename, size_t size, int quiet)
{
int file;
snprintf(filename, size, "/dev/i2c/%d", i2cbus);
filename[size - 1] = ‘\0‘;
file = open(filename, O_RDWR);//打开i2c设备
if (file < 0 && (errno == ENOENT || errno == ENOTDIR)) {
sprintf(filename, "/dev/i2c-%d", i2cbus);
file = open(filename, O_RDWR);
}
if (file < 0 && !quiet) {
if (errno == ENOENT) {
fprintf(stderr, "Error: Could not open file "
"`/dev/i2c-%d‘ or `/dev/i2c/%d‘: %s\n",
i2cbus, i2cbus, strerror(ENOENT));
} else {
fprintf(stderr, "Error: Could not open file "
"`%s‘: %s\n", filename, strerror(errno));
if (errno == EACCES)
fprintf(stderr, "Run as root?\n");
}
}
return file;
}
//向i2c设备写数据 (0, 0x58, , 9300, 0)
static int i2c_write_data(int i2cbus,int addr,int daddr,int set_val,int vmask)
{
int res, file;
int value;
char filename[20];
//打开设备描述符，可以自动寻找合适的设备名
file = open_i2c_dev(i2cbus, filename, sizeof(filename), 0);
if (file < 0 || set_slave_addr(file, addr, 1)) {
// perror("open i2c");
printf("open %s fail\n", filename);
return -1;
}
//掩码，可以单独设置某一位而不影响其他位
if (vmask) {
int oldvalue;
oldvalue = i2c_smbus_read_byte_data(file, daddr);
printf("write and read\n");
if (oldvalue < 0) {
fprintf(stderr, "Error: Failed to read old value\n");
close(file);
return -1;
}
value = (set_val & vmask) | (oldvalue & ~vmask);
} else {
value = set_val;
}
res = i2c_smbus_write_byte_data(file, daddr, value);
if (res < 0) {
fprintf(stderr, "Error: Write failed\n");
close(file);
return -1;
}
//res = i2c_smbus_read_byte_data(file, daddr);
close(file);
//if (res < 0) {
// printf("Warning - readback failed\n");
//} else
//if (res != value) {
// printf("Warning - data mismatch - wrote 0x%0*x, read back 0x%0*x\n",2,value,2, res);
//} else {
//printf("Value 0x%0*x written, readback matched\n",2, value);
return 0;//读到的值和写入的值一致，才返回成功
//}
//return -1;
}
//从i2c读取数据
static int i2c_read_data(int i2cbus,int addr,int daddr,unsigned char *read_data)
{
int file,res;
char filename[20] = {0};
//打开i2c设备
file = open_i2c_dev(0, filename, sizeof(filename), 0);
if (file < 0 || set_slave_addr(file, addr, 1))//设置从设备地址
return -1;
res = i2c_smbus_read_byte_data(file, daddr);
close(file);
if (res < 0) {
fprintf(stderr, "Error: Read failed\n");
return -1;
}
*read_data = res;
//printf("read data -- 0x%x\n",*read_data);
return 0;
}
/* qn8025 set "set_value" to register "daddr"
addr: slave address
daddr: register address
set_val: the value wiil put into register
vmask: value maske
*/
int qn8025_i2c_write_data(int addr, int daddr, int set_val, int vmask)
{
return i2c_write_data(0, addr, daddr, set_val, vmask);
}
/* qn8025 get "read_data" from register "daddr"
addr: slave address
daddr: register address
set_val: the value wiil get from register
*/
int qn8025_i2c_read_data(int addr, int daddr, unsigned char *read_data)
{
return i2c_read_data(0, addr, daddr, read_data);
}