在嵌入式中，I2C接口的意义非常重大，许多外围芯片控制接口都采用I2C。因此，了解在驱动模块中如何进行I2C总线通信是很有必要的。我们先看看I2C驱动代码的树形结构：
. |-- Kconfig |-- Makefile |-- algos | |-- Kconfig | |-- Makefile | |-- i2c-algo-bit.c | |-- i2c-algo-pca.c | |-- i2c-algo-pcf.c | `-- i2c-algo-pcf.h |-- busses | |-- Kconfig | |-- Makefile | |-- i2c-acorn.c | |-- ... | |-- i2c-gpio.c | |-- ... | |-- i2c-pxa.c | |-- i2c-s3c2410.c | `-- ... |-- chips | |-- Kconfig | |-- Makefile | |-- ds1682.c | |-- pca963x.c | `-- tsl2550.c |-- i2c-boardinfo.c |-- i2c-core.c |-- i2c-core.h `-- i2c-dev.c 3 directories, 86 files
其中i2c-core.c是I2C核心部分，提供接口函数，i2c_adapter和i2c_driver在模块初始化时分别调用i2c_add_numbered_adapter()和i2c_add_driver()将其注册到I2C Core中。
i2c-dev.c是I2C设备部分，实现字符设备访问接口，对硬件的具体访问是通过i2c_adapter来实现的。它在初始化时，需要向I2C Core注册一个i2c_driver。我认为这部分仅仅是让用户层访问I2C总线的，如果内核态访问I2C总线的话，则没有必要理会它；但是我们需要参考它来实现我们自己的i2c_driver，并且需要将这个i2c_driver注册到I2C Core。
busses目录下有许多个文件，为各种平台的I2C总线驱动，如s2c2410、pxa、mpc等等，跟硬件spec息息相关。一个总线驱动需要两个模块，用结构体i2c_adapter和i2c_algorithm来描述。
algos目录是对bus中的i2c_algorithm的补充，特别的i2c_algorithm可在这里实现。chips是I2C Slave芯片驱动代码，如I2C控制接口的eeprom、LED段码显示芯片等。
更详细的有关I2C驱动架构介绍的文章见：http://blog.chinaunix.net/space.php?uid=11134731&do=blog&id=33193
下面是正题，附上示范代码，碰到相关场合，直接将其复制过去使用即可。
#define AZURE_I2C_ADDR 0x1a
#define AZURE_I2C_BUS_NO 0
static struct i2c_client *azure_i2c;
static int azure_i2c_probe(struct i2c_client *i2c,
const struct i2c_device_id *id)
{
azure_i2c = i2c;
i2c->addr = AZURE_I2C_ADDR;
return 0;
}
static int azure_i2c_remove(struct i2c_client *client)
{
return 0;
}
static const struct i2c_device_id azure_i2c_id[] = {
{ "azure", 0 },
{ }
};
MODULE_DEVICE_TABLE(i2c, azure_i2c_id);
static struct i2c_driver azure_i2c_driver = {
.driver = {
.name = "AZURE I2C",
.owner = THIS_MODULE,
},
.probe = azure_i2c_probe,
.remove = azure_i2c_remove,
.id_table = azure_i2c_id,
};
static int azure_add_i2c_device()
{
struct i2c_board_info info;
struct i2c_adapter *adapter;
struct i2c_client *client;
int ret;
memset(&info, 0, sizeof(struct i2c_board_info));
info.addr = AZURE_I2C_ADDR;
strlcpy(info.type, "azure", I2C_NAME_SIZE);
adapter = i2c_get_adapter(AZURE_I2C_BUS_NO);
if (!adapter) {
printk(KERN_ERR "can't get i2c adapter %d/n", AZURE_I2C_BUS_NO);
goto err_driver;
}
client = i2c_new_device(adapter, &info);
i2c_put_adapter(adapter);
if (!client) {
printk(KERN_ERR "can't add i2c device at 0x%x/n", (unsigned int)info.addr);
goto err_driver;
}
ret = i2c_add_driver(&azure_i2c_driver);
if (ret != 0) {
printk(KERN_ERR "can't add i2c driver/n");
return ret;
}
return 0;
err_driver:
i2c_del_driver(&azure_i2c_driver);
return -ENODEV;
}
然后在模块初始化的时候调用azure_add_i2c_device()即将一个i2c_driver注册到I2C Core，之后便可调用i2c_master_recv()和i2c_master_send()进行I2C总线读写操作。i2c_master_send()/i2c_master_recv()第一个参数client由azure_i2c传入。
以上并不是聪明的做法。如果有N个模块要用到I2C，那么在N个模块就要添加基本一样的代码。更好的解决方法是为内核调用I2C写一个独立的模块，EXPORT_SYMBOL读写接口函数，在其他模块里就只需要调用这些读写接口函数就可以了。如下是模块代码：
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/init.h>
#include <linux/errno.h>
#include <linux/init.h>
#include <linux/list.h>
#include <linux/i2c.h>
#include <linux/smp_lock.h>
#define I2C_API_FAKE_ADDR 0x7f
#define I2C_MINORS 256
int i2c_api_attach(struct i2c_adapter *adapter);
int i2c_api_detach(struct i2c_adapter *adapter);
struct i2c_api {
struct list_head list;
struct i2c_client *client;
};
static LIST_HEAD(i2c_api_list);
static DEFINE_SPINLOCK(i2c_api_list_lock);
static const unsigned short normal_addr[] = { I2C_API_FAKE_ADDR, I2C_CLIENT_END };
static const unsigned short ignore[] = { I2C_CLIENT_END };
static struct i2c_client_address_data addr_data =
{
.normal_i2c = normal_addr,
.probe = ignore,
.ignore = ignore,
.forces = NULL,
};
static const struct i2c_device_id id[] = {
{"I2C-API", 0},
{}
};
MODULE_DEVICE_TABLE(i2c, id);
static struct i2c_driver i2c_api_driver = {
.id_table = id,
.attach_adapter = i2c_api_attach,
.detach_adapter = i2c_api_detach,
.command = NULL,
.driver = {
.name = "I2C-API",
.owner = THIS_MODULE,
},
.address_data = &addr_data,
};
static struct i2c_api *get_i2c_api(int bus_id)
{
struct i2c_api *i2c_api;
spin_lock(&i2c_api_list_lock);
list_for_each_entry(i2c_api, &i2c_api_list, list) {
if (i2c_api->client->adapter->nr == bus_id)
goto found;
}
i2c_api = NULL;
found:
spin_unlock(&i2c_api_list_lock);
return i2c_api;
}
static struct i2c_api *add_i2c_api(struct i2c_client *client)
{
struct i2c_api *i2c_api;
if (client->adapter->nr >= I2C_MINORS) {
printk(KERN_ERR "i2c_api: Out of device minors (%d)/n",
client->adapter->nr);
return NULL;
}
i2c_api = kzalloc(sizeof(*i2c_api), GFP_KERNEL);
if (!i2c_api)
return NULL;
i2c_api->client = client;
spin_lock(&i2c_api_list_lock);
list_add_tail(&i2c_api->list, &i2c_api_list);
spin_unlock(&i2c_api_list_lock);
return i2c_api;
}
static void del_i2c_api(struct i2c_api *i2c_api)
{
spin_lock(&i2c_api_list_lock);
list_del(&i2c_api->list);
spin_unlock(&i2c_api_list_lock);
kfree(i2c_api);
}
static int i2c_api_do_xfer(int bus_id, char chip_addr, char sub_addr, int mode,
char *buf, unsigned int size)
{
/** you could define more transfer mode here, implement it following. */
#define I2C_API_XFER_MODE_SEND 0x0 /* standard send */
#define I2C_API_XFER_MODE_RECV 0x1 /* standard receive */
#define I2C_API_XFER_MODE_SEND_NO_SUBADDR 0x2 /* send without sub-address */
#define I2C_API_XFER_MODE_RECV_NO_SUBADDR 0x3 /* receive without sub-address */
int ret = 0;
char *tmp;
struct i2c_api *i2c_api = get_i2c_api(bus_id);
if (!i2c_api)
return -ENODEV;
i2c_api->client->addr = chip_addr;
switch (mode) {
case I2C_API_XFER_MODE_SEND:
tmp = kmalloc(size + 1,GFP_KERNEL);
if (tmp == NULL)
return -ENOMEM;
tmp[0] = sub_addr;
memcpy(&tmp[1], buf, size);
ret = i2c_master_send(i2c_api->client, tmp, size + 1);
ret = (ret == size + 1) ? size : ret;
break;
case I2C_API_XFER_MODE_RECV:
ret = i2c_master_send(i2c_api->client, &sub_addr, 1);
if (ret < 0)
return ret;
ret = i2c_master_recv(i2c_api->client, buf, size);
break;
case I2C_API_XFER_MODE_SEND_NO_SUBADDR:
ret = i2c_master_send(i2c_api->client, buf, size);
break;
case I2C_API_XFER_MODE_RECV_NO_SUBADDR:
ret = i2c_master_recv(i2c_api->client, buf, size);
break;
default:
return -EINVAL;
}
return ret;
}
int i2c_api_do_send(int bus_id, char chip_addr, char sub_addr, char *buf, unsigned int size)
{
return i2c_api_do_xfer(bus_id, chip_addr, sub_addr, I2C_API_XFER_MODE_SEND, buf, size);
}
int i2c_api_do_recv(int bus_id, char chip_addr, char sub_addr, char *buf, unsigned int size)
{
return i2c_api_do_xfer(bus_id, chip_addr, sub_addr, I2C_API_XFER_MODE_RECV, buf, size);
}
int i2c_api_attach(struct i2c_adapter *adap)
{
struct i2c_board_info info;
struct i2c_client *client;
memset(&info, 0, sizeof(struct i2c_board_info));
strlcpy(info.type, "i2c_api", I2C_NAME_SIZE);
info.addr = I2C_API_FAKE_ADDR;
client = i2c_new_device(adap, &info);
if (client)
add_i2c_api(client);
printk(KERN_INFO "i2c_api_attach adap[%d]/n", adap->nr);
return 0;
}
int i2c_api_detach(struct i2c_adapter *adap)
{
struct i2c_api *i2c_api;
i2c_api = get_i2c_api(adap->nr);
if (i2c_api)
del_i2c_api(i2c_api);
return 0;
}
static int __init i2c_api_init(void)
{
int ret = i2c_add_driver(&i2c_api_driver);
if (ret) {
printk(KERN_ERR "[%s] Driver registration failed, module not inserted./n", __func__);
return ret;
}
return 0 ;
}
static void __exit i2c_api_exit(void)
{
i2c_del_driver(&i2c_api_driver);
}
MODULE_AUTHOR("Loon, <sepnic@gmail.com>");
MODULE_DESCRIPTION("I2C i2c_api Driver");
MODULE_LICENSE("GPL");
module_init(i2c_api_init);
module_exit(i2c_api_exit);
EXPORT_SYMBOL_GPL(i2c_api_do_send);
EXPORT_SYMBOL_GPL(i2c_api_do_recv);
内核版本是2.6.32，i2c_driver结构体成员attach_adapter和detach_adapter可能以后有增删，届时需要对模块里的i2c_api_attach()和i2c_api_detach()做调整。
加载该模块后，其他模块即可调用i2c_api_do_recv()/i2c_api_do_send()读写I2C总线。