驱动模块使用I2C总线范例

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, <[email protected]>");
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。