接下来以一个实际的例子来看I2C设备驱动，就以drivers/i2c/i2c-dev.c为例。

先看它的初始化和注销函数

static int __init i2c_dev_init(void)
{
	int res;

	printk(KERN_INFO "i2c /dev entries driver\n");

	res = register_chrdev(I2C_MAJOR, "i2c", &i2cdev_fops);
	if (res)
		goto out;

	i2c_dev_class = class_create(THIS_MODULE, "i2c-dev");
	if (IS_ERR(i2c_dev_class)) {
		res = PTR_ERR(i2c_dev_class);
		goto out_unreg_chrdev;
	}

	res = i2c_add_driver(&i2cdev_driver);
	if (res)
		goto out_unreg_class;

	return 0;

out_unreg_class:
	class_destroy(i2c_dev_class);
out_unreg_chrdev:
	unregister_chrdev(I2C_MAJOR, "i2c");
out:
	printk(KERN_ERR "%s: Driver Initialisation failed\n", __FILE__);
	return res;
}

static void __exit i2c_dev_exit(void)
{
	i2c_del_driver(&i2cdev_driver);
	class_destroy(i2c_dev_class);
	unregister_chrdev(I2C_MAJOR,"i2c");
}

首先调用register_chrdev注册了一个字符设备，这是老的字符驱动注册方式。然后到了接下来的主角，i2c_add_driver，在I2C子系统中，I2C设备驱动就是采用这个函数注册，注销一个I2C设备驱动使用下面的i2c_del_driver函数，那就具体看看这个I2C设备驱动注册函数。

static inline int i2c_add_driver(struct i2c_driver *driver)
{
	return i2c_register_driver(THIS_MODULE, driver);
}
int i2c_register_driver(struct module *owner, struct i2c_driver *driver)
{
	int res;

	/* Can't register until after driver model init */
	if (unlikely(WARN_ON(!i2c_bus_type.p)))
		return -EAGAIN;

	/* add the driver to the list of i2c drivers in the driver core */
	driver->driver.owner = owner;
	driver->driver.bus = &i2c_bus_type; /*指定驱动的总线类型*/

	/* When registration returns, the driver core
	 * will have called probe() for all matching-but-unbound devices.
	 */
	res = driver_register(&driver->driver); /*注册驱动*/
	if (res)
		return res;

	pr_debug("i2c-core: driver [%s] registered\n", driver->driver.name);

	INIT_LIST_HEAD(&driver->clients);
	/* Walk the adapters that are already present */
	mutex_lock(&core_lock);
	bus_for_each_dev(&i2c_bus_type, NULL, driver, __attach_adapter);
	mutex_unlock(&core_lock);

	return 0;
}

再来看看i2c设备驱动注销函数

void i2c_del_driver(struct i2c_driver *driver)
{
	mutex_lock(&core_lock);
	bus_for_each_dev(&i2c_bus_type, NULL, driver, __detach_adapter);
	mutex_unlock(&core_lock);

	driver_unregister(&driver->driver);
	pr_debug("i2c-core: driver [%s] unregistered\n", driver->driver.name);
}

也没什么，最后调用的就是驱动的注销函数driver_unregister函数。

来看传递给注册和注销i2c驱动函数的参数什么，i2cdev_driver它是structi2c_driver结构类型，i2c设备驱动就是使用这个结构类型描述，这个结构类型定义在include/linux/i2c.h

struct i2c_driver {
	unsigned int class;

	/* Notifies the driver that a new bus has appeared or is about to be
	 * removed. You should avoid using this if you can, it will probably
	 * be removed in a near future.
	 */
	int (*attach_adapter)(struct i2c_adapter *);
	int (*detach_adapter)(struct i2c_adapter *);

	/* Standard driver model interfaces */
	int (*probe)(struct i2c_client *, const struct i2c_device_id *);
	int (*remove)(struct i2c_client *);

	/* driver model interfaces that don't relate to enumeration  */
	void (*shutdown)(struct i2c_client *);
	int (*suspend)(struct i2c_client *, pm_message_t mesg);
	int (*resume)(struct i2c_client *);

	/* a ioctl like command that can be used to perform specific functions
	 * with the device.
	 */
	int (*command)(struct i2c_client *client, unsigned int cmd, void *arg);

	struct device_driver driver;
	const struct i2c_device_id *id_table;

	/* Device detection callback for automatic device creation */
	int (*detect)(struct i2c_client *, int kind, struct i2c_board_info *);
	const struct i2c_client_address_data *address_data;
	struct list_head clients;
};

来看i2c-dev.c中是怎么定义的

static struct i2c_driver i2cdev_driver = {
	.driver = {
		.name	= "dev_driver",
	},
	.attach_adapter	= i2cdev_attach_adapter,
	.detach_adapter	= i2cdev_detach_adapter,
};

这是老的方式，所以它只是给attach_adapter和detach_adapter赋了值，由于这里是老的方式，所以我们也就不去具体看这个函数了，我们直接去看它的数据传输部分吧。

static ssize_t i2cdev_read (struct file *file, char __user *buf, size_t count,
                            loff_t *offset)
{
	char *tmp;
	int ret;

	struct i2c_client *client = (struct i2c_client *)file->private_data;

	if (count > 8192)
		count = 8192;

	tmp = kmalloc(count,GFP_KERNEL);
	if (tmp==NULL)
		return -ENOMEM;

	pr_debug("i2c-dev: i2c-%d reading %zu bytes.\n",
		iminor(file->f_path.dentry->d_inode), count);

	ret = i2c_master_recv(client,tmp,count);
	if (ret >= 0)
		ret = copy_to_user(buf,tmp,count)?-EFAULT:ret;
	kfree(tmp);
	return ret;
}

这是i2c设备读函数，我们看它是调用的i2c_master_recv函数去操作的，去看这个函数

int i2c_master_recv(struct i2c_client *client, char *buf ,int count)
{
	struct i2c_adapter *adap=client->adapter;
	struct i2c_msg msg;
	int ret;

	msg.addr = client->addr;
	msg.flags = client->flags & I2C_M_TEN;
	msg.flags |= I2C_M_RD;
	msg.len = count;
	msg.buf = buf;

	ret = i2c_transfer(adap, &msg, 1);

	/* If everything went ok (i.e. 1 msg transmitted), return #bytes
	   transmitted, else error code. */
	return (ret == 1) ? count : ret;
}

i2c设备写函数

static ssize_t i2cdev_write (struct file *file, const char __user *buf, size_t count,
                             loff_t *offset)
{
	int ret;
	char *tmp;
	struct i2c_client *client = (struct i2c_client *)file->private_data;

	if (count > 8192)
		count = 8192;

	tmp = kmalloc(count,GFP_KERNEL);
	if (tmp==NULL)
		return -ENOMEM;
	if (copy_from_user(tmp,buf,count)) {
		kfree(tmp);
		return -EFAULT;
	}

	pr_debug("i2c-dev: i2c-%d writing %zu bytes.\n",
		iminor(file->f_path.dentry->d_inode), count);

	ret = i2c_master_send(client,tmp,count);
	kfree(tmp);
	return ret;
}
int i2c_master_send(struct i2c_client *client,const char *buf ,int count)
{
	int ret;
	struct i2c_adapter *adap=client->adapter;
	struct i2c_msg msg;

	msg.addr = client->addr;
	msg.flags = client->flags & I2C_M_TEN;
	msg.len = count;
	msg.buf = (char *)buf;

	ret = i2c_transfer(adap, &msg, 1);

	/* If everything went ok (i.e. 1 msg transmitted), return #bytes
	   transmitted, else error code. */
	return (ret == 1) ? count : ret;
}

这两个函数最终都是调用的i2c_transfer函数去完成数据的传输，只是他们的msg的flags不一样，读操作的flags要加上I2C_M_RD这个标志。

再看它们两个共同的i2c_transfer函数

int i2c_transfer(struct i2c_adapter *adap, struct i2c_msg *msgs, int num)
{
	unsigned long orig_jiffies;
	int ret, try;

	/* REVISIT the fault reporting model here is weak:
	 *
	 *  - When we get an error after receiving N bytes from a slave,
	 *    there is no way to report "N".
	 *
	 *  - When we get a NAK after transmitting N bytes to a slave,
	 *    there is no way to report "N" ... or to let the master
	 *    continue executing the rest of this combined message, if
	 *    that's the appropriate response.
	 *
	 *  - When for example "num" is two and we successfully complete
	 *    the first message but get an error part way through the
	 *    second, it's unclear whether that should be reported as
	 *    one (discarding status on the second message) or errno
	 *    (discarding status on the first one).
	 */

	if (adap->algo->master_xfer) {
#ifdef DEBUG
		for (ret = 0; ret < num; ret++) {
			dev_dbg(&adap->dev, "master_xfer[%d] %c, addr=0x%02x, "
				"len=%d%s\n", ret, (msgs[ret].flags & I2C_M_RD)
				? 'R' : 'W', msgs[ret].addr, msgs[ret].len,
				(msgs[ret].flags & I2C_M_RECV_LEN) ? "+" : "");
		}
#endif

		if (in_atomic() || irqs_disabled()) {
			ret = mutex_trylock(&adap->bus_lock);
			if (!ret)
				/* I2C activity is ongoing. */
				return -EAGAIN;
		} else {
			mutex_lock_nested(&adap->bus_lock, adap->level);
		}

		/* Retry automatically on arbitration loss */
		orig_jiffies = jiffies;
		for (ret = 0, try = 0; try <= adap->retries; try++) {
			ret = adap->algo->master_xfer(adap, msgs, num);
			if (ret != -EAGAIN)
				break;
			if (time_after(jiffies, orig_jiffies + adap->timeout))
				break;
		}
		mutex_unlock(&adap->bus_lock);

		return ret;
	} else {
		dev_dbg(&adap->dev, "I2C level transfers not supported\n");
		return -EOPNOTSUPP;
	}
}

我们看就是调用总线的master_xfer方法，我们在前面分析使用gpio模拟i2c总线时，看过这样一句 .master_xfer =bit_xfer, ，所以最终调用的是这个函数来完成数据传输。使用i2c_master_recv和i2c_master_send函数一次只能传输一个msg，由于它一次只能传输一个msg，所以它的传输方向不能改变，也就是一次只能完成读或写操作，并且读操作时还不能传递设备的基地址，所以通常是不会用这两个函数的，直接的做法时，构造两个msg，一个msg的数据为操作设备基地址，另外一个msg才是我们真正要读写的数据，最后调用i2c_transfer函数去完成数据的传送。