i2c的操作在内核中是当做字符设备来操作的，相关初始化在由i2c_dev_init函数来初始化。
并且i2c adapter的驱动通过i2cdev_driver这个通用驱动的attach方法来实现注册的。
下面具体分析整个过程。
1.i2c_dev_init函数
static int __init i2c_dev_init(void)
{。。。 。。。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);。。。 。。。
}函数首先调用register_chardev函数向内核注册主备号为I2C_MAJOR、操作集为i2cdev_fops的字符设备。
register_chrdev函数最终会向系统注册主设备为I2C_MAJOR，此设备号为0~255的设备。这表示系统最多
可以容纳256个i2c adapter，adapter的字符操作方法i2cdev_fops如下：
static const struct file_operations i2cdev_fops = {.owner = THIS_MODULE,.llseek = no_llseek,.read = i2cdev_read,.write = i2cdev_write,.unlocked_ioctl = i2cdev_ioctl,.open = i2cdev_open,.release = i2cdev_release,
};当read()、write()、open()、close()、ioctl()等系统调用发生时就会调用到这些函数。
但是i2cdev_fops其实是通用的的操作，应为不同的adapter对应的操作方法肯定有区别
所以这里的fops只是具体adapter操作方法的一层外壳，具体稍后分析。
字符设备注册完毕后通过class_create()函数初始化一个类i2c_dev_class，这个类稍后需要使用，用于在
/dev/i2c-0下自动创建设备，后面分析。
类初始化完毕后，然后调用函数i2c_add_driver函数注册i2c driver。这里所说的i2c其实对应的是系统中所有的i2c类设备。
通过i2c driver中的attach_adapter方法来实现将adapter和对应的驱动绑定。
static struct i2c_driver i2cdev_driver = {.driver = {.name = "dev_driver",},.attach_adapter = i2cdev_attach_adapter,.detach_adapter = i2cdev_detach_adapter,
};
下面具体分析i2c_add_driver注册i2cdev_driver的过程
2.i2c_add_driver
i2c_add_driver函数只是对i2c_register_driver做了简单的封装，下面直接分析i2c_register_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))){printk("Can't register until after driver model init\n");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;/* When registration returns, the driver core* will have called probe() for all matching-but-unbound devices.*/res = driver_register(&driver->driver);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, __process_new_driver);mutex_unlock(&core_lock);return 0;
}函数通过
driver->driver.bus = &i2c_bus_type;值得一提的是此处i2cdev_driver中的attach_adapter的执行机会很大，
通过bus_for_each_dev()函数
bus_for_each_dev(&i2c_bus_type, NULL, driver, __process_new_driver);此处的bus_for_each_dev函数主要功能就是循环查询一遍i2c总线上所有的dev，包括adapter device和client device。
然后依次将dev和driver作为__process_new_driver的参数并执行__process_new_driver函数，但是只有adapter device
才会执行后续的操作，否则返回继续轮询i2c总线上的dev。
static int __process_new_driver(struct device *dev, void *data)
{if (dev->type != &i2c_adapter_type)return 0;return i2c_do_add_adapter(data, to_i2c_adapter(dev));
}
可以发现__process_new_driver函数首先判断的是dev的类型，假如是adapter类型才会继续执行
后面的代码，假如不是则立即返回继续摘取下个dev然后循环执行__process_new_driver。因为对
我们来说，我们只需要注册adapter的驱动就可以了，i2c的所有操作是通过主机来完成的，从机只是
被动接受。由于之前已经通过i2cadd_numbered_adapter()注册过adapter到总线i2c_bus_type，
所以此处有机会执行后面的i2c_do_add_adapter函数。
static int i2c_do_add_adapter(struct i2c_driver *driver,struct i2c_adapter *adap)
{/* Detect supported devices on that bus, and instantiate them */i2c_detect(adap, driver);/* Let legacy drivers scan this bus for matching devices */if (driver->attach_adapter) {/* We ignore the return code; if it fails, too bad */driver->attach_adapter(adap);}return 0;
}可以发现在i2c_do_add_adapter函数主要执行的是i2c_dectect和driver->attach_adapter。
由于此处驱动并未初始化driver->detect，所以i2c_detect函数未执行有效操作就会退出。
接着通过传统方式执行driver->attach_adapter方法。
tatic int i2cdev_attach_adapter(struct i2c_adapter *adap)
{struct i2c_dev *i2c_dev;int res;i2c_dev = get_free_i2c_dev(adap);if (IS_ERR(i2c_dev))return PTR_ERR(i2c_dev);/* register this i2c device with the driver core */i2c_dev->dev = device_create(i2c_dev_class, &adap->dev,MKDEV(I2C_MAJOR, adap->nr), NULL,"i2c-%d", adap->nr);if (IS_ERR(i2c_dev->dev)) {res = PTR_ERR(i2c_dev->dev);goto error;}res = device_create_file(i2c_dev->dev, &dev_attr_name);if (res)goto error_destroy;pr_debug("i2c-dev: adapter [%s] registered as minor %d\n",adap->name, adap->nr);return 0;
error_destroy:device_destroy(i2c_dev_class, MKDEV(I2C_MAJOR, adap->nr));
error:return_i2c_dev(i2c_dev);return res;
}可见attach_adapter函数的作用就是调用device_create()函数 通过之前class_create的类信息在/dev下自动创建设备文件。
并且此设备的设备号是由固定的主设备号I2C_MAJOR 和 从设备号组成的，从设备号取的就是adapter的nr，此处为0。
并且可以推断出系统最多可以容纳0~255 总共256个i2c adapter。
到此i2c部分的初始化就完成了，可以通过read write来操作设备了。
补充：上面说的新方法好像在驱动里面就会detect client，然后把所有检测到的client放到一条链表里。