类子系统是向用户空间导出信息的最好方法，
cjjust@cjjust-laptop:/sys/class/i2c-adapter$ ls
i2c-0 i2c-1 i2c-2 i2c-3 i2c-4 i2c-5
cjjust@cjjust-laptop:/sys/class/i2c-adapter$ ls -l
total 0
lrwxrwxrwx 1 root root 0 2010-09-14 08:46 i2c-0 -> http://www.cnblogs.com/devices/pci0000:00/0000:00:01.0/0000:01:00.0/i2c-0
lrwxrwxrwx 1 root root 0 2010-09-14 08:46 i2c-1 -> http://www.cnblogs.com/devices/pci0000:00/0000:00:01.0/0000:01:00.0/i2c-1
lrwxrwxrwx 1 root root 0 2010-09-14 08:46 i2c-2 -> http://www.cnblogs.com/devices/pci0000:00/0000:00:01.0/0000:01:00.0/i2c-2
lrwxrwxrwx 1 root root 0 2010-09-14 08:46 i2c-3 -> http://www.cnblogs.com/devices/pci0000:00/0000:00:01.0/0000:01:00.0/i2c-3
lrwxrwxrwx 1 root root 0 2010-09-14 08:46 i2c-4 -> http://www.cnblogs.com/devices/pci0000:00/0000:00:01.0/0000:01:00.0/i2c-4
lrwxrwxrwx 1 root root 0 2010-09-14 08:46 i2c-5 -> http://www.cnblogs.com/devices/pci0000:00/0000:00:01.0/0000:01:00.0/i2c-5
你希望看到的肯定是上面这部分的信息，而不是 sys/devices/pci0000:00/0000:00:01.0/0000:01:00.0/i2c-5之类的信息。
类有两个接口
class_simple接口和完整的类接口
struct class_simple *class_simple_create
struct class_device *class_simple_device_add
int class_simple_set_hotplug(struct class_simple *cs, int (*hotplug)(...));
完整类接口
struct class{
char *name;
struct class_attribute*
struct class_device_attribute*
int (*hotplug)
void (*release)
void (*class_release)
};
类设备：
struct class_device{
struct kobject kobj;
struct class *class;
struct device *dev;
void *class_data;
char class_id[BUS_ID_SIZE]
};
类接口:把一个类添加到类接口包含的类中，会调用add函数。
struct class_interface{
struct class *class;
int (*add) (struct class_device* cd);
void (*remove) (struct class_device *cd);
};
