开始学习驱动的时候，是将驱动程序编译成模块然后用mknod命令手动建立设备节点以提供给应用程序调用。这对于刚开始调试驱动程序的时候常用的一种方法。但是，当有种需要必须在系统启动的时候就将驱动程序就绪，来供应用层程序调用。这时就不能再手动的建立设备节点了，而必须自动的创建设备节点（不需要人为的操作）。

★注册类

注册类的目的是为了使mdev可以在/dev/目录下建立设备节点。

首先要定义一个类，利用struct class结构体。这个结构体定义在头文件include/linux/device.h中

struct class {
	const char		* name;
	struct module		* owner;

	struct subsystem	subsys;
	struct list_head	children;
	struct list_head	devices;
	struct list_head	interfaces;
	struct semaphore	sem;	/* locks both the children and interfaces lists */

	struct kobject		*virtual_dir;

	struct class_attribute		* class_attrs;
	struct class_device_attribute	* class_dev_attrs;
	struct device_attribute		* dev_attrs;

	int	(*uevent)(struct class_device *dev, char **envp,
			   int num_envp, char *buffer, int buffer_size);
	int	(*dev_uevent)(struct device *dev, char **envp, int num_envp,
				char *buffer, int buffer_size);

	void	(*release)(struct class_device *dev);
	void	(*class_release)(struct class *class);
	void	(*dev_release)(struct device *dev);

	int	(*suspend)(struct device *, pm_message_t state);
	int	(*resume)(struct device *);
}

然后使用

完成对类的注册。其中第一个参数一般为：THIS_MODULE。第二个参数为：设备节点的名称

举个例子：

★创建设备节点

创建设备节点的函数：

<pre name="code" class="cpp">struct device *device_create(struct class *class, struct device *parent,dev_t devt, const char *fmt, ...)
{
	va_list args;
	struct device *dev = NULL;
	int retval = -ENODEV;

	if (class == NULL || IS_ERR(class))
		goto error;

	dev = kzalloc(sizeof(*dev), GFP_KERNEL);
	if (!dev) {
		retval = -ENOMEM;
		goto error;
	}

	dev->devt = devt;
	dev->class = class;
	dev->parent = parent;
	dev->release = device_create_release;

	va_start(args, fmt);
	vsnprintf(dev->bus_id, BUS_ID_SIZE, fmt, args);
	va_end(args);
	retval = device_register(dev);
	if (retval)
		goto error;

	return dev;

error:
	kfree(dev);
	return ERR_PTR(retval);
}

★销毁类和设备节点

注意不要忘记了还要销毁类和销毁设备节点。

销毁类：参数为用struct class结构体定义的变量

void class_destroy(struct class *cls)
{
	if ((cls == NULL) || (IS_ERR(cls)))
		return;

	class_unregister(cls);
}

void device_destroy(struct class *class, dev_t devt)
{
	struct device *dev = NULL;
	struct device *dev_tmp;

	down(&class->sem);
	list_for_each_entry(dev_tmp, &class->devices, node) {
		if (dev_tmp->devt == devt) {
			dev = dev_tmp;
			break;
		}
	}
	up(&class->sem);

	if (dev)
		device_unregister(dev);
}

★例子（自己写的延时驱动）

#include <linux/miscdevice.h>    
#include <linux/delay.h>    
#include <asm/irq.h>
#include <linux/kernel.h>    
#include <linux/module.h>
#include <linux/delay.h>
#include <linux/unistd.h>
#include <linux/string.h>
#include <linux/fcntl.h>
#include <asm/uaccess.h>
#include <linux/kdev_t.h>
#include <linux/fs.h>
#include <linux/init.h>
#include <linux/version.h>
#include <linux/config.h>
#include <linux/kernel.h>
#include <linux/interrupt.h>
#include <linux/sched.h>
#include <linux/device.h> 

#define NAME	"ralink_drive_delay"
#define RALINK_GPIO_DEVNAME "my_delay" 

#define delay_us 0     //most least is 10 US
#define delay_ms 1     //Ms

int delay_MAJOR = 109;
MODULE_LICENSE("Dual BSD/GPL");

static long Ralink_delay_ioctl(struct inode * inode, struct file * file, unsigned int cmd,unsigned long arg)
{
    
    switch(cmd)
    {
            case delay_us: 
                udelay(10 * arg);
                return 0;
            case delay_ms:
                udelay(1000);
                return 0;
            default:
                return -1;
    }
}

static struct file_operations My_delay_fops = 
{
    .owner = THIS_MODULE,
    .ioctl = Ralink_delay_ioctl,
};

static struct class *delay_class;

static int __init my_delay_init(void)
{
    int ret = 0;
    ret = register_chrdev(delay_MAJOR, RALINK_GPIO_DEVNAME,&My_delay_fops);
    if(ret < 0)
    {
        printk("unable to register character device\n");
        return ret;
    }
    if (delay_MAJOR == 0) 
    {
	    delay_MAJOR = ret;
	    printk(KERN_DEBUG NAME ": got dynamic major %d\n", ret);
    }
     
    //注册一个类，使mdev可以在"/dev/目录下建立设备节点"
    delay_class = class_create(THIS_MODULE, RALINK_GPIO_DEVNAME);
    if(IS_ERR(delay_class))
    {
        printk("failed in My_led class.\n");
        return -1;                       
    } 
    device_create(delay_class, NULL, MKDEV(delay_MAJOR, 0),RALINK_GPIO_DEVNAME 0);//
    //第一个参数是所要创建的设备所从属的类
    //第二个参数是这个设备的父节点，没有指定就是NULL 
    //第三个参数是设备号
    //第四个参数是设备名称
    //第五个参数是从设备号 
    printk("my_delay driver initialized\n");
    return 0;
}

void __exit my_delay_exit(void)
{
    unregister_chrdev(delay_MAJOR,RALINK_GPIO_DEVNAME);
    device_destroy(delay_class,MKDEV(delay_MAJOR,0));//注销设备节点 
    class_destroy(delay_class);//销毁类 
    printk("my_delay driver exited\n");
    
}

module_init(my_delay_init);
module_exit(my_delay_exit);