class_device_create
	class_device_register
		class_device_add
			kobject_uevent(&class_dev->kobj, KOBJ_ADD);
				kobject_uevent_env(kobj, action, NULL);

					// action_string = "add";
					action_string = action_to_string(action);

					/* 分配保存环境变量的内存 */
					/* environment values */
					buffer = kmalloc(BUFFER_SIZE, GFP_KERNEL);
					
					/* 设置环境变量 */
					envp [i++] = scratch;
					scratch += sprintf(scratch, "ACTION=%s", action_string) + 1;
					envp [i++] = scratch;
					scratch += sprintf (scratch, "DEVPATH=%s", devpath) + 1;
					envp [i++] = scratch;
					scratch += sprintf(scratch, "SUBSYSTEM=%s", subsystem) + 1;
					
					/* 调用应用程序: 比如mdev */
					/* 启动脚本 echo /sbin/mdev > /proc/sys/kernel/hotplug 
					 * 设置了uevent_helper为“/sbin/mdev“
					 */
					
					argv [0] = uevent_helper;  // = "/sbin/mdev"
					argv [1] = (char *)subsystem;
					argv [2] = NULL;
					call_usermodehelper (argv[0], argv, envp, 0);

分析: busybox mdev.c  

100ask: uevent_helper = /sbin/mdev
envp[0] = HOME=/
envp[1] = PATH=/sbin:/bin:/usr/sbin:/usr/bin
envp[2] = ACTION=add
envp[3] = DEVPATH=/class/sixth_drv/buttons
envp[4] = SUBSYSTEM=sixth_drv
envp[5] = SEQNUM=720
envp[6] = MAJOR=252
envp[7] = MINOR=0

mdev_main
	temp = /sys/class/sixth_drv/buttons
	make_device(temp, 0);
		/* 确定设备文件名,类型,主次设备号 */
		device_name = bb_basename(path);  = "buttons"
		
		
		'c' == > 字符设备节点
		根据"/sys/class/sixth_drv/buttons/dev"的内容确定主次设备号
		
		mknod(device_name, mode | type, makedev(major, minor)

我接上U盘，想自动挂载，怎么办？
mdev.conf的格式：
<device regex> <uid>:<gid> <octal permissions> [<@|$|*> <command>]

device regex：正则表达式，表示哪一个设备
uid: owner
gid: 组ID
octal permissions：以八进制表示的属性
@：创建设备节点之后执行命令
$：删除设备节点之前执行命令
*: 创建设备节点之后 和 删除设备节点之前 执行命令
command：要执行的命令

写mdev.conf
1.
leds 0:0 777
led1 0:0 777
led2 0:0 777
led3 0:0 777

2.
leds?[123]? 0:0 777

3.
leds?[123]? 0:0 777 @ echo create /dev/$MDEV > /dev/console

4.
leds?[123]? 0:0 777 * if [ $ACTION = "add" ]; then echo create /dev/$MDEV > /dev/console; else echo remove /dev/$MDEV > /dev/console; fi

5. 
leds?[123]? 0:0 777 * /bin/add_remove_led.sh

把命令写入一个脚本：
add_remove_led.sh

#!/bin/sh
if [ $ACTION = "add" ]; 
then 
	echo create /dev/$MDEV > /dev/console; 
else 
	echo remove /dev/$MDEV > /dev/console; 
fi

6. U盘自动加载
sda[1-9]+ 0:0 777 * if [ $ACTION = "add" ]; then mount /dev/$MDEV /mnt; else umount /mnt; fi

7.
sda[1-9]+ 0:0 777 * /bin/add_remove_udisk.sh

add_remove_udisk.sh
#!/bin/sh
if [ $ACTION = "add" ]; 
then 
	mount /dev/$MDEV /mnt; 
else 
	umount /mnt; 
fi