uevent

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：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

leds?[123]? 0:0 777

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

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

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

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