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


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


mknod(device_name, mode | type, makedev(major, minor)



: [ ]

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 和 删除设备节点之前 执行命令


写mdev.conf放在/etc/目下,sh脚本文件放在bin目录下,device regex用正则表达式




leds 0:0 777

led1 0:0 777

led2 0:0 777

led3 0:0 777

dma 0:0 777


leds?[123]? 0:0 777

/*For your convenience, the shell env var $MDEV is set to the device name. So if

the device "hdc" was matched, MDEV would be set to "hdc".*/


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/



if [ $ACTION = "add" ];


echo create /dev/$MDEV > /dev/console;


echo remove /dev/$MDEV > /dev/console;


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/


if [ $ACTION = "add" ];


mount /dev/$MDEV /tmp;


umount /tmp;




MDEV Primer


For those of us who know how to use mdev, a primer might seem lame. For

everyone else, mdev is a weird black box that they hear is awesome, but can't

seem to get their head around how it works. Thus, a primer.


Basic Use


Mdev has two primary uses: initial population and dynamic updates. Both

require sysfs support in the kernel and have it mounted at /sys. For dynamic

updates, you also need to have hotplugging enabled in your kernel.

Here's a typical code snippet from the init script:

[0] mount -t proc proc /proc

[1] mount -t sysfs sysfs /sys

[2] echo /sbin/mdev > /proc/sys/kernel/hotplug

[3] mdev -s

Alternatively, without procfs the above becomes:

[1] mount -t sysfs sysfs /sys

[2] sysctl -w kernel.hotplug=/sbin/mdev

[3] mdev -s

Of course, a more "full" setup would entail executing this before the previous

code snippet:

[4] mount -t tmpfs -o size=64k,mode=0755 tmpfs /dev

[5] mkdir /dev/pts

[6] mount -t devpts devpts /dev/pts

The simple explanation here is that [1] you need to have /sys mounted before

executing mdev. Then you [2] instruct the kernel to execute /sbin/mdev whenever

a device is added or removed so that the device node can be created or

destroyed. Then you [3] seed /dev with all the device nodes that were created

while the system was booting.

For the "full" setup, you want to [4] make sure /dev is a tmpfs filesystem

(assuming you're running out of flash). Then you want to [5] create the

/dev/pts mount point and finally [6] mount the devpts filesystem on it.


MDEV Config (/etc/mdev.conf)


Mdev has an optional config file for controlling ownership/permissions of

device nodes if your system needs something more than the default root/root

660 permissions.

The file has the format:


or @ :

For example:

hd[a-z][0-9]* 0:3 660

The config file parsing stops at the first matching line. If no line is

matched, then the default of 0:0 660 is used. To set your own default, simply

create your own total match like so:

.* 1:1 777

You can rename/move device nodes by using the next optional field.



So if you want to place the device node into a subdirectory, make sure the path

has a trailing /. If you want to rename the device node, just place the name.

hda 0:3 660 =drives/

This will move "hda" into the drives/ subdirectory.

hdb 0:3 660 =cdrom

This will rename "hdb" to "cdrom".

Similarly, ">path" renames/moves the device but it also creates

a direct symlink /dev/DEVNAME to the renamed/moved device.

You can also prevent creation of device nodes with the 4th field as "!":

tty[a-z]. 0:0 660 !

pty[a-z]. 0:0 660 !

If you also enable support for executing your own commands, then the file has

the format:


[=path] [@|$|*]



[>path] [@|$|*]



[!] [@|$|*]

For example:


# block devices

([hs]d[a-z]) root:disk660>disk/%1/0

([hs]d[a-z])([0-9]+) root:disk660>disk/%1/%2

mmcblk([0-9]+) root:disk660>disk/mmc/%1/0

mmcblk([0-9]+)p([0-9]+) root:disk660>disk/mmc/%1/%2

# network devices

(tun|tap) root:network660>net/%1


The special characters have the meaning:

@ Run after creating the device.

$ Run before removing the device.

* Run both after creating and before removing the device.

The command is executed via the system() function (which means you're giving a

command to the shell), so make sure you have a shell installed at /bin/sh. You

should also keep in mind that the kernel executes hotplug helpers with stdin,

stdout, and stderr connected to /dev/null.

For your convenience, the shell env var $MDEV is set to the device name. So if

the device "hdc" was matched, MDEV would be set to "hdc".




Some kernel device drivers need to request firmware at runtime in order to

properly initialize a device. Place all such firmware files into the

/lib/firmware/ directory. At runtime, the kernel will invoke mdev with the

filename of the firmware which mdev will load out of /lib/firmware/ and into

the kernel via the sysfs interface. The exact filename is hardcoded in the

kernel, so look there if you need to know how to name the file in userspace.




Kernel does not serialize hotplug events. It increments SEQNUM environmental

variable for each successive hotplug invocation. Normally, mdev doesn't care.

This may reorder hotplug and hot-unplug events, with typical symptoms of

device nodes sometimes not created as expected.

However, if /dev/mdev.seq file is found, mdev will compare its

contents with SEQNUM. It will retry up to two seconds, waiting for them

to match. If they match exactly (not even trailing '\n' is allowed),

or if two seconds pass, mdev runs as usual, then it rewrites /dev/mdev.seq

with SEQNUM+1.

IOW: this will serialize concurrent mdev invocations.

If you want to activate this feature, execute "echo >/dev/mdev.seq" prior to

setting mdev to be the hotplug handler. This writes single '\n' to the file.

NB: mdev recognizes /dev/mdev.seq consisting of single '\n' character

as a special case. IOW: this will not make your first hotplug event

to stall for two seconds.

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