Linux系统之热插拨事件uevent

文章最后面我会附上medv.conf配置文档medv.txt，是英文的，部分朋友的英语不是很好，但还是希望大家要硬着头皮去读，不要有畏惧心理，相信你读完了后会给你带来很大的帮助。你要相信外文资料讲得很地道，翻译过来没有那种感觉，以上只是个人看法。

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放在/etc/目下，sh脚本文件放在bin目录下，device regex用正则表达式
写相对方便一点，command可以写在脚本文件中，但是要注意sh脚本文件是可以执行的。

下面是几个例子：
1.
leds 0:0 777
led1 0:0 777
led2 0:0 777
led3 0:0 777
dma 0:0 777
2.
leds?[123]? 0:0 777

/*For your convenience, the shell env var $MDEV is set to the device name. So if
the device "hdc" was matched, MDEV would be set to "hdc".*/
3.
leds?[123]? 0:0 777 @ echo create /dev/$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 /tmp;
else
umount /tmp;

mdev.txt

-------------
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:
<device regex> <uid>:<gid> <permissions>
or @<maj[,min1[-min2]]> <uid>:<gid> <permissions>

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.
<device regex> <uid>:<gid> <permissions> [=path]
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:
<device regex> <uid>:<gid> <permissions> [=path] [@|$|*<command>]
or
<device regex> <uid>:<gid> <permissions> [>path] [@|$|*<command>]
or
<device regex> <uid>:<gid> <permissions> [!] [@|$|*<command>]

For example:
---8<---
# 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
---8<---

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".

----------
FIRMWARE
----------

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.

------------
SEQUENCING
------------

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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