[linux] mdev和udev功能

最新推荐文章于 2024-08-19 23:18:54 发布

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最新推荐文章于 2024-08-19 23:18:54 发布 · 1.2k 阅读

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本文介绍了Linux内核中的设备管理器udev和简化版的mdev，重点讲解了mdev在嵌入式系统中的作用，包括初始化、动态更新设备节点，以及在系统启动和热插拔时创建或删除设备节点的过程。同时提到了busybox中的mdev工具配置和使用方法，以及如何通过mdev和sysfs实现设备节点的自动化管理。

【linux kernel】mdev详解_iriczhao的博客-优快云博客

udev

udev 是Linux kernel 2.6系列的设备管理器。它主要的功能是管理/dev目录底下的设备节点。它同时也用来接替devfs及热插拔的功能，这意味着它要在添加/删除硬件时处理/dev目录以及所有用户空间的行为

mdev

mdev概述

mdev是busybox提供的一个工具，在嵌入式系统中，相当于简化版的udev，作用是：在系统启动、热插拔和动态加载驱动程序时，自动创建设备节点。文件系统中的/dev目录下的设备节点都是由mdev创建的。在加载驱动过程中，根据驱动程序，在/dev下自动创建设备节点。

以下内容摘自busybox-1.23.1的mdev.txt文件，该文件中描述了mdev工具的功能和用法：

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（该功能依赖于内核的hotpluggin功能）.

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