udev :应用层的守护进程，由启动脚本加载，负责建立热拨插的接点
cdev :建立字符设备接口
platform device :相关平台直接总线建立的设备，主要出现需要自己直接挂到平台的时候，因为很多驱动都挂到相应的总线上的
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如果没有udevd守护进程,那么/dev下的很多设备文件必须自己mknod
如果没有platform device也没有多少关系,看你的具体的硬件驱动挂那个子系统上,或者总线上
在前面，我们看到了/dev、devfs、udev和sysfs等概念，估计读者也会乱了思路了，下面这段文章摘自udev的说明文档，来理清一下它们之间的关系，这里我就不翻译了，我只在后面说明一下。
On typical Linux-based systems, the /dev directory is used to store file-like device nodes which refer to certain devices in the system. Each node points to a part of the system (a device), which might or might not exist. Userspace applications can use these device nodes to interface with the systems hardware, for example, the X server will "listen to" /dev/input/mice so that it can relate the user's mouse movements to moving the visual mouse pointer.
The original /dev directories were just populated with every device that might possibly appear in the system. /dev directories were typically very large because of this. devfs came along to provide a more manageable approach (noticeably, it only populated /dev with hardware that is plugged into the system), as well as some other functionality, but the system proved to have problems which could not be easily fixed.
udev is the "new" way of managing /dev directories, designed to clear up some issues with previous /dev implementations, and provide a robust path forward. In order to create and name /dev device nodes corresponding to devices that are present in the system, udev relies on matching information provided by sysfs with rules provided by the user. This documentation aims to detail the process of rule-writing, one of the only udev-related tasks that must (optionally) be performed by the user.
sysfs is a new filesystem to the 2.6 kernels. It is managed by the kernel, and exports basic information about the devices currently plugged into your system. udev can use this information to create device nodes corresponding to your hardware. sysfs is mounted at /sys and is browseable. You may wish to investigate some of the files stored there before getting to grips with udev. Throughout this document, I will use the terms /sys and sysfs interchangeably.
我这里简单说明一下。在以前呢/dev存储的是linux可以认识的所有的文件设备名，无论这个设备有没有在系统里。为了解决/dev目录过大的问题，就产生了devfs，但devfs是基因内核的，有很多无法解决的问题（参考前面的文章），所以就出现了udev。而udev对设备在/dev目录下的命名依赖于sysfs所提供的规则信息，这些规则信息由用户提供。sysfs是在2.6内核时加进linux内核的。sysfs就是我们系统加载的/sys。