文档介绍：http://lxr.linux.no/linux+v2.6.37/Documentation/filesystems/sysfs.txt
Sysfs was developed initially as an extension of the driver model. Sysfs is a mechanism for representing kernel objects, their attributes, and their relationships with each other. It provides two components: a kernel programming interface for exporting these items via sysfs, and a user interface to view and manipulate these items that maps back to the kernel objects which they represent. The table below shows the mapping between internel (kernel) constructs and their external (userspace) sysfs mappings.
Internal External
Kernel Objects Directories
Object Attributes Regular Files
Object Relationships Symbolic Links
This information is strictly organized and usually formatted simply in ASCII, making it very accessible to users and applications. It provides a clear window into the kernel data structures and the physical or virtual objects that they control.
sysfs呈现给用户的信息大部分是ASCII普通文件，此外还有二进制文件（Binary Files）和属性组（Attribute Groups）。
history
sysfs is an in-memory filesystem that was originally based on ramfs. ramfs was written around the time the 2.4.0 kernel was being stabilized. It was an exercise in elegance, as it showed just how easy it was to write a simple filesystem using the then-new VFS layer. Because of its simplicity and use of the VFS, it provided a good base from which to derive other in-memory based filesystems.
sysfs was originally called ddfs (Device Driver Filesystem) and was written to debug the new driver model as it was being written. That debug code had originally used procfs to export a device tree, but under strict urging from Linus Torvalds, it was converted to use a new filesystem based on ramfs.
sys/devices
The devices directory contains the global device hierarchy. This contains every physical device that has been discovered by the bus types registered with the kernel. It represents them in an ancestrally correct way—each device is shown as a subordinate device of the device that it is physically (electrically) subordinate to.像物理从属关系一样呈现设备分层。
两类设备除外：platform设备和system设备。There are two types of devices that are exceptions to this representation: platform devices and system devices. Platform devices are peripheral devices that are inherent to a particular platform. They usually have some I/O ports, or MMIO, that exists at a known, fixed location. Examples of platform devices are legacy x86 devices like a serial controller or a floppy controller, or the embedded devices of a SoC solution.
System devices are non-peripheral devices that are integral components of the system. In many ways, they are nothing like any other device. They may have some hardware register access for configuration, but do not have the capability to transfer data. They usually do not have drivers which can be bound to them. But, at least for those represented through sysfs, have some architecture-specific code that configures them and treats them enough as objects to export them. Examples of system devices are CPUs, APICs, and timers.
linux设备驱动开发详解理论
linux 2.6的内核引入了sysfs文件系统，sysfs被看作是与proc、devfs和devpty同类别的文件系统，它可以产生一个包括所有系统硬件的层级视图，与提供进程和状态信息的proc文件系统十分类似。
sysfs把连接在系统上的设备和总线组织成一个分级的文件，他们可以由用户空间存取、向用户空间导出内核数据结构以及他们的属性。sysfs的一个目的就是展示设备驱动模型中各组件的层次关系，其顶级目录包括block、device、bus、drivers、class、power和firmware。
linux2.6内核开发了全新的设备、总线、类和驱动环环相扣的设备模型。linux内核中，分别使用bus_type、device_driver和device来描述总线、驱动和设备，这3个结构体定义于include/linux/device.h头文件中。
device_driver和device分别表示驱动和设备，而这两者必须依附于一种总线，因此都包含struct bus_type指针。在linux内核中，设备和驱动是分开注册的，注册1个设备的时候，并不需要驱动已经存在，而1个驱动被注册时，也不需要驱动的设备已经被注册。设备和驱动各自涌向内核，而每个设备和驱动涌入的时候，都会去寻找自己的另一半。茫茫人海，何处寻觅？正是bus_type的match()成员函数将两者捆绑在一起。
总线、驱动和设备都可以认为是kobject的派生类（device结构体直接包含了kobject kobj成员，bus_type和device_driver则通过bus_type_private、driver_private间接包含kobject），kobject可看作所有总线、设备和驱动的抽象基类，1个kobject对应sysfs中的1个目录。
总线、设备和驱动中的各个attribute则直接落实为sysfs中的1个文件，attribute会伴随着show()和store()这两个函数，分别用于读写该attribute对应的sysfs文件结点。
事实上，udev规则中各信息的来源实际上就是bus_type、device_driver、device以及attribute等所对应的sysfs节点。