The Linux Device Model
Many Linux subsystems, such as the /dev filesystem, hotplug, module autoload, and microcode download have undergone significant changes with the introduction of the new device model. Learn about udev, sysfs, kobjects, classes, and more.
Tuesday, August 15th, 2006
The Linux 2.6 kernel features a new device model built around concepts like sysfs, kobjects, device classes, and udev. The new model makes it easier to develop device drivers by introducing C++- like abstractions that distill commonalities from device drivers into bus and core layers. It also weeds policies out of kernel space and pushes them to user space, which has resulted in a total revamp of many features, including /dev node management, hotplug, coldplug, module autoload and firmware download.
This month, let’s look at the different pieces that constiture the device model and how each piece affects key kernel subsystems. But let’s start by learning about udev with the help of some examples.
Tinkering with Udev
Years ago, when Linux was young, it wasn’t fun to administer device nodes. All the needed nodes (which could run into thousands) had to be statically created under the /dev directory. With the advent of the 2.4 kernels came devfs, which introduced dynamic device node creation. Devfs provided kernel interfaces to request generation of device nodes in an in-memory filesystem, but the onus of naming the nodes still rested with device drivers. However, device naming policy is administrative and doesn’t mix well with the kernel. Udev arrived on the scene to push device management entirely to user space.
Udev depends on the following to do its work:
