chapter-17.md

Chapter 17: Devices and Modules

• Device types: Classifications used in all Unix systems to unify behavior of common devices
• Modules: The mechanism by which the Linux kernel can load and unload object code on demand
• Kernel objects: Support for adding simple object-oriented behavior and a parent/child relationship to kernel data structures
• Sysfs: A filesystem representation of the system's device tree

Device Types

In Linux, as with all Unix systems, devices are classified into one of three types:

• Block devices: Often abbreviated blkdevs, block devices are addressable in device-specified chunks called blocks and generally support seeking, the random access of data.
• Character devices: Often abbreviated cdevs, character devices are generally not addressable, providing access to data only as a stream, generally of characters (bytes).
• Network devices: Sometimes called Ethernet devices after the most common type of network devices, network devices provide access to a network via a physical adapter and a specific protocol.

Modules

The Linux kernel is modular, supporting the dynamic insertion and removal of code from itself at runtime. Related subroutines, data, and entry and exit points are grouped together in a single binary image, a loadable kernel object, called a module.

The Device Model

A significant new feature in the 2.6 Linux kernel is the addition of a unified device model. The device model provides a single mechanism for representing devices and describing their topology in the system.

sysfs

The sysfs filesystem is an in-memory virtual filesystem that provides a view of the kobject hierarchy. It enables users to view the device topology of their system as a simple filesystem. Using attributes, kobjects can export files that enable kernel variables to be read from and optionally written to.

The magic behind sysfs is simply tying kobjects to directory entries via the dentry member inside each kobject.