Understanding the Linux Kernel, Second Edition

Chapter 13. Managing I/O Devices

The Virtual File System in the last chapter depends on lower-level functions to carry out each read, write, or other operation in a manner suited to each device. The previous chapter included a brief discussion of how operations are handled by different filesystems. In this chapter, we look at how the kernel invokes the operations on actual devices.

In Section 13.1, we give a brief survey of the 80 × 86 I/O architecture. In Section 13.2, we show how the VFS associates a special file called “device file” with each different hardware device so that application programs can use all kinds of devices in the same way. Finally, in Section 13.3, we illustrate the overall organization of device drivers in Linux. Readers interested in developing device drivers on their own may want to refer to Alessandro Rubini and Jonathan Corbet’s Linux Device Drivers (O’Reilly).

I/O Architecture

To make a computer work properly, data paths must be provided that let information flow between CPU(s), RAM, and the score of I/O devices that can be connected to a personal computer. These data paths, which are denoted collectively as the bus , act as the primary communication channel inside the computer.

Several types of buses, such as the ISA, EISA, PCI, and MCA, are currently in use. In this section, we discuss the functional characteristics common to all PC architectures, without giving details about a specific bus type.

In fact, what is commonly denoted as a bus consists ...