Linux Device Drivers, 3rd Edition

Chapter 3. Char Drivers

The goal of this chapter is to write a complete char device driver. We develop a character driver because this class is suitable for most simple hardware devices. Char drivers are also easier to understand than block drivers or network drivers (which we get to in later chapters). Our ultimate aim is to write a modularized char driver, but we won’t talk about modularization issues in this chapter.

Throughout the chapter, we present code fragments extracted from a real device driver: scull (Simple Character Utility for Loading Localities). scull is a char driver that acts on a memory area as though it were a device. In this chapter, because of that peculiarity of scull, we use the word device interchangeably with “the memory area used by scull.”

The advantage of scull is that it isn’t hardware dependent. scull just acts on some memory, allocated from the kernel. Anyone can compile and run scull, and scull is portable across the computer architectures on which Linux runs. On the other hand, the device doesn’t do anything “useful” other than demonstrate the interface between the kernel and char drivers and allow the user to run some tests.

The Design of scull

The first step of driver writing is defining the capabilities (the mechanism) the driver will offer to user programs. Since our “device” is part of the computer’s memory, we’re free to do what we want with it. It can be a sequential or random-access device, one device or many, and so on.

To make scull useful ...