An Introduction to Device Drivers

              One of the many advantages of free operating systems, as typified by Linux, is that their internals are open for all to view. The operating system, once a dark and mysterious area whose code was restricted to a small number of programmers, can now be
readily examined, understood, and modified by anybody with the requisite skills. 
Linux has helped to democratize operating systems. The Linux kernel remains a large and complex body of code, however, and would-be kernel hackers need an entry point where they can approach the code without being overwhelmed by complexity. Often, device drivers provide that gateway

 

Device drivers take on a special role in the Linux kernel. They are distinct “black
boxes” that make a particular piece of hardware respond to a well-defined internal
programming interface; they hide completely the details of how the device works.
User activities are performed by means of a set of standardized calls that are indepen-
dent of the specific driver; mapping those calls to device-specific operations that act
on real hardware is then the role of the device driver. This programming interface is
such that drivers can be built separately from the rest of the kernel and “plugged in”
at runtime when needed. This modularity makes Linux drivers easy to write, to the
point that there are now hundreds of them available.
There are a number of reasons to be interested in the writing of Linux device drivers.
The rate at which new hardware becomes available (and obsolete!) alone guarantees
that driver writers will be busy for the foreseeable future. Individuals may need to
know about drivers in order to gain access to a particular device that is of interest to
them. Hardware vendors, by making a Linux driver available for their products, can
add the large and growing Linux user base to their potential markets. And the open
source nature of the Linux system means that if the driver writer wishes, the source
to a driver can be quickly disseminated to millions of users.
This book teaches you how to write your own drivers and how to hack around in
related parts of the kernel. We have taken a device-independent approach; the pro-
gramming techniques and interfaces are presented, whenever possible, without being
tied to any specific device. Each driver is different; as a driver writer, you need to
understand your specific device well. But most of the principles and basic tech-
niques are the same for all drivers. This book cannot teach you about your device,
but it gives you a handle on the background you need to make your device work.
As you learn to write drivers, you find out a lot about the Linux kernel in general;
this may help you understand how your machine works and why things aren’t
always as fast as you expect or don’t do quite what you want. We introduce new
ideas gradually, starting off with very simple drivers and building on them; every new
concept is accompanied by sample code that doesn’t need special hardware to be
tested.
This chapter doesn’t actually get into writing code. However, we introduce some
background concepts about the Linux kernel that you’ll be glad you know later,
when we do launch into programming.