O'Reilly - Linux Device Drivers 2nd Edition.pdf - LINUX - antonif

P REFACE

This is, on the surface, a book about writing device drivers for the Linux system.

That is a worthy goal, of course; the ﬂow of new hardware products is not likely

to slow down anytime soon, and somebody is going to have to make all those

new gadgets work with Linux. But this book is also about how the Linux kernel

works and how to adapt its workings to your needs or interests. Linux is an open

system; with this book, we hope, it will be more open and accessible to a larger

community of developers.

Much has changed with Linux since the ﬁrst edition of this book came out. Linux

now runs on many more processors and supports a much wider variety of hard-

ware. Many of the internal programming interfaces have changed signiﬁcantly.

Thus, the second edition. This book covers the 2.4 kernel, with all of the new fea-

tures that it provides, while still giving a look backward to earlier releases for

those who need to support them.

We hope you’ll enjoy reading this book as much as we have enjoyed writing it.

Alessandro’s Introduction

As an electronic engineer and a do-it-yourself kind of person, I have always

enjoyed using the computer to control external hardware. Ever since the days of

my father’s Apple IIe, I have been looking for another platform where I could con-

nect my custom circuitry and write my own driver software. Unfortunately, the PC

of the 1980s wasn’t powerful enough, at either the software or the hardware level:

the internal design of the PC is much worse than that of the Apple II, and the

available documentation has long been unsatisfying. But then Linux appeared, and

I decided to give it a try by buying an expensive 386 motherboard and no propri-

etary software at all.

Preface

At the time, I was using Unix systems at the university and was greatly excited by

the smart operating system, in particular when supplemented by the even smarter

utilities that the GNU project donates to the user base. Running the Linux kernel

on my own PC motherboard has always been an interesting experience, and I

could even write my own device drivers and play with the soldering iron once

again. I continue to tell people, “When I grow up, I wanna be a hacker,” and

GNU/Linux is the perfect platform for such dreams. That said, I don’t know if I

will ever grow up.

As Linux matures, more and more people get interested in writing drivers for cus-

tom circuitry and for commercial devices. As Linus Torvalds noted, “We’re back to

the times when men were men and wrote their own device drivers.”

Back in 1996, I was hacking with my own toy device drivers that let me play with

some loaned, donated, or even home-built hardware. I already had contributed a

few pages to the Ker nel Hacker’s Guide , by Michael Johnson, and began writing

kernel-related articles for Linux Journal , the magazine Michael founded and

directed. Michael put me in touch with Andy Oram at O’Reilly; he expressed an

interest in having me write a whole book about device drivers, and I accepted this

task, which kept me pretty busy for quite a lot of time.

In 1999 it was clear I couldn’t ﬁnd the energy to update the book by myself: my

family had grown and I had enough programming work to keep busy producing

exclusively GPL’d software. Besides, the kernel had grown bigger and supported

more diverse platforms than it used to, and the API had turned more broad and

more mature. That’s when Jonathan offered to help: he had just the right skills and

enthusiasm to start the update and to force me to stay on track with the sched-

ule—which slipped quite a lot anyway. He’s been an invaluable mate in the pro-

cess, which he pushed forward with good skills and dedication, deﬁnitely more

than I could put in. I really enjoyed working with him, both on a technical and

personal level.

Jon’s Introduction

I ﬁrst started actively playing with Linux early in 1994, when I convinced my

employer to buy me a laptop from a company called, then, Fintronic Systems.

Having been a Unix user since the beginning of the 1980s, and having played

around in the source since about then, I was immediately hooked. Even in 1994,

Linux was a highly capable system, and the ﬁrst truly free system that I had ever

been able to work with. I lost almost all my interest in working with proprietary

systems at that point.

I didn’t ever really plan to get into writing about Linux, though. Instead, when I

started talking with O’Reilly about helping with the second edition of this book, I

had recently quit my job of 18 years to start a Linux consulting company. As a way

xii

Preface

of attracting attention to ourselves, we launched a Linux news site, Linux Weekly

News ( http://lwn.net ), which, among other things, covered kernel development. As

Linux exploded in popularity, the web site did too, and the consulting business

was eventually forgotten.

But my ﬁrst interest has always been systems programming. In the early days, that

interest took the form of “ﬁxing” the original BSD Unix paging code (which has to

have been a horrible hack job) or making recalcitrant tape drives work on a

VAX/VMS system (where source was available, if you didn’t mind the fact that it

was in assembly and Bliss, and came on microﬁche only). As time passed, I got to

hack drivers on systems with names like Alliant, Ardent, and Sun, before moving

into tasks such as deploying Linux as a real-time radar data collection system or, in

the process of writing this book, ﬁxing the I/O request queue locking in the Linux

ﬂoppy driver.

So I welcomed the opportunity to work on this book for several reasons. As much

as anything, it was a chance to get deeply into the code and to help others with a

similar goal. Linux has always been intended to be fun as well as useful, and play-

ing around with the kernel is one of the most fun parts of all—at least, for those

with a certain warped sense of fun. Working with Alessandro has been a joy, and I

must thank him for trusting me to hack on his excellent text, being patient with

me as I came up to speed and as I broke things, and for that jet-lagged bicycle

tour of Pavia. Writing this book has been a great time.

Audience of This Book

On the technical side, this text should offer a hands-on approach to understanding

the kernel internals and some of the design choices made by the Linux develop-

ers. Although the main, ofﬁcial target of the book is teaching how to write device

drivers, the material should give an interesting overview of the kernel implementa-

tion as well.

Although real hackers can ﬁnd all the necessary information in the ofﬁcial kernel

sources, usually a written text can be helpful in developing programming skills.

The text you are approaching is the result of hours of patient grepping through

the kernel sources, and we hope the ﬁnal result is worth the effort it took.

This book should be an interesting source of information both for people who

want to experiment with their computer and for technical programmers who face

the need to deal with the inner levels of a Linux box. Note that “a Linux box” is a

wider concept than “a PC running Linux,” as many platforms are supported by our

operating system, and kernel programming is by no means bound to a speciﬁc

platform. We hope this book will be useful as a starting point for people who

want to become kernel hackers but don’t know where to start.

xiii

Preface

The Linux enthusiast should ﬁnd in this book enough food for her mind to start

playing with the code base and should be able to join the group of developers

that is continuously working on new capabilities and performance enhancements.

This book does not cover the Linux kernel in its entirety, of course, but Linux

device driver authors need to know how to work with many of the kernel’s sub-

systems. It thus makes a good introduction to kernel programming in general.

Linux is still a work in progress, and there’s always a place for new programmers

to jump into the game.

If, on the other hand, you are just trying to write a device driver for your own

device, and you don’t want to muck with the kernel internals, the text should be

modularized enough to ﬁt your needs as well. If you don’t want to go deep into

the details, you can just skip the most technical sections and stick to the standard

API used by device drivers to seamlessly integrate with the rest of the kernel.

The main target of this book is writing kernel modules for version 2.4 of the Linux

kernel. A module is object code that can be loaded at runtime to add new func-

tionality to a running kernel. Wherever possible, however, our sample code also

runs on versions 2.2 and 2.0 of the kernel, and we point out where things have

changed along the way.

Organization of the Material

The book introduces its topics in ascending order of complexity and is divided

into two parts. The ﬁrst part (Chapters 1 to 10) begins with the proper setup of

kernel modules and goes on to describe the various aspects of programming that

you’ll need in order to write a full-featured driver for a char-oriented device. Every

chapter covers a distinct problem and includes a “symbol table” at the end, which

can be used as a reference during actual development.

Throughout the ﬁrst part of the book, the organization of the material moves

roughly from the software-oriented concepts to the hardware-related ones. This

organization is meant to allow you to test the software on your own computer as

far as possible without the need to plug external hardware into the machine. Every

chapter includes source code and points to sample drivers that you can run on any

Linux computer. In Chapter 8 and Chapter 9, however, we’ll ask you to connect an

inch of wire to the parallel port in order to test out hardware handling, but this

requirement should be manageable by everyone.

The second half of the book describes block drivers and network interfaces and

goes deeper into more advanced topics. Many driver authors will not need this

material, but we encourage you to go on reading anyway. Much of the material

found there is interesting as a view into how the Linux kernel works, even if you

do not need it for a speciﬁc project.

xiv

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