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Writing UNIX® Device Drivers
George Pajari
Addison-Wesley Publishing Company, Inc.
Reading, Massachusetts Menlo Park, California New York
Don Mills, Ontario Wokingham, England Amsterdam Bonn
Sydney Singapore Tokyo Madrid San Juan
Paris Seoul Milan Mexico City Taipei
Many of the designations used by manufacturers and sellers to distin¬
guish their products are claimed as trademarks. Where those designa¬
tions appear in this book and Addison-Wesley was aware of a trademark
claim, the designations have been printed in initial capital letters.
The authors and publishers have taken care in preparation of this book,
but make no expressed or implied warranty of any kind and assume no
responsibility for errors or omissions. No liability is assumed for inciden¬
tal or consequential damages in connection with or arising out of the use
of the information or programs contained herein.
Library of Congress Cataloging-in-Publication Data
Pajari, George.
Writing UNIX device drivers / by George Pajari.
p. cm.
Includes index.
ISBN 0-201-52374-4
1. UNIX device drivers (Computer programs) I. Title.
QA76.76.D49P35 1991
005.4’3—dc20 91-42361
Copyright © 1992 by George Pajari
All rights reserved. No part of this publication may be reproduced, stored
in a retrieval system, or transmitted, in any form or by any means,
electronic, mechanical, photocopying, recording, or otherwise, without
the prior written permission of the publisher. Printed in the United
States of America. Published simultaneously in Canada.
ISBN 0-201-52374-4
Managing Editor: Amorette Pedersen
Set in 11-point New Century Schoolbook by Benchmark Productions
2 3 4 5 6 7 8 9-MW-95949392
Second printing, April 1992
This book is dedicated to Carol, who has taken the fun out of being alone.
Acknowledgments
«
To my colleagues at Driver Design Labs: Jeff Tate, who checked that it
made sense and helped make it somewhat understandable; Carolanne
Reynolds, who spent many hours reading and revising drafts trying to
convert it into something much closer to English; and to Frank Pronk who
provided a useful reality check. The unseen corrections and improvements
are theirs (many remaining problems and errors are mine).
To Harry Henderson, Mitch Waite, Rebecca Thomas, David Flack, Tom
Ward, Ben Smith, and Jane Tazelaar; all of whom have helped with one
or more of my previously published works on device drivers.
To Chris Williams and Amy Pedersen who, as editors of this book, exhib¬
ited patience and understanding above and beyond the call. No author
could ask for more, and aspiring authors of technical books are urged to
contact them.
To Jeff Kulick and the Department of Computing and Information Science
at Queen’s University at Kingston: both gave me the opportunity to play
with the UNIX kernel well over a decade ago and both have to take at
least some responsibility for all of this.
To Ed Palmer and the organizers of the annual UniForum Conference
and Exhibition, where the author has had the pleasure of delivering the
Writing UNIX Device Drivers tutorial for several years. The course notes
for those tutorials formed the basis for this book.
To the Scottish & Newcastle Breweries PLC Edinburgh, whose McEwan’s
Scotch Malt Liquor had a very enjoyable, if detrimental, effect on the
writing of this book.
To my parents and grandparents who have always encouraged and sup¬
ported me.
And most importantly to my wife, Carolanne Reynolds, for more reasons
than there is room in these notes to list.
in
Table of Contents
Chapter 1
y
What Is A Device Driver? 1
The Grand Design 1
The Details 3
Types of Device Drivers 6
The Gross Anatomy of a Device Driver 10
General Programming Considerations 12
Summary 13
Chapter 2
Character Driver I: A Test Data Generator 15
The Design Issues 16
The Driver 19
The Recapitulation 29
Summary 31
Exercises 32
Chapter 3
Character Drivers II: An A/D Converter 35
The Design Issues 37
The Driver 37
Exercises 54
iv
Table of Contents u
Chapter 4
Character Drivers III: A Line Printer 55
The Device 56
The Driver 56
Summary 78
Exercises 78
Chapter 5
Block Drivers I: A Test Data Generator 81
The Design Issues 83
The Driver 84
Summary 92
Exercises 92
Chapter 6
Block Drivers III: A RAM Disk Driver 93
The Design Issues 93
The Driver 94
Summary 106
Exercises 107
7
Chapter
Block Drivers III: A SCSI Disk Driver 109
The Design Issues 109
The Device 110
Summary 145
Exercises 145
Chapter 8
Character Drivers IV: The Raw Disk Driver 147
DMA and Character Drivers 148
The General Structure 151
Exercises 153
vi WRITING UNIX DEVICE DRIVERS
Chapter 9
Terminal Drivers I: The C0M1 Port 155
Line Disciplines 156
The Device 156
The Driver 157
Beyond Line Disciplines 185
Summary 185
Exercises 186
Chapter 10
Character Drivers V: A Tape Drive 187
A SCSI Tape Drive 187
The Driver 189
Possible Enhancements 207
Summary 209
Exercises 209
Chapter 11
STREAMS Drivers I: A Loop-Back Driver 211
The Design Issues 219
A Loop-Back STREAMS Driver 219
Summary 230
Exercises 231
Chapter 12
Streams Drivers II: The COM1 Port (Revisited) 233
The Driver 234
Suggestions for Improvements 273
Summary 274
Exercises 274
Table of Contents vii
Chapter 13
Driver Installation 275
Compiling the Driver 276
Configuring the Kernel 276
Installing the New Driver 279
Building a New Kernel 280
Creating Entries in /dev 280
Rebooting the New Kernel 281
Installation Example 282
Advanced Driver Installation 284
Exercises 285
Chapter 14
Zen and the Art of Device Driver Writing 287
Preparing to Write a Driver 289
Debugging Device Drivers 291
Driver Performance 296
Unusual Driver Applications 300
Exercises 301
Chapter 15
Writing Drivers for System V Release 4 303
The Good News 304
The New Order 304
Sample SVR4 Drivers 307
Summary 311
Exercises 312
Index 313
Preface
So you want to write a UNIX device driver. Or perhaps you just want to
learn a bit more about a topic that has historically been the exclusive
domain of systems gurus and programming wizards. In either case, this
book is written expressly for you.
Writing UNIX Device Drivers
Writing device drivers is one of the most challenging and interesting
types of programming. In contrast to applications programming, the
device driver writer has to be concerned with hardware details at the low¬
est levels as well as the problems of concurrency. These additional com¬
plexities not only make device driver programming much more difficult
and frustrating, they make it far more interesting and challenging.
Let us consider some of the problems that regularly arise during the
development of a device driver—problems which, by the way, rarely if
ever affect applications programmers.
Device drivers interface with the kernel using an entirely different set of
routines than those used by applications software. This programming
interface is all too often incompletely (or incorrectly) documented, fre¬
quently leaving the device driver programmer to debug the documenta¬
tion as well as the driver.
Device drivers usually interface with hardware (i.e., disk drives or tape
drives). Not only is this interface almost always incompletely documented,
it usually suffers from idiosyncrasies that are very different from software
interfaces. Some hardware devices will malfunction if their control regis¬
ters are written in the wrong order, or if written before being read, or if
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