Linux for Embedded and Real-Time Applications

272 Pages·2003·1.288 MB·English

by  Doug Abbott

Linux for Embedded and
Real-time Applications
.
Linux for Embedded and
Real-time Applications
by Doug Abbott
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Contents
Preface................................................................................................... ix
Audience and Prerequisites................................................................. x
Personal Biases.................................................................................. xi
Chapter 1: The Embedded and Real-time Space.....................................1
What Is Embedded? ...........................................................................1
What Is Real-time?.............................................................................2
How and Why Does Linux Fit in? ........................................................3
Resources...........................................................................................7
Chapter 2: Introducing Linux .................................................................9
Features.............................................................................................9
Protected Mode Architecture............................................................11
The Linux Process Model ..................................................................16
The Linux Filesystem.........................................................................21
The “root” User................................................................................23
The /usr hierarchy ............................................................................28
The Shell..........................................................................................29
Resources.........................................................................................29
Chapter 3: The Host Development Environment..................................31
Cross-Development Tools—the GNU Tool Chain ...............................32
Configuring and Building the Kernel .................................................33
Summary .........................................................................................46
Resources.........................................................................................46
Chapter 4: BlueCat Linux.....................................................................49
The “Less Is More” Philosophy..........................................................49
Installing BlueCat Linux....................................................................50
X86 Target for Blue Cat Linux ...........................................................52
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Linux for Embedded and Real-time Applications
Configuring the Workstation ............................................................55
First Test Program............................................................................58
Directories .......................................................................................58
Configuration Files...........................................................................59
Makefile...........................................................................................59
Target Files.......................................................................................59
Resources.........................................................................................63
Chapter 5: Debugging Embedded Software .........................................65
The Target Setup..............................................................................65
GDB ................................................................................................66
Debugging a Sample Program...........................................................68
The Host as a Debug Environment....................................................73
Adding Programmable Setpoint and Limit ........................................76
Resources.........................................................................................79
Chapter 6: Kernel Modules and Device Drivers ....................................81
Kernel Modules ................................................................................81
What’s a Device Driver Anyway?........................................................86
Linux Device Drivers..........................................................................87
Internal Driver Structure...................................................................90
The Hardware ..................................................................................95
The Target Version of Thermostat.....................................................96
Debugging Kernel Code....................................................................96
Building Your Driver into the Kernel ................................................100
An Alternative—uCLinux .................................................................103
Resources.......................................................................................104
Chapter 7: Embedded Networking.....................................................105
Sockets ..........................................................................................105
A Simple Example...........................................................................108
A Remote Thermostat ....................................................................111
Embedded Web Servers..................................................................113
Resources.......................................................................................116
vi
Contents
Chapter 8: Introduction to Real-time Programming...........................117
Polling vs. Interrupts.......................................................................118
Tasks..............................................................................................125
Scheduling .....................................................................................128
Kernel Services................................................................................132
Inter-task Communication..............................................................134
Problems with Solving the Resource Sharing Problem—
Priority Inversion ............................................................................141
Interrupts and Exceptions...............................................................143
Critical Sections .............................................................................144
Resources.......................................................................................145
Chapter 9: Linux and Real-time..........................................................147
Why Linux Isn’t Real-time ...............................................................147
Two Approaches ............................................................................150
Resources—Obtaining Real-time Linux Implementations..................153
Chapter 10: The RTAI Environment...................................................155
Installing RTAI................................................................................155
The RTAI Architecture ....................................................................159
Intertask Communication and Synchronization...............................162
Communicating with Linux Processes..............................................163
Real-time in User Space—LXRT .......................................................164
One Shot vs. Periodic Timing..........................................................167
Moving to Kernel Space..................................................................170
Real-time FIFOs and Shared Memory..............................................171
Suggested Exercises.........................................................................173
Chapter 11: Posix Threads.................................................................175
Threads..........................................................................................176
Synchronization—Mutexes ..............................................................178
Communication—Condition Variables.............................................181
Pthreads in User Space ...................................................................182
Moving to Kernel Space..................................................................188
Message Queues.............................................................................190
Suggestions for Further Exploration ................................................192
Resources.......................................................................................193
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Linux for Embedded and Real-time Applications
Appendix A: RTAI Application Programming Interface (API).............195
Appendix B: Posix Threads (Pthreads) Application Programming
Interface........................................................................................227
Appendix C: Why Software Should Not Have Owners .......................243
Index...................................................................................................251
viii
Preface
“ ’You are in a maze of twisty little passages, all alike’
Before you looms one of the most complex and utterly intimidating systems ever
written. Linux, the free UNIX clone for the personal computer, produced by a
mishmash team of UNIX gurus, hackers, and the occasional loon. The system
itself reflects this complex heritage, and although the development of Linux may
appear to be a disorganized volunteer effort, the system is powerful, fast, and
free. It is a true 32-bit operating system solution.”1
I have a confession to make. Until about three years ago, I didn’t like Unix
and avoided it as much as possible. I always considered it deliberately obscure
and difficult to use. I still do. Working with Linux has been one of the most
frustrating experiences in my long career as a computer engineer. I can do a
Windows installation in about 15 minutes without ever referring to a
manual. I can’t do that with Linux.
But, while Linux is far from being ready for prime time in the world of con-
sumer computing, there are some good things about it that have forced me to
soften my bias and grin and bear it. In the embedded space where I work,
Linux can no longer be ignored or avoided, nor should it be.
Linux is indeed complex and, unless you’re already a Unix guru, the learning
curve is quite steep. The information is out there on the web but it is often
neither easy to find nor readable. There are probably hundreds of books in
print on Linux covering every aspect from beginners’ guides to the internal
workings of the kernel. But until recently little has been written about Linux
in embedded or real-time environments.
1 Linux Installation and Getting Started, Matt Welsh, et al.
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Linux for Embedded and Real-time Applications
I decided to climb the Linux learning curve partly because I saw it as an
emerging market opportunity and partly because I was intrigued by the Open
Source development model. The idea of programmers all over the world
contributing to the development of a highly sophisticated operating system
just for the fun of it is truly mind-boggling. Having the complete source code
not only allows you to modify it to your heart’s content, it allows you (in
principle at least) to understand how the code works. Unfortunately, my
experience has been that a lot of Linux code is “write-only.” Someone obvi-
ously wrote it, but no one else can read it.
Open Source has the potential to be a major paradigm shift in how our
society conducts business because it demonstrates that cooperation can be as
useful in developing solutions to problems as competition. Yet at the time
this book is being written, serious questions are being raised concerning
whether or not it is possible to actually make money with Open Source
software. Is there a business model that works? The jury is still out.
Audience and Prerequisites
This book is directed at two different audiences:
■ The primary audience is embedded programmers who need an intro-
duction to Linux in the embedded space. This is where I came from
and how I got into Linux so it seems like a reasonable way to struc-
ture the book.
■ The other audience is Linux programmers who need an introduction
to the concepts of embedded and real-time programming.
Consequently, each group will see some material that is review although it
may be presented with a fresh perspective.
This book is not a beginners’ guide. I assume that you have successfully
installed a Linux system and have at least played around with it some. You
know how to log in, you’ve experimented with some of the command utilities
and have probably fired up X-windows. Chapter 2 is a cursory introduction to
some of the features and characteristics of Linux that are of interest to
embedded and real-time programmers.
x