Preface
Linux has been the mainstay of embedded computing for many years. And yet, there are remarkably few books that cover the topic as a whole: this book is intended to fill that gap. The term embedded Linux is not well-defined, and can be applied to the operating system inside a wide range of devices ranging from thermostats to Wi-Fi routers to industrial control units. However, they are all built on the same basic open source software. Those are the technologies that I describe in this book, based on my experience as an engineer and the materials I have developed for my training courses.
Technology does not stand still. The industry based around embedded computing is just as susceptible to Moore's law as mainstream computing. The exponential growth that this implies has meant that a surprisingly large number of things have changed since the first edition of this book was published. This second edition is fully revised to use the latest versions of the major open source components, which include Linux 4.9, Yocto Project 2.2 Morty, and Buildroot 2017.02. Since it is clear that embedded Linux will play an important part in the Internet of Things, there is a new chapter on the updating of devices in the field, including Over the Air updates. Another trend is the quest to reduce power consumption, both to extend the battery life of mobile devices and to reduce energy costs. The chapter on power management shows how this is done.
Mastering Embedded Linux Programming covers the topics in roughly the order that you will encounter them in a real-life project. The first 6 chapters are concerned with the early stages of the project, covering basics such as selecting the toolchain, the bootloader, and the kernel. At the conclusion of this this section, I introduce the idea of using an embedded build tool, using Buildroot and the Yocto Project as examples.
The middle part of the book, chapters 7 through to 13, will help you in the implementation phase of the project. It covers the topics of filesystems, the init program, multithreaded programming, software update, and power management. The third section, chapters 14 and 15, show you how to make effective use of the many debug and profiling tools that Linux has to offer in order to detect problems and identify bottlenecks. The final chapter brings together several threads to explain how Linux can be used in real-time applications.
Each chapter introduces a major area of embedded Linux. It describes the background so that you can learn the general principles, but it also includes detailed worked examples that illustrate each of these areas. You can treat this as a book of theory, or a book of examples. It works best if you do both: understand the theory and try it out in real life.
What this book covers
Chapter 1, Starting Out, sets the scene by describing the embedded Linux ecosystem and the choices available to you as you start your project.
Chapter 2, Learning About Toolchains, describes the components of a toolchain and shows you how to create a toolchain for cross-compiling code for the target board. It describes where to get a toolchain and provides details on how to build one from the source code.
Chapter 3, All About Bootloaders, explains the role of the bootloader in loading the Linux kernel into memory, and uses U-Boot and Bareboot as examples. It also introduces device trees as the mechanism used to encode the details of hardware in almost all embedded Linux systems.
Chapter 4, Configuring and Building the Kernel, provides information on how to select a Linux kernel for an embedded system and configure it for the hardware within the device. It also covers how to port Linux to the new hardware.
Chapter 5, Building a Root Filesystem, introduces the ideas behind the user space part of an embedded Linux implementation by means of a step-by-step guide on how to configure a root filesystem.
Chapter 6, Selecting a Build System, covers two commonly used embedded Linux build systems, Buildroot and Yocto Project, which automate the steps described in the previous four chapters.
Chapter 7, Creating a Storage Strategy, discusses the challenges created by managing flash memory, including raw flash chips and embedded MMC (eMMC) packages. It describes the filesystems that are applicable to each type of technology.
Chapter 8, Updating Software in the Field, examines various ways of updating the software after the device has been deployed, and includes fully managed Over the Air (OTA) updates. The key topics under discussion are reliability and security.
Chapter 9, Interfacing with Device Drivers, describes how kernel device drivers interact with the hardware with worked examples of a simple driver. It also describes the various ways of calling device drivers from the user space.
Chapter 10, Starting Up – The Init Program, shows how the first user space program--init--starts the rest of the system. It describes the three versions of the init program, each suitable for a different group of embedded systems, ranging from the simplicity of the BusyBox init, through System V init, to the current state-of-the-art, systemd.
Chapter 11, Managing Power, considers the various ways that Linux can be tuned to reduce power consumption, including Dynamic Frequency and Voltage scaling, selecting deeper idle states, and system suspend. The aim is to make devices that run for longer on a battery charge and also run cooler.
Chapter 12, Learning About Processes and Threads, describes embedded systems from the point of view of the application programmer. This chapter looks at processes and threads, inter-process communications, and scheduling policies
Chapter 13, Managing Memory, introduces the ideas behind virtual memory and how the address space is divided into memory mappings. It also describes how to measure memory usage accurately and how to detect memory leaks.
Chapter 14, Debugging with GDB, shows you how to use the GNU debugger, GDB, together with the debug agent, gdbserver, to debug applications running remotely on the target device. It goes on to show how you can extend this model to debug kernel code, making use of the kernel debug stubs, KGDB.
Chapter 15, Profiling and Tracing, covers the techniques available to measure the system performance, starting from whole system profiles and then zeroing in on particular areas where bottlenecks are causing poor performance. It also describes how to use Valgrind to check the correctness of an application's use of thread synchronization and memory allocation.
Chapter 16, Real-Time Programming, provides a detailed guide to real-time programming on Linux, including the configuration of the kernel and the PREEMPT_RT real-time kernel patch. The kernel trace tool, Ftrace, is used to measure kernel latencies and show the effect of the various kernel configurations.
What you need for this book
The software used in this book is entirely open source. In almost all cases, I have used the latest stable versions available at the time of writing. While I have tried to describe the main features in a manner that is not version-specific, it is inevitable that some of the examples will need adaptation to work with later software.
Embedded development involves two systems: the host, which is used for developing the programs, and the target, which runs them. For the host system, I have used Ubuntu 16.04, but most Linux distributions will work with just a little modification. You may decide to run Linux as a guest in a virtual machine, but you should be aware that some tasks, such as building a distribution using the Yocto Project, are quite demanding and are better run on a native installation of Linux.
I chose two exemplar targets: the QEMU emulator and the BeagleBone Black. Using QEMU means that you can try out most of the examples without having to invest in any additional hardware. On the other hand, some things work better if you do have real hardware, for which, I have chosen the BeagleBone Black because it is not expensive, it is widely available, and it has very good community support. Of course, you are not limited to just these two targets. The idea behind the book is to provide you with general solutions to problems so that you can apply them to a wide range of target boards.
Who this book is for
This book is written for developers who have an interest in embedded computing and Linux, and want to extend their knowledge into the various branches of the subject. In writing the book, I assume a basic understanding of the Linux command line, and in the programming examples, a working knowledge of the C language. Several chapters focus on the hardware that goes into an embedded target board, and, so, a familiarity with hardware and hardware interfaces will be a definite advantage in these cases.
Conventions
In this book, you will find a number of text styles that distinguish between different kinds of information. Here are some examples of these styles and an explanation of their meaning. Code words in text, database table names, folder names, filenames, file extensions, pathnames, dummy URLs, user input, and Twitter handles are shown as follows: "You configure tap0 in exactly the same way as any other interface."
A block of code is set as follows:
/ {
#address-cells = <2>;
#size-cells = <2>;
memory@80000000 {
device_type = "memory";
reg = <0x00000000 0x80000000 0 0x80000000>;
};
};Any command-line input or output is written as follows:
$ mipsel-unkown-linux-gnu-gcc -dumpmachine
milsel-unknown-linux-gnuNew terms and important words are shown in bold.
Note
Warnings or important notes appear in a box like this.
Note
Tips and tricks appear like this.
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