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Mastering Embedded Linux Programming
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The examples in this book are intended to be generic, but to make them relevant and easy to follow, I have had to choose specific hardware. I have chosen three exemplar devices: the Raspberry Pi 4, the BeagleBone Black, and QEMU. The first is by far the most popular Arm-based single board computer on the market. The second is a widely available and cheap development board that can be used in serious embedded hardware. The third is a machine emulator that can be used to create a range of systems that are typical of embedded hardware. It was tempting to use QEMU exclusively, but, like all emulations, it is not quite the same as the real thing. Using the Raspberry Pi 4 and BeagleBone Black, you have the satisfaction of interacting with real hardware and seeing real LEDs flash. While the BeagleBone Black is several years old now, it remains open source hardware (unlike the Raspberry Pi). This means that the board design materials are freely available for anyone to build a BeagleBone Black or derivative into their products.
In any case, I encourage you to try out as many of the examples as you can, using either of these three platforms, or indeed any embedded hardware you may have to hand.
At the time of writing, the Raspberry Pi 4 Model B is the flagship tiny, dual-display, desktop computer produced by the Raspberry Pi Foundation. Their website is
https://raspberrypi.org/. The Pi 4's technical specs include the following:
In addition, there is a 40-pin expansion header for which there are a great variety of daughter boards, known as HATs (Hardware Attached on Top), that allow you to adapt the board to do many different things. However, you will not need any HATs for the examples in this book. Instead, you will make use of the Pi 4's built-in Wi-Fi and Bluetooth (which the BeagleBone Black lacks).
In addition to the board itself, you will require the following:
Next is the BeagleBone Black.
The BeagleBone and the later BeagleBone Black are open hardware designs for a small, credit card-sized development board produced by CircuitCo LLC. The main repository of information is at https://beagleboard.org/. The main points of the specifications are as follows:
In addition, there are two 46-pin expansion headers for which there are a great variety of daughter boards, known as capes, which allow you to adapt the board to do many different things. However, you do not need to fit any capes for the examples in this book.
In addition to the board itself, you will require the following:
In addition to the above, Chapter 12, Prototyping with Breakout Boards, also requires
the following:
QEMU is a machine emulator. It comes in a number of different flavors, each of which can emulate a processor architecture and a number of boards built using that architecture. For example, we have the following:
qemu-system-arm
: 32-bit Armqemu-system-mips
: MIPSqemu-system-ppc
: PowerPCqemu-system-x86
: x86 and x86_64For each architecture, QEMU emulates a range of hardware, which you can see by using the -machine help
option. Each machine emulates most of the hardware that would normally be found on that board. There are options to link hardware to local resources, such as using a local file for the emulated disk drive.
Here is a concrete example:
$ qemu-system-arm -machine vexpress-a9 -m 256M -drive file=rootfs.ext4,sd -net nic -net use -kernel zImage -dtb vexpress- v2p-ca9.dtb -append "console=ttyAMA0,115200 root=/dev/mmcblk0" -serial stdio -net nic,model=lan9118 -net tap,ifname=tap0
The options used in the preceding command line are as follows:
-machine vexpress-a9
: Creates an emulation of an Arm Versatile Express development board with a Cortex A-9 processor-m 256M
: Populates it with 256 MiB of RAM-drive file=rootfs.ext4,sd
: Connects the SD interface to the local file rootfs.ext4
(which contains a filesystem image)-kernel zImage
: Loads the Linux kernel from the local file named zImage
-dtb vexpress-v2p- ca9.dtb
: Loads the device tree from the local file vexpress-v2p-ca9.dtb
-append "..."
: Appends this string as the kernel command line-serial stdio
: Connects the serial port to the terminal that launched QEMU, usually so that you can log on to the emulated machine via the serial console-net nic,model=lan9118
: Creates a network interface-net tap,ifname=tap0
: Connects the network interface to the virtual network interface, tap0
To configure the host side of the network, you need the tunctl
command from the User Mode Linux (UML) project; on Debian and Ubuntu, the package is named uml-utilites
:
$ sudo tunctl -u $(whoami) -t tap0
This creates a network interface named tap0
that is connected to the network controller in the emulated QEMU machine. You configure tap0
in exactly the same way as any other interface.
All of these options are described in detail in the following chapters. I will be using Versatile Express for most of my examples, but it should be easy to use a different machine or architecture.