The bootloader is the second element of embedded Linux. It is the part that starts the system up and loads the operating system kernel. In this chapter, I will look at the role of the bootloader and, in particular, how it passes control from itself to the kernel using a data structure called a device tree, also known as a flattened device tree or FDT. I will cover the basics of device trees, so that you will be able to follow the connections described in a device tree and relate it to real hardware.
I will look at the popular open source bootloader, U-Boot, and show you how to use it to boot a target device, and also how to customize it to run on a new device, using the BeagleBone Black as an example. Finally, I will take a quick look at Barebox, a bootloader that shares its past with U-Boot, but which has, arguably, a cleaner design.
In this chapter, we...
