Chapter 3. Bootloaders

In this chapter, you will be presented with one of the most important components necessary for using a Linux system in an embedded environment. Here, I am referring to the bootloader, a piece of software that offers the possibility of initializing a platform and making it ready to boot a Linux operating system. In this chapter, the benefits and roles of bootloaders will be presented. This chapter mainly focuses on the U-Boot bootloaders, but readers are encouraged to have a look at others, such as Barebox, RedBoot, and so on. All these bootloaders have their respective features and there isn't one in particular that suits every need; therefore, experimentation and curiosity are welcome when this chapter. You have already been introduced to the the Yocto Project reference in the last chapter; hence, you will now be able to understand how this development environment works with various bootloaders, and especially the ones available inside a Board Support Package (BSP).

The main purpose of this chapter is to present the main properties of embedded bootloaders and firmware, their booting mechanisms, and the problems that appear when firmware is updated or modified. We will also discuss the problems related to safety, installation, or fault tolerance. With regard to bootloader and firmware notions, we have multiple definitions available and a number of them refer to traditional desktop systems, which we are not interested in.

A firmware usually represents a fixed and small program that is used on a system to control hardware. It performs low-level operations and is usually stored on flash, ROM, EPROM, and so on. It is not changed very often. Since there have been situations where this term has confused people and was sometimes used only to define hardware devices or represent data and its instructions, it was avoided altogether. It represents a combination of the two: computer data and information, along with the hardware device combined in a read-only piece of software available on the device.

The bootloader represents the piece of software that is first executed during system initialization. It is used to load, decompress, and execute one or more binary applications, such as a Linux kernel or a root filesystem. Its role involves adding the system in a state where it can execute its primary functions. This is done after loading and starting the correct binary applications that it receives or has already saved on the internal memory. Upon initializing, the hardware bootloader may need to initialize the phase-locked loop (PLL), set the clocks, or enable access to the RAM memory and other peripherals. However, these initializations are done on a basic level; the rest are done by kernels drivers and other applications.

Today, a number of bootloaders are available. Due to limited space available for this topic, and also the fact that their number is high, we will only discuss the most popular ones. U-Boot is one of the most popular bootloaders available for architectures, such as PowerPC, ARM, MIPS, and others. It will constitute the primary focus of this chapter.