When you turn on an embedded device such as a Chromebook, the firmware is the first software that runs. It sets up the machine so that it can load the operating system and then transfers control to the OS. This general pattern applies to any architecture, but there are differences depending on the exact system architecture. For example, on ARM SoCs, it is common to have a boot ROM within the SoC that is capable of booting from various media and may set up basic clocks and peripherals.
On x86 systems, only the boot system is in SPI flash, and the firmware must deal with every aspect of setting up the machine. Typically, a first stage firmware such as Coreboot (not covered in this guide) is used on x86 to deal with these complexities. On x86, part of the firmware stays resident even after the OS runs, whereas on ARM and most other architectures, the firmware is no longer in memory once control transfers to the OS.
The firmware sets up the basic peripherals such as the keyboard, display, mass storage, security chip (TPM), and SPI flash for its own use while it runs the steps to set up the machine. In Chrome OS, the firmware can be updated, so the system first checks whether an update exists and, if so, it loads the update and jumps to execute it. In normal cases, the OS is available and is loaded and started. If a problem exists (for example, the OS is corrupt), the firmware enters recovery mode, which allows the user to insert media with a new OS. In developer mode, users can bypass Chrome OS security, or even the entire OS.
U-Boot (Universal Boot) is a useful basis for firmware because it has wide architecture support and contains many drivers and subsystems. Chrome OS uses an upstream version of U-Boot and adds its own verified boot infrastructure, which is linked to U-Boot. Many of the drivers and features in U-Boot are used by verified boot. Some of the advantages of U-Boot are
U-Boot was started in 2002 by Wolfgang Denk, who based his work on previous Power PC development (PPCBoot) by Magnus Damm. Wolfgang Denk is the primary maintainer of U-Boot source code (see www.denx.de). U-Boot is available in the Git tree. Patchwork is used to manage patches to the tree. All development is done in patches sent to the u-boot mailing list.
This guide describes how to port U-Boot to a new Chrome OS platform. Although it specifically describes using U-Boot with Chrome OS, the general process, information, and examples apply to using U-Boot with any Chromium OS. The guide provides a conceptual framework for U-Boot porting tasks as well as task-oriented guidance for the porting process.
Appendix A. Using nv-U-Boot on the Samsung Chromebook