1. Overview of the Porting Process

Basic Tasks

U-Boot is a highly portable boot loader, supporting many high-end ARM SoCs. It contains a vast array of features and drivers to help speed up the porting process.  When porting U-Boot to a new board, follow these basic steps:

  1. Add support for your SoC. U-Boot supports many common SoCs, and in many cases, a new SoC is a variant of an existing one that can serve as a starting point for the port.
  2. Add a board configuration file and a board implementation file containing code specific to your board.
  3. Add or modify any drivers you need for your platform.
  4. Bring in the kernel device tree file if available (or write a new device tree).

This guide describes these steps in some detail, with a focus on Chrome OS-specific implementations.

Note:  The following information was written for Tegra in 2011 and is somewhat out of date.

Development Flow

Chromium OS uses U-Boot as its boot loader on ARM and x86. This page describes the development flow, requirements for commits submitted to the project, upstreaming responsibilities and Chromium OS-specific features.

Before reading this document, please see http://www.denx.de/wiki/U-Boot and at the very least, read:

Except during a rebase period, Chromium OS has a single U-Boot repository, called ‘u-boot.git’, containing all U-Boot source code. There is no
split between different SOCs, no split for Chromium OS-specific code, no split at all. We deal with the different types of commits using commit tags.


  • Formalized commit structure rules (see ‘Commit Rules’ below)
  • All Chromium OS source is in its own subdirectory (see ‘Chromium Source Subdirectory’ below)
  • Encourage compliance with pre-submit checks and reverts
  • Tag every commit to indicate its intended destination
  • Each commit must be producing a source tree which builds and runs (bisectability)


We plan to upstream most or all of our code. Any code that you write will likely either be dropped or sent upstream. Please consider when writing code that it should fit with U-Boot conventions
Let’s call the person who is upstreaming a particular feature the ‘canoe captain’.
  • The canoe captain (with upstreaming responsibility) lies with:
    • Committer for patches against upstream code
    • SOC Vendor for patches against vendor code
  • Clearly mark commits that are destined for upstream according to destination
  • Open an ‘upstream needed’ tracking bug when you begin upstreaming work. There should be one bug per group of related commits. For example ‘upstream I2C driver for Tegra’.
  • Patches should be sent upstream within one month
  • Canoe captain is responsible for rebasing those commits which are not upstream
  • Code should be accepted upstream in the next merge cycle (i.e. within 3 months)
  • Should upstream activity produce the need to adjust or rewrite the commit, then this responsibility lies with the canoe captain.

Code Review

  • Add 2-3 reviewers, including 1-2 original code authors
    • For vendor code, at least one reviewer or the author should be with the vendor company
    • At least one reviewer should be a Chromium OS U-Boot committer
    • Vendor code requires an LGTM from the both types of reviewers; other code just needs an LGTM from the latter
  • Please first carefully check you have followed the rules on commit tags, BUG and TEST
  • Initial reviews should be completed within one day during the working week
    • If not possible, then please add a comment to the commit letting the author know there will be a delay, and offer an alternative
  • Rebases and minor don’t need a fresh LGTM. So if you get an LGTM on patch 3, and just fix requested nits or do a rebase to get patch 4, you can push it.

Code Refactors

  • We encourage discussion of planned refactors using the issue tracker (crbug.com). Those which are in upstream code should be discussed on the U-Boot mailing list


Most testing is currently manual. It is important that people can repeat your test steps later without a deep knowledge of your environment, board, commit and mind.

You may find the crosfw script useful for building and testing U-Boot.

Mandatory testing:
  • Build and boot through to kernel (verified or non-verified) on your chosen hardware
    • Check that you change has not affected operation of peripherals related to your change
    • Add a note that you did this (and the result) in your TEST= description (see below)
  • If you change modifies code accessible from another architecture, build for that architecture too
    • So if your chosen hardware is ARM, but you are not changing ARM-specific code, you should build for x86
    • Add a note about this to TEST=

Suggested testing:

  • If changing generic code, you should ensure that U-Boot continues to build for all targets (run MAKEALL)
    • Add a note about this to TEST=

Commit Tags

All commits should have exactly one tag from the following table at the start of the subject line. The tag indicates the intended destination for the commit. Note that standard upstream commit tags are encouraged also (i2c, usb, net, lcd, spi) and these follow the primary tag. Tags are always lower case.

Commits cannot normally span more than one of the paths below, so commits in multiple areas must be split into different commits. The idea to keep in mind is that this is like having separate repositories, but with less admin/productivity overhead.

Where a change requires that (say) Chromium, Tegra and Samsung code must change at the same time to avoid a build breakage, the ‘split’ tag should be used first, and the rest in alphabetical order as in ‘split: chromium: exynos: tegra: fiddle with something’.

Tag: Meaning Canoe Captain Path
cros: Local patches, will not conceivably go upstream. Use for verified boot mainly - cros/
tegra: Tegra SOC, board and driver code Nvidia arch/arm/cpu/armv7/tegra*/
exynos: Samsung SOC, board and driver code Samsung arch/arm/cpu/armv7/exynos*/
arm: ARM-generic patches, committer mans the canoe Committer arch/arm
(but not in an soc subdir)
x86: x86-generic patches, committer mans the canoe Committer
upstream: Pulled in from upstream - any
drop: Urgent / unclean / factory patches which we will drop on next rebase - any
split: Indicates a commit which must be split to send upstream

gen: Generic (not architecture-specific) commit which should go upstream Committer any. Note that when sending upstream this tag is dropped
 config:  Change to a board configuration file
 -  include/configs/

Pre-upload Checks

Before uploading a commit for review, please do the following checks:

  • The commit is ‘checkpatch’ clean (repo upload checks this)
  • The commit has tags indicated its upstream destination
  • The commit has a valid bug referenced, by a tag on a line by itself
  • The commit has a valid test case.


  • For public bugs: BUG=chromium-os:12435
  • For private bugs: BUG=chrome-os-partner:1234
  • It is possible to use BUG=none in some cases. We should use this rarely. Examples where is it useful are when:
    • The commit is a small clean-up not related to a particular subsystem. Examples are fixing a comment or code style
    • The commit fixes compilation warnings
  • In most other cases, if you cannot find a bug to reference, you should file a new public bug and reference that.


This should be detailed enough that another person without particular knowledge of your code can run the procedure you detail, and verify your commit. The information should specific. Examples are:

  • TEST=build and run on daisy; run ‘bootp’ command, see that it finds an IP address
  • TEST=manual
         . after enabling configuration necessary for this code to compile
           in, added a printout statement in main.c to report the clock
           frequency and to print a few strings at get_tbclk() tick
           intervals. The clock frequency was correctly reported at 1800 MHz
           and the printouts came out one second apart.
  • TEST=boot from SD card; run sd_to_spi; see that it completes; then boot from
       SPI and see that the U-Boot timestamp is correct.
  • TEST=build okay and boot on Daisy

Some bad examples of test information are:

  • TEST=tested on daisy (how? need detail on what was done and what result as obtained)
  • TEST=none (surely you are least built it, did you run it on any board?)
  • TEST=verified that it worked well (how?)

Chromium Source Subdirectory

Our local code commits can be broken into several areas, but almost all code is in a ‘cros/’ subdirectory.  Here is the current split:

Directory Used for Previously
cros/ All Chromium OS commits Various
cros/tegra Implementation of Chrome OS low-level API on Tegra SOCs board/nvidia/chromeos
cros/exynos Implementation of Chrome OS low-level API on Exynos SOCs -
cros/coreboot Implementation of Chrome OS low-level API on Coreboot board/chromebook-x86/chromeos
cros/lib Chrome OS library functions lib/chromeos
cros/vboot Verified boot exported functions lib/vbexport
cros/cmd Chrome OS commands common/cmd_....c
cros/include Chrome OS headers include/chromeos

Other sections in the U-Boot Porting Guide

  1.  Overview of the Porting Process (this page)
  2.  Concepts