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Das U-Boot is the boot loader used by Chromium OS on ARM. This page provide some basic information about our use of it. See the official U-Boot page for more information.


Chrome OS uses a single variant of U-Boot for all Tegra2 boards and all purposes. The same applies for Exynos, although this is a different image from Tegra2. This is a departure from the previous approach of having different variants for each purpose (stub, developer, recovery, normal, flasher) and each board (Seaboard, Aebl, Kaen). Both of these changes are enabled by run-time configuration in U-Boot. Traditionally, U-Boot's configuration has been set by CONFIG_ options. We still use CONFIG_ options to select which functionality / drivers is included in an image. However, we avoid using CONFIG_ options that simply configure that functionality. For example, CONFIG_USB_EHCI is used to include the EHCI functionality for USB, but we do not use CONFIG_TEGRA2_USB0to specify the physical address of the first USB port. We use flattened device tree (FDT) to hold this run-time configuration. FDT consists of a number of nodes each with a list of properties. It is easy to add new nodes and properties, and a libfdt library exists for reading and writing them. For more information about FDT, see the specification document before you continue reading here. You might also find the U-Boot doc/README.fdt-control file useful. Note: as of March 2012 we still use 'seaboard' (rather than 'generic') as the name of our generic Tegra2 board. This may change in the future.


We have various types of nodes in the FDT and they can be categorized as follows:

FDT-based configuration is enabled with the CONFIG_OF_CONTROL option. FDT files are stored in board/nvidia/dts or board/samsung/dts. FDT can be packaged with U-Boot in two ways:

We use the second approach for Chrome OS, since it allows cros_bundle_firmware to create an image for any board without needing to look around inside U-Boot to change the FDT.


Initially, we used a library called fdt_decode to decode FDT blobs on behalf of different classes of drivers. That approach may become partially useful in future, but we decided to drop it for upstream. There is a still an fdtdec library which helps a lot, but drivers must decode their own nodes.

Header Files

U-Boot uses header files for configuration. Since the FDT change this has become very simple. We have:


TODO: Add description of cros_choose_profile to a more generic page when one is created. In order to simplify the handling of different boards and board variants, a cros_choose_profile tool is used:

cros_choose_profile --help
USAGE: /usr/bin/cros_choose_profile [flags] args
  --board:  The name of the board to set up. (default: 'tegra2_seaboard')
  --build_root:  The root location for board sysroots. (default: '/build')
  --board_overlay:  Location of the board overlay. (default: '')
  --variant:  Board variant. (default: '')
  --profile:  The portage configuration profile to use. (default: '')
  -h,--[no]help:  show this help (default: false)

The cros_choose_profile utility selects which board and profile we are using. It is normally run by the setup_board script.


The main U-Boot-related ebuilds are in src/third_party/chromiumos-overlay/sys-boot:

Add New Boards to Ebuilds

The variables U_BOOT_CONFIG_USE and U_BOOT_FDT_USE are currently used to specific the FDT file for U-Boot. These variables are set in the make.defaults file for the board. For example, for tegra2 boards this is in src/overlays/overlay-tegra2/profiles/base/make.defaults.

There is now no need to figure out the FDT or config in the ebuild - you can just use these variables directly.

To check the settings for your board:

emerge-tegra2_seaboard --info |tr ' ' '\n' |grep U_BOOT

Building U-Boot

There are two main U-Boot repositories on the server: the stable u-boot.git and the unstable u-boot-next.git. Normally you will use the stable repo.

From ebuild

U-Boot is built from an ebuild like this:

emerge-${BOARD} chromeos-u-boot

For example for the Seaboard:

emerge-tegra2_seaboard  chromeos-u-boot

This builds it from scratch which takes about 40s on a fast machine.

Incremental build

If you have run cros_workon start chromeos-u-boot (followed by repo sync u-boot) then you can use the incremental build option which takes perhaps 12s.

bin/cros_workon_make chromeos-u-boot

This is the recommended approach since it sets up the options for your board automatically. If you look in the output of this command you should see the make command it is using. If you really want to, you can use this command, which takes the build time down to around 6s.

Build script

NOTE: Advanced U-Boot coders only! This can reduce an incremental build to about 1s on a fast machine (16 core). If you are working in u-boot and constantly flashing U-Boot for different Tegra2 boards then you might find this script useful. Run it within your U-Boot source tree. Beware this is not for beginners. Before you use this script, read it through and compare it with the U-Boot ebuild. You can select an FDT to use to change the board. Note: this script could be enhanced to output the resulting u-boot binary to a subdirectory with make ...O=<target_dir>.

#! /bin/sh
# Script to build U-Boot without an ebuild and with upstream config files,
# then write it to SPI on your board.
# It builds for seaboard by default - use -b to change the board.
# For an incremental board, run with no arguments. To clean out and reconfig,
# pass the -c flag.
if grep -sq Gentoo /etc/lsb-release; then
    # location of src within chroot
    # Change this to point to your source dir
    export PATH=${BIN}:${PATH}
    echo $PATH
. "${SRC_ROOT}/scripts/lib/shflags/shflags"
DEFINE_boolean config "${FLAGS_FALSE}" "Reconfigure and clean" c
DEFINE_boolean good "${FLAGS_FALSE}" "Use known-good U-Boot" g
DEFINE_string board "seaboard" \
    "Select config: chromeos_tegra2_<board>_<config>_config" b
DEFINE_boolean separate "${FLAGS_TRUE}" "Separate device tree" s
DEFINE_boolean verified "${FLAGS_FALSE}" "Enable verified boot" V
DEFINE_string dt "seaboard" \
    "Select device tree: seaboard, kaen, aebl" d
DEFINE_boolean build "${FLAGS_TRUE}" "Build U-Boot" B
# Parse command line
FLAGS "$@" || exit 1
eval set -- "${FLAGS_ARGV}"
if [ "$FLAGS_build" -eq "${FLAGS_FALSE}" ]; then
    if [ "$FLAGS_config" -eq "${FLAGS_TRUE}" ]; then
        echo "Warning: disabled -c since -B was given"
cd $SRC_ROOT/third_party/u-boot/files
if [ -n "${FLAGS_ARGV}" ]; then
if [ "$FLAGS_config" -eq "${FLAGS_FALSE}" ]; then
    # Get the correct board for cros_write_firmware
    board=$(grep BOARD include/ | awk '{print $3}')
    if [ -z "${FLAGS_board}" -a "$board" != "${FLAGS_board}" ]; then
        echo "Warning: U-Boot is configured for $board, " \
            "forcing reconfigure"
BASE+="-s "
BASE+="-j15 "
BASE+="--no-print-directory "
BASE+="HOSTCC=$gcc HOSTSTRIP=true VBOOT=/build/tegra2_${FLAGS_board}/usr "
if  [ "$FLAGS_separate" -eq "${FLAGS_TRUE}" ]; then
BASE+="DEV_TREE_SRC=tegra2-${FLAGS_dt} "
if [ "${FLAGS_verified}" -eq "${FLAGS_TRUE}" ]; then
# echo $BASE
if [ "$FLAGS_config" -eq "${FLAGS_TRUE}" ]; then
    make $BASE distclean
    make $BASE ${FLAGS_board}_config || { echo "Make failed"; exit 1; }
    make $BASE dep
if [ "$FLAGS_good" -eq "${FLAGS_TRUE}" ]; then
    if [ "$FLAGS_build" -eq "${FLAGS_TRUE}" ]; then
        make $BASE $target || { echo "Make failed"; exit 1; }
if  [ "$FLAGS_separate" -eq "${FLAGS_TRUE}" ]; then
    cat $IMAGE u-boot.dtb >u-boot.bin.dtb
cros_bundle_firmware -w -u u-boot.bin -s -b tegra2_aebl --bootsecure \
    --bootcmd vboot_twostop --bmpblk ${bmp}
#--add-config-int silent_console 1

Flashing U-Boot

The method here depends on the platform you are using. Please see the documentation on cros_bundle_firmware also.

NVidia Tegra2x

There is a firmware writing utility available which can reflash U-Boot on your board. For example, for Seaboard you can write the recovery U-Boot with:

# Connect USB A-A cable to top USB port, reset with recovery button held down
If you have a T20-seaboard (no camera attachment on top), please build/flash your firmware with:
USE=tegra20-bct emerge-tegra2_seaboard tegra-bct chromeos-bootimage
cros_write_firmware -b tegra2_seaboard \
                    -i /build/tegra2_seaboard/u-boot/legacy_image.bin
If you have a T25-seaboard (black camera box above the screen), please build/flash your firmware with:
emerge-tegra2_seaboard tegra-bct chromeos-bootimage
cros_write_firmware -b tegra2_seaboard \
                    -i /build/tegra2_seaboard/u-boot/legacy_image.bin

The BCT controls early memory initialization on Tegra2 CPUs. It is something that is customized for every variant of every board.

We need a different BCT for T20 and T25 Seaboards. Up until this point we have just used an old version of the T20 BCT for both products. This is not such a great thing for various reasons, one of which is problems with DVFS. Since we are planning to enable DVFS realsoonnow(TM), you will need this change.

One thing to note: you probably shouldn't put "official" builds onto your T20 Seaboard, since they will (I think) clobber the firmware with the T25 version (the default). You've been warned.

Configuring U-Boot Environment

U-boot accepts user commands to set up its execution environment. The problem with the current implementation is that at the default console baud rate of 115200 u-boot is not running fast enough and the UART gets easily overrun in case a user tries pasting text through terminal connected to the console.

The attached expect script allows to work around the problem. The script is just an example, which can be used it as follows. Start its execution on the workstation which has a USB port connected to the target console

./ttyusb <file with u-boot script>

and then hit reset on the target (if it is not already in u-boot console mode). <file with u-boot script> is a text file including a set of u-boot commands, one per line. Lines starting with # are ignored. ttyusb will send the commands to the console, character by character, making sure that all characters are echoed and the CrOS> prompt is returned after each command. Once all commands are executed, the terminal session becomes the console terminal for the target. To exit the console session type ^a. The serial port device is hardcoded in the script to /dev/ttyUSB1, edit your copy of to get a different tty device used, if necessary.

Memory Map

Tegra 2x

Current (as of 2011-09-13):

We have 0x4000 0000 (1GB) of memory in our machines. RAM starts at physical address 0x0.

Address 0x Approx. Size Defined where Notes
0000 0100 0.25KB 1KB gd->bd->bi_boot_params (TODO: used booting non-FIT images?) Kernel boot parameters area (ATAGs)
0000 8000 32KB 4MB zreladdr Not used by u-boot, but good to keep in mind that this is the location the kernel will eventually decompress / relocate itself to (TODO: will it put any initramdisk here too?). It's important there's room so the decompressed kernel won't clobber the FDT.
0010 0000 1MB 8MB CHROMEOS_KERNEL_LOADADDR CHROMEOS_KERNEL_BUFSIZE vboot loads the FIT image here; FDT and zImage will then be relocated elsewhere
0040 8000 4MB + 32KB 4MB CONFIG_LOADADDR Area to load scripts or kernel uImage (prior to decompression); used by legacy u-boot. TODO: Why not use 0x00100000?
00e0 8000 14MB + 32KB 1MB TEXT_BASE Start of U-Boot code region. Data appears immediately after
01ff c000 32MB - 16KB 16KB CONFIG_SYS_BOOTMAPSZ appears to control the 32MB (can be overridden by environment variables). The 16KB comes from the padded size of the kernel's FDT (as the FDT grows, this should move). U-boot ends up relocating the kernel's copy of the FDT to here.
0200 0000 32MB 512KB CONFIG_CHROMEOS_RAMOOPS_RAM_START CONFIG_CHROMEOS_RAMOOPS_RAM_SIZE Kernel preserved area (kcrashmem). This is preserved_start in the kernel, or kcrashmem= environment variable in U-Boot.
0300 0000 48MB ?MB kernel_fdt.its Image is copied here by u-boot before jumping to.
1c40 6000 512MB - 192MB + 4MB + 24KB 8KB TEGRA_LP0_ADDR TEGRA_LP0_SIZE Used in suspend / resume.
3768 0000 1GB - ~144MB varies; ~2MB frame-buffer lcd_get_size()