The wiki is being retired!

Documentation is now handled by the same processes we use for code: Add something to the Documentation/ directory in the coreboot repo, and it will be rendered to https://doc.coreboot.org/. Contributions welcome!

Introduction

The fallback mecanism permits to have 2 different romstage,ramstage,payload in the same images under a different prefix. The switch between both can be governed by an nvram configuration parameter.

Howto

Mandatory configuration (in make menuconfig)

Pseudo-diff

$ diff -u defconfig-fallback defconfig-normal
--- defconfig-fallback	2013-10-26 22:27:19.471326092 +0200
+++ defconfig-normal	2013-10-26 22:26:44.471328732 +0200

-CONFIG_LOCALVERSION="fallback-mode"
-CONFIG_CBFS_PREFIX="fallback"
+CONFIG_LOCALVERSION="normal-mode"
+CONFIG_CBFS_PREFIX="normal"
-# CONFIG_UPDATE_IMAGE is not set
+CONFIG_UPDATE_IMAGE=y

Compilation

Build script

#!/bin/sh
# In the cases where this work is copyrightable, it falls under the GPLv2
# or later license that is available here:
# https://www.gnu.org/licenses/gpl-2.0.txt

die() {
	echo
	echo "!!!! Compilation failed !!!!"
	exit 1
}

success() {
	echo
	echo "!!!! Compilation finished !!!!"
	echo
}

separator() {
	echo
	echo "!!!! First prefix compilation finished !!!!"
	echo
}

make clean || die

#fallback image
cp defconfig-fallback .config  || die
make  || die
./build/cbfstool ./build/coreboot.rom add -f .config -n config-fallback -t raw  || die

#because it will be re-included it in the second build...
#./build/cbfstool ./build/coreboot.rom remove -n etc/ps2-keyboard-spinup  || die
./build/cbfstool ./build/coreboot.rom remove -n pci8086,109a.rom  || die

cp ./build/coreboot.rom ./build-save/coreboot.rom.fallback || die
make clean || die
mkdir -p build/
cp ./build-save/coreboot.rom.fallback ./build/coreboot.rom || die
separator

#second prefix's build.

#normal image
cp defconfig-normal .config  || die
make  || die
./build/cbfstool ./build/coreboot.rom add -f .config -n config-normal -t raw  || die

success

Old Howto (will be replaced)

build the coreboot image as usual, it will produce an image in build/coreboot.rom
After the first build run:

make menuconfig

Optionally change the payload.
Go in

General setup  --->

Change:

(fallback) CBFS prefix to use

To:

(normal) CBFS prefix to use

Go back to the main menu and select:

Architecture (x86)  --->

select the following option:

[*] Update existing coreboot.rom image

Exit and save and rebuild...

The image will then have fallback and normal:

Name                           Offset     Type         Size
cmos_layout.bin                0x0        cmos_layout  1776
pci1002,9710.rom               0x740      optionrom    60928
fallback/romstage              0xf580     stage        92823
fallback/coreboot_ram          0x26080    stage        66639
fallback/payload               0x36540    payload      54976
config                         0x43c40    raw          4455
normal/romstage                0x44e00    stage        92823
normal/coreboot_ram            0x5b8c0    stage        68820
normal/payload                 0x6c600    payload      159949
(empty)                        0x93700    null         442136

Fallback mechanism