Fallback mechanism

Introduction

This mechanism permits to test and recover from certain non-booting coreboot images.

This works by having two coreboot images in the same flash chip:

• One fallback/ image: The working image.
• One normal/ image: The image to be tested.

This feature is not widely tested on all boards. It also requires it to have a reboot_bits exported in the CMOS layout.

This also doesn't protect against human errors when using such feature, or bugs in the code responsible for switching between the two images.

Uses cases

• Test new images way faster: if the image doesn't boot it will fallback on the old known-working image and save a long reflashing procedure. Handy for bisecting faster.
• Test new images more safely: Despite of the recommendations of having a way to externally reflash, many new user don't. Still, this method is not totally foolproof.
• More compact testing setup: Since reflashing tools are not mandatory anymore, the tests can be done with less hardware, very useful when traveling.

How it works (summary)

Coreboot increments a reboot count at each boot but never clears it. What runs after coreboot is responsible for that.

That way, the count can be cleared by the OS once it's fully booted.

If a certain threshold<ref>Defined by CONFIG_MAX_REBOOT_CNT, typically 3</ref> is attained at boot, coreboot will boot the fallback image.

Using it

Prerequisites

• Make sure that your fallback/ image has the fallback switch mecanism enabled
• Build the fallback image as you would build an image usually

Building the coreboot.rom image

Building normal/ (normal.sh)

This scrpit takes an existing coreboot image path as argument.

#!/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

image="$1"
if [ $# -ne 1 ] ; then
	echo "Usage $0 <image>"
	exit 1
fi

die()
{
  echo "$1 Failed"
  exit 1
}

cbfs_add()
{
  name=$1
  file=$2
  cbfs_remove ${name}
  ./util/cbfstool/cbfstool ./build/coreboot.rom add -n ${name} -t raw -f ${file}
}

cbfs_remove()
{
  name=$1
  ./util/cbfstool/cbfstool ./build/coreboot.rom remove -n ${name}
}

cbfs_reuse_payload()
{
  ./util/cbfstool/cbfstool ./build/coreboot.rom extract -f ./build/payload.elf -n fallback/payload
  ./util/cbfstool/cbfstool ./build/coreboot.rom add -f ./build/payload.elf -n normal/payload -t payload
}

check_config()
{
  grep "^CONFIG_CBFS_PREFIX=\"normal\"$" .config > /dev/null || die "Not using normal cbfs prefix"
  grep "^CONFIG_UPDATE_IMAGE=y$" .config > /dev/null || die "Not using CONFIG_UPDATE_IMAGE"
  grep "^CONFIG_SKIP_MAX_REBOOT_CNT_CLEAR=y" .config > /dev/null || die "Not using CONFIG_SKIP_MAX_REBOOT_CNT_CLEAR"
}

check_config
make oldconfig || die "make oldconfig"
make clean || die "clean"
mkdir build/ || die "mkdir build"

cp ${image} ./build/coreboot.rom || die "cp"

cbfs_remove normal/romstage
cbfs_remove normal/ramstage
cbfs_remove normal/payload
cbfs_remove normal/dsdt.aml
cbfs_remove config
cbfs_remove revision

# it now adds it automatically
cbfs_remove etc/ps2-keyboard-spinup
 
make || die "make"

# uncomment if you want to reuse fallback's payload
# cbfs_reuse_payload

./util/cbfstool/cbfstool ./build/coreboot.rom print

OS configuration examples

The configurations below assume that the user wants to keep booting on normal/ if the boot doesn't fail.

Example scripts

The most simple way to do it is to run some nvramtool commands, they are described in the scripts below. set-normal-0.sh has to be run:

• After the boot is completed and is declared a success.
• After the resuming is completed.

The way to make them run at boot and after suspend is not described here yet.

set-fallback-1.sh

#!/bin/sh
nvramtool -w boot_option=Fallback
nvramtool -w last_boot=Fallback
nvramtool -w reboot_bits=1

set-normal-0.sh

#!/bin/sh
nvramtool -w boot_option=Normal
nvramtool -w last_boot=Normal
nvramtool -w reboot_bits=0

get-nvram.sh

#!/bin/sh
nvramtool -a | grep -e boot_option -e last_boot -e reboot_bits

With systemd

Systemd setup

Requirements:

• nvramtool has to be in the path.

Limitations:

• This setup doesn't needs to run that systemd unit when resuming from suspend to ram, but it's not described yet here.

The unit file below has to be activated with:

systemctl enable coreboot-booted-ok
systemctl start coreboot-booted-ok

/etc/systemd/system/coreboot-booted-ok.service:

#  This file is not part of systemd.
#
#  this file is free software; you can redistribute it and/or modify it
#  under the terms of the GNU Lesser General Public License as published by
#  the Free Software Foundation; either version 2.1 of the License, or
#  (at your option) any later version.

[Unit]
Description=Tell coreboot that the computer booted fine.
DefaultDependencies=no
Wants=display-manager.service
After=display-manager.service

[Service]
Type=oneshot
RemainAfterExit=yes
ExecStart=/usr/local/sbin/nvramtool -w boot_option=Normal
ExecStart=/usr/local/sbin/nvramtool -w last_boot=Normal
ExecStart=/usr/local/sbin/nvramtool -w reboot_bits=0

[Install]
WantedBy=multi-user.target

Current limitations

• Example script exist only for systemd. Still, they are trivial to adapt to other init systems.
• Some issues can arrise when the nvram layout is not the same between normal/ and fallback/
• The number of failed boot is fixed at compilation time.
• In order to fully boot, some boards do reset conditionally during the boot process resulting in a non-predictable increment of the boot count.
• Payloads sometime have fixed default locations when loading things from cbfs:
  • When using grub as a payload, grub.cfg is at etc/grub.cfg by default, so if you want to test grub as a payload, remember to change grub.cfg's path not to interfer with the fallback's grub configuration.
  • Changing the path of what SeaBIOS loads from cbfs is probably configurable with SeaBIOS cbfs symlinks but not yet tested/documented with the use of the fallback mecanism
  • The user may wrongly identify which image booted, and because of that, end up reflashing a non-working image.

references

<references/>