Lenovo T420 Coreboot W/Raspberry Pi

Coreboot is an open source bios replacement. This guide will describe the steps needed to install it on a Lenovo T420.

Before you start you should be comfortable using a Linux terminal as well as disassembling your laptop.

There is a chance that this will brick your laptop you do this at your own risk.

• Ponoma 5250 Test Clip - For connecting to the bios chip.
• Female to Female Breadboard Jumper Cables - Also known as Dupont wires.
• Phillips Screwdriver
• Small Pliers, or 5.0 mm hex bit.
• Thermal compound
• Isopropyl alcohol
• Cotton Swabs

• Lenovo T420
• Computer running Linux. "Main PC"
• Raspberry Pi(3 or 4) - running the latest version or Raspberry Pi OS - Instructions on installing can be found here.
• T420 Hardware Maintenance Manual

It is a good idea to update the Embedded Controller to the latest version. The easiest way to do this is install the latest version of the factory bios. Coreboot is unable to touch the EC. You will be unable to update it after flashing unless you revert to the factory bios.

In order to read/write to the bios chip you need to enable some kernel modules.

Access the raspberry pi config utility.

sudo raspi-config

Under interface options enable :

• P2 SSH - if you will be running the pi headless
• P4 SPI
• P5 I2C
• P8 Remote GPIO - If using ssh to connect to the pi

First thing to do is install the dependencies needed to build coreboot.

For a Debian based system

sudo apt install git build-essential gnat flex bison libncurses5-dev wget zlib1g-dev

For an Arch based system

sudo pacman -S base-devel gcc-ada flex bison ncurses wget zlib git

Make a directory in your home dir to work in. For this example I will be calling it 'work'. You will also want a directory to store the factory images. I will call that directory 'roms' You can do this in one line to save time

mkdir -p ~/work/roms

Move into the work directory

cd ~/work

Download the latest version of ME_Cleaner from github

git clone https://github.com/corna/me_cleaner

Download the latest version of Coreboot

git clone  https://review.coreboot.org/coreboot

Move into the coreboot directory

cd ~/work/coreboot

Download the required submodules

git submodule update --init --checkout

Make a directory to hold some files specific to your T420 it will be needed later.

mkdir -p ~/work/coreboot/3rdparty/blobs/mainboard/lenovo/t420

Build the ifd tool. This will be used to split the factory bios into it's different regions.

cd ~/work/coreboot/utils/ifdtool
make

Use the 6 female to female wire to connect the clip to the Pi

Bios 1 > Pi 24

Bios 2 > Pi 21

Bios 4 > Pi 25

Bios 5 > Pi 19

Bios 7 > Pi 23

Bios 8 > Pi 17

Pins 3 and 7 on the Bios are not used.

The bios chip is located under the roll cage. In order to access it you will need to remove the mother board.

The Hardware Maintenance Manual can provide instructions if you are having trouble figuring it out.

I have included my teardown images.They we're never intended for public viewing (my handwriting is horrible sorry) but what the heck they may help.

With the Pi powered OFF connect the clip to the bios chip.

Power on the Pi

Create a roms directory and move to it .

mkdir -p ~/work/roms

cd ~/work/roms

To read and write the chip you will need to use a program called Flashrom. First make sure it is installed

sudo apt install flashrom

Use flashrom to probe the chip and make sure it is connected

flashrom -p linux_spi:dev=/dev/spidev0.0,spispeed=128

Read the factory bios off the chip 3 times and save them as factory1.rom factory2.rom factory3.rom

Use the -c option to specify your flash chip. Make sure to enter everything between the quotes

Each read will take some time depending on the chip it could be between 30-45 min each read. Dont worry if it seems like the pi is hung.

flashrom -p linux_spi:dev=/dev/spidev0.0,spispeed=128 -c <chip name> -r factory1.rom

flashrom -p linux_spi:dev=/dev/spidev0.0,spispeed=128 -c <chip name> -r factory2.rom

flashrom -p linux_spi:dev=/dev/spidev0.0,spispeed=128 -c <chip name> -r factory3.rom

Next you want to compare the 3 files to make sure you had a good read / connections

sha512sum factory*.rom

If they all match copy them to the main computer in the ~/work/roms directory.

Power off the Pi. You can leave the clip connected.

Move to ~/work/roms

cd ~/work/roms

The factory roms should not be edited. Make a copy of one of them to clean.

cp factory1.rom cleaned.rom

Clean the IME on cleaned .rom

~/work/me_cleaner/me_cleaner.py -S cleaned.rom

The bios chip is split into 4 regions. You need to split the cleaned.rom image into its different regions with the ifd tool provided by coreboot

~/work/coreboot/utils/ifdtool/ifdtool -x cleaned.rom

This will produce 4 files. We need to rename 3 of them and can delete 1

Rename the descriptor region

mv flashregion_0_flashdescriptor.bin descriptor.bin

Delete the bios region - It will be replaced with coreboot.

rm flashregion_1_bios.bin

Rename the GBE region

mv flashregion_2_gbe.bin gbe.bin

Rename the ME region

mv flashregion_3_me.bin me.bin

Copy the files to the coreboot directory

cp descriptor.bin gbe.bin me.bin ~/work/coreboot/3rdparty/blobs/mainboard/lenovo/t420/

Move to the coreboot directory

cd ~/work/coreboot

Configure coreboot.

make nconfig

This will bring up the Coreboot config editor. Most of the default settings are fine, but there are a few that can be added. This is a very basic config. More advanced options such as splash screens, vga roms , alternate payloads are available. These options are beyond the scope of this guide.

General Setup

• Use CMOS for configuration values

Mainboard

• Mainboard vendor >>> Select >> Lenovo
• Mainboard model >>> Select >>> T420

Chipset

• Add Intel descriptor.bin file
• Add Intel ME/TXE firmware
• Add gigabit ethernet configuration

Devices

• Enable PCIe Clock Power Management
• Enable PCIe ASPM L1 SubState

Generic Driver

• PS/2 keyboard init

Time to compile!

First built the gcc toolchain

make crossgcc-i386 CPUS=X

X = the number of threads your CPU has.

Build coreboot

make iasl
make

This will produce a file ~/work/coreboot/build/coreboot.rom.

Power on the Pi and copy that file to your ~/work/roms directory.

Move to the roms directory

cd ~/work/roms

Probe the chip to make sure its detected

flashrom -p linux_spi:dev=/dev/spidev0.0,spispeed=128

Write the coreboot image. This will take longer then reading the image.

flashrom -p linux_spi:dev=/dev/spidev0.0,spispeed=128 -c <chip name> -w coreboot.rom

After the write is verified power off the pi. Remove the Clip and reassemble the T420.

Congrats you have just flashed Coreboot.