Reverting coreboot installation on Thinkpad X220 without an original SPI dump

Congratulations! You forgot to back up your original BIOS dump, and now you're stuck with coreboot forever...right? Not so fast.
Even without the SPI dump, you can still go back to the original BIOS, which is really useful if you experience some problems or decide to install macOS.

Requirements

• $01CB000.FL1 file from this repository
• X220 running Linux with flashrom package installed, as well as kernel option iomem=relaxed set in bootloader

WARNING

Only do this if you know what you're doing, be sure to have a Raspberry Pi and a Pomona clip handy to re-flash your SPI chip in case you mess up. I am not responsible for any damage your computer may suffer during this process.

The process

Take the $01CB000.FL1 file, and truncate it to exactly 8MByte:

dd if=\$01CB000.FL1 of=x220_spi.bin bs=$((0x800000)) count=1

Backup the old content and put it on an external storage:

flashrom -p internal:laptop=force_I_want_a_brick -r file.rom

You may need to tell the tool which flash IC it is with the "-c" option:

flashrom -p internal:laptop=force_I_want_a_brick -c MX25L6405 -r file.rom

As we want to write only the BIOS part in the SPI flash starting at offset 0x500000, we need a layout file:

echo -e "000000:4fffff dummy\n500000:7fffff bios" > x220.layout

Update the BIOS portion in the SPI flash

flashrom -p internal:laptop=force_I_want_a_brick -c MX25L6405 --layout x220.layout -i bios -w x220_spi.bin

Done!

This guide is mostly based on this wiki page: http://thinkwiki.de/UEFI_BIOS_T420_BIOS_Structure