Recently I have decided to give a try to coreboot for first time and
flashed my ThinkPad T420, but a few weeks ago I have swapped the USB
controller on the back, next to the battery, with a FireWire/USB controller
(40GAB5809-G200) from another T420. Nothing special, since some models have
been shipped like this. The controller is no longer accessible on my laptop.
It seems like it may have been detected as an "SD Host Controller" or not
detected at all. I will probably have to remove the chip and compare the
output of lspci and lshw. If nothing has changed, I will probably have to
return the stock BIOS and compare the results again. I have also tried to
load some of the firewire kernel modules manually with modprobe.
The operating systems I have tested so far are Arch Linux and Xubuntu. I am
willing to provide more useful information, boot into a fresh Windows
install, flash the chip again or whatever else. Correct me if I am wrong,
but if I go back to the stock BIOS, the next time I flash, I will have to
disassemble the laptop again and otherwise I must be fine with flashing