I am using T420 and I also experience hangs in OS. But I have pretty old coreboot. In some free time I am going to try main branch again and see if it helps. If not then I am on the same boat as OP.

On January 17, 2024 6:54:28 AM UTC, Patrick Rudolph <coreboot@fe80.eu> wrote:
Issue #121 has been updated by Patrick Rudolph.

It might be possible that testing on my side was a false positive and I was just lucky that the issue didn't appear within time.
As it's unclear what the issue is about I'm just poking in the dark, comparing the reference code (MRC.bin) with coreboot's native code.

Anything that could help narrowing it down as described here: https://ticket.coreboot.org/issues/121#note-80 would help to fix it.
Bug #121: T520: Hangs in OS

* Author: Firstname Lastname
* Status: In Progress
* Priority: Normal
* Category: chipset configuration
* Start date: 2017-06-09
* Affected hardware: SNB, IVY
* Affected OS: -
I have been running coreboot since 2017.04.15 and have experienced hangs ever since then. It was suggested by folk on the IRC that I run memtest to check for incorrect raminit causing errors, however I have run memtest for 12 hours straight with no errors.

Due to the ambiguous nature of the hangs (immediate freeze with no warning signs, audio gets stuck repeating the last 50ms or so of noise, not sure what this effect is called) I don't have much useful information other than the .config and dmesg. However one thing I can say with high confidence is that the hangs occur significantly more frequently in Linux (*buntu distros) than Windows 10. Within an hour of launching Linux a hang is likely, whereas Windows typically runs for many hours before a hang occurs. I considered this an insignificant anecdotal anomaly at first but over the course of the nearly 2 months I have been running coreboot it seems to be a solid trend. The hangs occur anywhere, typically during mere desktop usage or basic web browsing.

Additionally there is another form of hang I experience where the screen goes black except for some sort of graphical corruption down the left side (http://i.imgur.com/4zWrlpX.jpg), whether this is related to the more common total freeze hangs I don't know but I figured I should include it nonetheless. These hangs only occur about 1:20 compared to the regular hangs.

config (20.7 KB)
dmesg.txt (57.3 KB)
cbmem-raminit.txt (62 KB)
lspci.txt (29.6 KB)
cpuinfo.txt (3.94 KB)
defconfig (1023 Bytes)
defconfig (699 Bytes)