39-1 severely worsens nouveau driver issues

Summary

I initially erroneously reported this as a bug in linux-hardened ( archlinux/packaging/packages/linux-hardened#6 (closed)), but after thoroughly testing across package combinations and reboots, I believe mkinitcpio 39-1 is the root cause of the issue. With 39-1, I have only managed to successfully get a working dGPU setup in one out of ten boots. The others have all resulted a black greeter screen with only a cursor, any program using the dGPU rendering as random pixels, and the kernel log being filled with an endless stream of the following error:

May 09 16:07:03 skidbladnir kernel: nouveau 0000:01:00.0: gr: DATA_ERROR 0000009c [] ch 5 [00ff976000 WebKitWebProces[1002]] subc 0 class c197 mthd 0d78 data 00000004

I have been able to reproduce this issue on 38.1-1, however with the probabilities flipped: nine out of ten reboots started with a working dGPU, greeter, etc, and no DATA_ERROR messages in the logs.

More details

Setting pacman to point to the package archive from 2024-05-03 (which appears to have been made before 39-1 landed) results in a mostly working system (8/10 working boots), with 2024-05-04 resulting in a mostly broken system (3/10 working boots).

Downgrading mkinitcpio to 38.1-1 and adding it to IgnorePkg, and then installing every other update from 2024-05-04 keeps the system working (9/10 working boots on average). And I have confirmed this to remain the case (9/10 working boots) with mkinitcpio fixed to 38.1-1 and all the latest updates as of today (2024-05-09T14:52:39 UTC). With that final setup, upgrading to 39-1 results in 1/10 working boots.