(In reply to hiwatari.seiji from comment #7)
> Are you sure, that b7d91c915290ab0bfbab84a0fb9c9eae57816982 is definitely
> not the cause?

At least 99% sure. Pure merge commits are almost never a correct bisection
result.

> - For me, when booting the machine (BIOS) - the screen resolution of GRUB is
> quite random. Sometimes, it's using the native screen resolution, sometimes
> it is using a very reduced resolution. Sometimes, it's only displaying on
> one of the two screens, sometimes it's displaying on both. But that wasn't a
> problem before linux-4.9.

This is before the Linux kernel is even loaded, so it can hardly be directly
related to it.