So I spent an evening bisecting the issue from upstream sources. The problem was introduced with > 7b192ec4c term/ns8250: Use ACPI SPCR table when available to configure serial
The old behaviour can be regained by setting the unit explicitly grub.cfg > serial --unit=0 --speed=115200 This solves my issue, so you can go ahead and close the bug report. I assume the ACPI lookup logic triggered a bug in my particular firmware. /Morten
