Hello,
first I'd like to thank Ben for providing me his ACPI tables.
This morning I digged through the disassembled DSDT and got the feeling that this AMD platforms depends on an ACPI-capable OS calling the _PIC and _PRT methods for IRQ setup. This is the call trace I discovered
call _PIC to inform AML of the interrupt model in use _PIC calls _SB.IRQC in APIC mode _SB.IRQC configures several SystemIO fields (e.g., PIRA, PIRB)
_PRT reports the routing table as usual referencing AR00 in APIC mode so this part should be working already. I suggest further investigation of the IRQ-related setup done in the DST AML code. Unfortunately, I'm not familiar enough with acpica to know if it could be easily adapted to try a full ACPI setup (incl. IRQs) on the platform.
Regards