Re: [PATCH v3 RESEND] acpi,pci: warn about duplicate IRQ routing entries returned from _PRT

[Bjorn Helgaas <helgaas@kernel.org>]

On Mon, Jan 23, 2023 at 10:00:43PM +0100, Mateusz Jończyk wrote:
> W dniu 23.01.2023 o 21:33, Bjorn Helgaas pisze:
> > On Sat, Jan 21, 2023 at 04:33:14PM +0100, Mateusz Jończyk wrote:
> >> On some platforms, the ACPI _PRT function returns duplicate interrupt
> >> routing entries. Linux uses the first matching entry, but sometimes the
> >> second matching entry contains the correct interrupt vector.
> >>
> >> Print an error to dmesg if duplicate interrupt routing entries are
> >> present, so that we could check how many models are affected.
> >
> > It shouldn't be too hard to use qemu to figure out whether Windows
> > uses the last matching entry, i.e., treating _PRT entries as
> > assignments.  If so, maybe Linux could just do the same.
> >
> > Is anybody up for that?
> 
> The hardware in question has a working Windows XP installation,
> and I could in theory check which interrupt vector it uses - but
> I think that such reverse engineering is forbidden by Windows' EULA.

I'm not talking about any sort of disassembly or anything like that;
just that we can observe what Windows does given the _PRT contents.
You've already figured out that on your particular hardware, the _PRT
has two entries, and Linux uses the first one while Windows uses the
second one, right?

On qemu, we have control over the BIOS and can easily update _PRT to
whatever we want, and then we could boot Windows and see what it uses.
But I guess maybe that wouldn't tell us anything more than what you
already discovered.

So my inclination would be to make Linux use the last matching entry.

Bjorn