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

[Rafael J. Wysocki]

On Mon, Jan 23, 2023 at 10:44 PM Bjorn Helgaas <helgaas@xxxxxxxxxx> wrote:
>
> 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.

But it would be able to log a diagnostic message anyway IMO.

So maybe two steps can be taken here, (1) adding the message printout
(this patch) and (2) changing the behavior?