Date:   Mon, 30 Jan 2023 11:45:30 -0600
From:   Bjorn Helgaas <helgaas@kernel.org>
To:     "Rafael J. Wysocki" <rafael@kernel.org>
Cc:     Mateusz =?utf-8?Q?Jo=C5=84czyk?= <mat.jonczyk@o2.pl>,
        linux-kernel@vger.kernel.org, linux-pci@vger.kernel.org,
        linux-acpi@vger.kernel.org, linux-i2c@vger.kernel.org,
        Len Brown <lenb@kernel.org>, Borislav Petkov <bp@suse.de>,
        Jean Delvare <jdelvare@suse.com>,
        Jean Delvare <jdelvare@suse.de>
Subject: Re: [PATCH v3 RESEND] acpi,pci: warn about duplicate IRQ routing
 entries returned from _PRT
Message-ID: <20230130174530.GA1690323@bhelgaas>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
In-Reply-To: <CAJZ5v0hTXAdM5oC_5cFF0pZU5QKaaqcsY4rADXJhAkAa0s=4Kw@mail.gmail.com>
Precedence: bulk

On Mon, Jan 30, 2023 at 04:56:20PM +0100, Rafael J. Wysocki wrote:
> On Mon, Jan 23, 2023 at 10:44 PM Bjorn Helgaas <helgaas@kernel.org> 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?

Yep, makes sense to me.

Bjorn