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From:   Thomas Gleixner <tglx@linutronix.de>
To:     Maximilian Luz <luzmaximilian@gmail.com>,
        David Laight <David.Laight@ACULAB.COM>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>
Cc:     "H. Peter Anvin" <hpa@zytor.com>, Sachi King <nakato@nakato.io>,
        "x86\@kernel.org" <x86@kernel.org>,
        "linux-kernel\@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "stable\@vger.kernel.org" <stable@vger.kernel.org>
Subject: Re: [PATCH] x86/i8259: Work around buggy legacy PIC
In-Reply-To: <e7dbd4d1-f23f-42f0-e912-032ba32f9ec8@gmail.com>
References: <20210512210459.1983026-1-luzmaximilian@gmail.com> <9b70d8113c084848b8d9293c4428d71b@AcuMS.aculab.com> <e7dbd4d1-f23f-42f0-e912-032ba32f9ec8@gmail.com>
Date:   Fri, 14 May 2021 15:44:57 +0200
Message-ID: <87r1i94eg6.ffs@nanos.tec.linutronix.de>
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Max,

On Thu, May 13 2021 at 12:11, Maximilian Luz wrote:
> And lastly, if that's any help at all: The PIC device is described in
> ACPI in [3]. The Surface Laptop 3 also has an AMD CPU (although a prior
> generation) and has the PIC described in the exact same way (can also be
> found in that repository), but doesn't exhibit that behavior (and
> doesn't show the "Using NULL legacy PIC" line). I expect there's not
> much you can change to that definition so that's probably irrelevant
> here.
>
> Again, I don't really know anything about these devices, so my guess
> would be bad hardware revision or bad firmware revision. All I know is
> that retrying seems to "fix" it.

That might be a a power optimization thing.

Except for older systems the PIC is not really required for IOAPiC to
work. But there is some historical code which makes assumptions. We can
change that, but that needs some careful thoughts.

Thanks,

        tglx