linux-kernel - Re: [PATCH 2/2] acpi: Disable IRQ 0 through I/O APIC for some HP systems

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Date:	Mon, 7 Jul 2008 15:01:31 +0100 (BST)
From:	"Maciej W. Rozycki" <macro@...ux-mips.org>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
cc:	Ingo Molnar <mingo@...e.hu>, Matthew Garrett <mjg59@...f.ucam.org>,
	Len Brown <lenb@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-next@...r.kernel.org, linux-acpi@...r.kernel.org,
	linux-kernel@...r.kernel.org, Andi Kleen <andi@...stfloor.org>
Subject: Re: [PATCH 2/2] acpi: Disable IRQ 0 through I/O APIC for some HP
 systems

On Mon, 7 Jul 2008, Rafael J. Wysocki wrote:

> >  It makes absolutely no sense and should be harmful to call
> > clear_IO_APIC_pin(apic1, pin1) here, because both apic1 and pin1 should be
> > equal to -1 here.  If it has to be called, then I suppose the DMI matching 
> > did not work and the workaround has not been enabled.
> 
> Do you realize that the clear_IO_APIC_pin(apic1, pin1) thing is _only_ called
> _IF_ the DMI matching did work?

 Well, it is the very intent of the DMI quirk to set apic1 and pin1 both
to -1, as a result of IRQ0 being absent from our I/O APIC interrupt
routing table.  Therefore if the quirk did indeed work, a call to
clear_IO_APIC_pin() is useless and likely harmful as its callees don't do
range checking (my understanding of code is it results in random poking at
the local APIC through the FIX_APIC_BASE fixmap).  There should be nothing
to clear too, as interrupt redirection entries for all the I/O APIC inputs
are cleared (the mask is set to 1 and the remaining fields zeroed) when
clear_IO_APIC() is called from enable_IO_APIC() upon initialization and
all the unused ones (not referred to from anywhere in the interrupt
routing table) are never touched afterwards.

  Maciej
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