linux-kernel - Re: [patch 0/6] AMD C1E aware idle support

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Date:	Tue, 5 Aug 2008 19:42:18 +0200
From:	Pavel Machek <pavel@...e.cz>
To:	Andreas Herrmann <andreas.herrmann3@....com>
Cc:	Len Brown <lenb@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Arjan van de Veen <arjan@...radead.org>,
	"Maciej W. Rozycki" <macro@...ux-mips.org>
Subject: Re: [patch 0/6] AMD C1E aware idle support

Hi!

> > > AMD CPUs with C1E support are currently excluded from high resolution
> > > timers and NOHZ support. The reason is that C1E is a BIOS controlled
> > > C3 power state which switches off TSC and the local APIC timer. The
> > > ACPI C-State control manages the TSC/local APIC timer wreckage, but
> > > this does not include the C1 based ("halt" instruction) C1E mode. The
> > > BIOS/SMM controlled C1E state works on most systems even without
> > > enabling ACPI C-State control.
> > 
> > What a mess.

Yep, seems like AMD is breaking C1 semantics. Is it even valid from
ACPI spec point of view?

> > What is the measured power savings that justifies this effort?
> 
> IMHO the power savings are not that important when such a kernel runs
> on bare metal:

Ok, so maybe we should disable C1E to work around its misdesign?

It would be certainly nice to have noc1e command line option...

>   But overall no measurable difference in power usage was seen.

							Pavel
-- 
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