linux-kernel - Re: [linux-pm] [PATCH] cpuidle: avoid singing capacitors

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Message-ID: <20080310100008.GA9520@elf.ucw.cz>
Date:	Mon, 10 Mar 2008 11:00:08 +0100
From:	Pavel Machek <pavel@....cz>
To:	Pierre Ossman <drzeus-list@...eus.cx>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	linux-pm@...ts.linux-foundation.org,
	"Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com>,
	Dave Jones <davej@...emonkey.org.uk>,
	Andi Kleen <andi@...stfloor.org>,
	Alan Stern <stern@...land.harvard.edu>,
	Adam Belay <abelay@...ell.com>,
	Lee Revell <rlrevell@...-job.com>
Subject: Re: [linux-pm] [PATCH] cpuidle: avoid singing capacitors

On Sun 2008-03-09 15:16:59, Pierre Ossman wrote:
> I'm beginning to think this is a lost cause. I've tried several variants, all without satisfactory results.
> 
> In case anyone else has any more ideas, I'll detail what I've found influences the noise:
> 
> 1. C state
> 
> This is the big one. There is no noise as long as C3 is avoided (processor.max_cstate).
> 
> 2. uhci_hcd driver
> 
> USB is somehow involved in this problem. Unloading the uhci_hcd driver almost entirely kills the noise on a 1000 HZ NO_HZ kernel. On a 100 HZ, no NO_HZ kernel, the effect is very small, but still there.

USB keeps processor out of C3 in many cases.

> 4. Battery and AC
> 
> The noise increases with the battery present and also when the AC supply is removed.
>

On battery, some machines swap C3 for "secret" C4, which is slower but
saves power.
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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