linux-kernel - Re: CONFIG_SUSPEND and power consumption

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Message-ID: <20070820172907.GC3975@ucw.cz>
Date:	Mon, 20 Aug 2007 17:29:07 +0000
From:	Pavel Machek <pavel@....cz>
To:	Oliver Neukum <oliver@...kum.org>
Cc:	Jean Delvare <khali@...ux-fr.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: CONFIG_SUSPEND and power consumption

Hi!

> > If I rmmod "ehci-hcd" then the power consumption is back to 69 W. This
> > confirms that this is really USB-related. I have to admit that I did
> > not expect an external drive to eat that much power from the system,
> > especially when not used. I am told that VIA chips are notoriously bad
> > at this kind of things. I'll try the same external drive on an Intel
> > system later today.
> > 
> > The last mystery remaining is how USB "activity" can cause my CPU to
> > heat. I would expect the south bridge to heat, not the CPU.
> 
> USB, or strictly speaking EHCI, OHCI and UHCI, use DMA. To allow
> that the cache coherency logic has to be active. Therefore your CPU
> cannot go to C3. Therefore it draws more power. The problem we are
> facing in USB is that to get great savings, our coverage has to be perfect.
> One device that cannot be autosuspended and we lose most savings.

Ok.. but CONFIG_USB_SUSPEND should not really have anything to do with
CONFIG_SUSPEND (= s2ram). Perhaps it should depend on CONFIG_PM
instead?
							Pavel

-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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