linux-kernel - Re: CONFIG_SUSPEND and power consumption

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Date:	Mon, 20 Aug 2007 17:34:50 +0200
From:	Jean Delvare <khali@...ux-fr.org>
To:	Oliver Neukum <oliver@...kum.org>
Cc:	"Rafael J. Wysocki" <rjw@...k.pl>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: CONFIG_SUSPEND and power consumption

On Mon, 20 Aug 2007 12:11:34 +0200, Oliver Neukum wrote:
> Am Montag 20 August 2007 schrieb Jean Delvare:
> > 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.

Ah, OK, thanks for the clarification, it explains a lot.

I've made some more tests on two Intel boards and another VIA board.
The bottom line is that both VIA boards see a bump in power consumption
when plugging my USB 2.0 hard disk drive (10 W on one board, 4 W on the
other) while none of the Intel boards exhibit any change in power
consumption. I wonder if I should blame VIA for eating extra power when
the disk is plugged, or thank them for saving power when it's not. Or
maybe I am looking at things the wrong way, and I should thank AMD for
saving more power in C3 than Intel does?

-- 
Jean Delvare
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