linux-kernel - Re: CONFIG_SUSPEND and power consumption

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Message-Id: <200708201211.34698.oliver@neukum.org>
Date:	Mon, 20 Aug 2007 12:11:34 +0200
From:	Oliver Neukum <oliver@...kum.org>
To:	Jean Delvare <khali@...ux-fr.org>
Cc:	"Rafael J. Wysocki" <rjw@...k.pl>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: CONFIG_SUSPEND and power consumption

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.

	Regards
		Oliver

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