linux-kernel - Re: CONFIG_SUSPEND and power consumption

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Message-Id: <200708202108.35674.rjw@sisk.pl>
Date:	Mon, 20 Aug 2007 21:08:35 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Pavel Machek <pavel@....cz>
Cc:	Oliver Neukum <oliver@...kum.org>,
	Jean Delvare <khali@...ux-fr.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: CONFIG_SUSPEND and power consumption

On Monday, 20 August 2007 19:29, Pavel Machek wrote:
> 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?

CONFIG_USB_SUSPEND doesn't depend on CONFIG_SUSPEND.

Greetings,
Rafael
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