linux-kernel - Re: CONFIG_SUSPEND and power consumption

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Message-Id: <200708202348.57596.rjw@sisk.pl>
Date:	Mon, 20 Aug 2007 23:48:57 +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 22:19, 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.
> 
> Strange... what is going on here, then?
> 
> config USB_SUSPEND
>         bool "USB selective suspend/resume and wakeup (EXPERIMENTAL)"
>         depends on USB && PM && EXPERIMENTAL
>         help
>           If you say Y here, you can use driver calls or the sysfs
>           "power/state" file to suspend or resume individual USB
>           peripherals.
> 
>           Also, USB "remote wakeup" signaling is supported, whereby
> some
>           USB devices (like keyboards and network adapters) can wake
> up
>           their parent hub.  That wakeup cascades up the USB tree, and
>           could wake the system from states like suspend-to-RAM.
> 
>           If you are unsure about this, say N here.
> 
> config USB_OTG
>         bool
>         depends on USB && EXPERIMENTAL
>         select USB_SUSPEND
>         default n
> 
> hmmm, it looks like USB_OTG can be selected without CONFIG_PM, but it
> selects USB_SUSPEND. Is that okay?

Well, I don't think so.  It should depend on the same things as USB_SUSPEND,
IMO.

Greetings,
Rafael
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