linux-kernel - Re: [GIT PULL] PM updates for 2.6.33

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Date:	Sun, 6 Dec 2009 11:35:55 -0800
From:	Arjan van de Ven <arjan@...radead.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	"Rafael J. Wysocki" <rjw@...k.pl>,
	LKML <linux-kernel@...r.kernel.org>,
	ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
	pm list <linux-pm@...ts.linux-foundation.org>,
	Alan Stern <stern@...land.harvard.edu>
Subject: Re: [GIT PULL] PM updates for 2.6.33

On Sat, 5 Dec 2009 18:05:14 -0800 (PST)
Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> 
> 
> On Sun, 6 Dec 2009, Rafael J. Wysocki wrote:
> > 
> > While the current settings are probably unsafe (like enabling PCI
> > devices to be suspended asynchronously by default if there are not
> > any direct dependences between them), there are provisions to make
> > eveything safe, if we have enough information (which also is needed
> > to put the required logic into the drivers).
> 
> I disagree.
> 
> Think of a situation that we already handle pretty poorly: USB mass 
> storage devices over a suspend/resume.
> 
> > The device tree represents a good deal of the dependences
> > between devices and the other dependences may be represented as PM
> > links enforcing specific ordering of the PM callbacks.
> 
> The device tree means nothing at all, because it may need to be
> entirely rebuilt at resume time. 


btw I instrumented both the suspend and resume, and made graphs out of
it for my laptop (modern laptop with Intel cpu/wifi/graphics of course).

http://www.fenrus.org/graphs/suspend.svg
http://www.fenrus.org/graphs/resume.svg

(also attached for convenience)

the resume clearly shows that all this talking about PCI stuff is
completely without practical merit.. it's the USB stuff where the time
is spent.

in suspend, there's a PCI device (:1b) that does take some time, which
is the audio controller. The bulk of the time is in the serio driver
though..


As an "interested bystander" to this thread.... sounds like Linus'
arguments have merit, and that solving the USB resume to go async in
some form will fix pretty much all we want solving...

[and that at least we need to do this stuff data/measurement driven,
and not just based on how we THINK things work]





-- 
Arjan van de Ven 	Intel Open Source Technology Centre
For development, discussion and tips for power savings, 
visit http://www.lesswatts.org

Download attachment "suspend.svgz" of type "image/svg+xml-compressed" (2733 bytes)

Download attachment "resume.svgz" of type "image/svg+xml-compressed" (2852 bytes)