linux-kernel - Re: [RFD] Automatic suspend

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Date:	Wed, 18 Feb 2009 00:21:58 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	"Woodruff, Richard" <r-woodruff2@...com>,
	Arjan van de Ven <arjan@...radead.org>,
	Kyle Moffett <kyle@...fetthome.net>,
	Oliver Neukum <oliver@...kum.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	pm list <linux-pm@...ts.linux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Arve Hjønnevåg <arve@...roid.com>,
	Pavel Machek <pavel@....cz>,
	Nigel Cunningham <nigel@...el.suspend2.net>,
	Matthew Garrett <mjg59@...f.ucam.org>,
	mark gross <mgross@...ux.intel.com>,
	Uli Luckas <u.luckas@...d.de>,
	Igor Stoppa <igor.stoppa@...ia.com>,
	Brian Swetland <swetland@...gle.com>,
	Len Brown <lenb@...nel.org>
Subject: Re: [RFD] Automatic suspend

On Tuesday 17 February 2009, Alan Stern wrote:
> On Tue, 17 Feb 2009, Rafael J. Wysocki wrote:
> 
> > Phase 1: I agree that system-auto-suspend-on, system-auto-suspend-off would be
> > useful, but I don't like the wakelocks interface.  Do you think there is an
> > alternative way/mechanism of doing this?
> 
> I rather like the suggestions Matthew Garrett has been making.  They 
> show how to improve the wakelock interface without losing any function.
> 
> Really, the idea behind wakelocks comes down to the question of how to
> determine when the system is sufficiently idle to go into auto-suspend.  
> There may be occasions when no task is runnable but userspace knows
> that the system should not go to sleep because some work will be done
> in the near future.  (Arve's example of a non-empty input buffer is
> such a case.)  How should userspace let the kernel know whether it's
> okay to suspend at these times?  That is the problem userspace
> wakelocks are meant to solve.

Still, do we really need multiple user space wakelocks (I'd prefer to call them
sleeplocks)?  It seems that one such lock and a user space manager controlling
it should be sufficient.

> Kernel wakelocks are a separate matter.  They are more like a form of 
> optimization, preventing the kernel from starting an auto-suspend when 
> some driver knows beforehand that it will return -EBUSY.

I think kernel-side autosuspend (or rather autosleep) should only happen
after certain subset of devices have been suspended using a per-device
run-time autosuspend mechanism.

> > Phase 3: Probably explicit control left to open/close.
> 
> While that's generally a good idea, it's important to recognize that 
> some devices should be runtime-suspended even while they are open.

>From the kernel side, yes (and that should be transparent to the user space
having them open).  By the user space, no.
 
> Basically, any device that is "always open" falls in this category.  
> Some examples are the screen, the keyboard, the mouse, and disk drives.  
> And of course, some of these things already have runtime power 
> management.

Thanks,
Rafael
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