linux-kernel - Re: Attempted summary of suspend-blockers LKML thread

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Date:	Thu, 5 Aug 2010 02:10:40 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	david@...g.hm
Cc:	Arve Hjønnevåg <arve@...roid.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Arjan van de Ven <arjan@...radead.org>,
	"Ted Ts'o" <tytso@....edu>, linux-pm@...ts.linux-foundation.org,
	"linux-kernel" <linux-kernel@...r.kernel.org>, mjg59@...f.ucam.org,
	pavel@....cz, florian@...kler.org, stern@...land.harvard.edu,
	swetland@...gle.com, peterz@...radead.org, tglx@...utronix.de,
	alan@...rguk.ukuu.org.uk
Subject: Re: Attempted summary of suspend-blockers LKML thread

On Thursday, August 05, 2010, david@...g.hm wrote:
> On Thu, 5 Aug 2010, Rafael J. Wysocki wrote:
> 
> > On Thursday, August 05, 2010, david@...g.hm wrote:
> >> On Wed, 4 Aug 2010, Rafael J. Wysocki wrote:
> >>
> >>> Subject: Re: Attempted summary of suspend-blockers LKML thread
> >>>
> >>> On Wednesday, August 04, 2010, david@...g.hm wrote:
> >>>> On Wed, 4 Aug 2010, Rafael J. Wysocki wrote:
> >>>>> In the suspend case, when you have frozen all applications, you can
> >>>>> sequentially disable all interrupts except for a few selected ("wakeup") ones
> >>>>> in a safe way.   By disabling them, you ensure that the CPU will only be
> >>>>> "revived" by a limited set of events and that allows the system to stay
> >>>>> low-power for extended time intervals.
> >>>>
> >>>> the benifit of this will depend on what wakeups you are able to avoid by
> >>>> putting the hardware to sleep. Depending on the hardware, this may be not
> >>>> matter that much.
> >>>
> >>> That's correct, but evidently it does make a difference with the hardware
> >>> Android commonly runs on.
> >>
> >> Ok, but is there a way to put some of this to sleep without involving a
> >> full suspend?
> >
> > Technically, maybe, but we have no generic infrastructure in the kernel for that.
> > There may be SoC-specific implementations, but nothing general enough.
> 
> well, I know that we have specific cases of this (drive spin-down, cpu 
> speed, display backlight for a few examples), is it worth trying to define 
> a generic way to do this sort of thing? or should it be left as a 
> per-device thing (with per-device knobs to control it)

The ability to put specific devices into low-power states in certain
well-defined situations is clearly not sufficient.  For one example, usually
you can easily put an Ethernet adapter into a low-power state when the network
cable is detached from it.  It is not clear, however, what criteria should be
used for deciding to put that adapter into the low-power state when the cable
is attached to it (and open() has been called).

To mimic suspend you'll have to be able to put _all_ devices into low-power
states and shut down the interrupts that allow the monotonic clock to advance.
That's much more than simple runtime power management of selected devices.

> I thought I had seen discussion on how to define such a generic power 
> management interface, and I thought the results had been acceptable.

If you have a pointer to that discussion, I'm interested. :-)

Thanks,
Rafael
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