linux-kernel - Re: [PATCH 1/8] PM: Opportunistic suspend support.

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Date:	Thu, 27 May 2010 16:48:42 -0700
From:	Dmitry Torokhov <dmitry.torokhov@...il.com>
To:	Arve Hjønnevåg <arve@...roid.com>
Cc:	Alan Stern <stern@...land.harvard.edu>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Linux-pm mailing list <linux-pm@...ts.linux-foundation.org>,
	Kernel development list <linux-kernel@...r.kernel.org>,
	Len Brown <len.brown@...el.com>, Pavel Machek <pavel@....cz>,
	Randy Dunlap <rdunlap@...otime.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andi Kleen <ak@...ux.intel.com>,
	Cornelia Huck <cornelia.huck@...ibm.com>,
	Tejun Heo <tj@...nel.org>,
	Jesse Barnes <jbarnes@...tuousgeek.org>,
	Nigel Cunningham <nigel@...onice.net>,
	Ming Lei <tom.leiming@...il.com>,
	Wu Fengguang <fengguang.wu@...el.com>,
	Maxim Levitsky <maximlevitsky@...il.com>,
	linux-doc@...r.kernel.org
Subject: Re: [PATCH 1/8] PM: Opportunistic suspend support.

On Thu, May 27, 2010 at 04:36:28PM -0700, Arve Hjønnevåg wrote:
> 2010/5/27 Dmitry Torokhov <dmitry.torokhov@...il.com>:
> > On Wed, May 26, 2010 at 05:52:40PM -0700, Arve Hjønnevåg wrote:
> >> 2010/5/26 Alan Stern <stern@...land.harvard.edu>:
> >> > On Wed, 26 May 2010, Arve Hjønnevåg wrote:
> >> >
> >> >> > I must be missing something.  In Arve's patch 1/8, if the system is in
> >> >> > opportunistic suspend, and a wakeup event occurs but no suspend
> >> >> > blockers get enabled by the handler, what causes the system to go back
> >> >> > into suspend after the event is handled?  Isn't that a loop of some
> >> >> > sort?
> >> >> >
> >> >>
> >> >> Yes it is a loop. I think what you are missing is that it only loops
> >> >> repeatedly if the driver that aborts suspend does not use a suspend
> >> >> blocker.
> >> >
> >> > You mean "the driver that handles the wakeup event".  I was asking what
> >> > happened if suspend succeeded and then a wakeup occurred.  But yes, if
> >> > a suspend blocker is used then its release causes another suspend
> >> > attempt, with no looping.
> >> >
> >> >> > And even if it isn't, so what?  What's wrong with looping behavior?
> >> >>
> >> >> It is a significant power drain.
> >> >
> >> > Not in the situation I was discussing.
> >> >
> >>
> >> If you meant it spend most of the time suspended, then I agree. It
> >> only wastes power when a driver blocks suspend by returning an error
> >> from its suspend hook and we are forced to loop doing no useful work.
> >>
> >
> > If driver refuses to suspend that means there are events that need
> > processing. I fail to see why it would be called "looping doing no
> > useful work".
> 
> Because the useful work is done in another thread. All the loop does
> is check if the useful work has completed which most likely will slow
> down the useful work.

Or useful work could signal when it is done processing critical section.

> Blocking suspend with a suspend blocker until
> the useful work is done is more efficient.
> 

-- 
Dmitry
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