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

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Date:	Tue, 25 May 2010 15:05:19 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Dmitry Torokhov <dmitry.torokhov@...il.com>
cc:	Arve Hjønnevåg <arve@...roid.com>,
	Linux-pm mailing list <linux-pm@...ts.linux-foundation.org>,
	Kernel development list <linux-kernel@...r.kernel.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>, 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 Tue, 25 May 2010, Dmitry Torokhov wrote:

> > > What you describe can be done in userspace though, via a "suspend manager" 
> > > process. Tasks reading input events will post "busy" events to stop the 
> > > manager process from sending system into suspend. But this can be confined to 
> > > Android userspace, leaving the kernel as is (well, kernel needs to be modified 
> > > to not go into suspend with full queues, but that is using existing kernel 
> > > APIs).
> > 
> > I think that could be made to work.  And it might remove the need for 
> > the userspace suspend-blocker API, which would be an advantage.  It 
> > could even remove the need for the opportunistic-suspend workqueue -- 
> > opportunistic suspends would be initiated by the "suspend manager" 
> > process instead of by the kernel.
> > 
> > However you still have the issue of modifying the kernel drivers to 
> > disallow opportunistic suspend if their queues are non-empty.  Doing 
> > that is more or less equivalent to implementing kernel-level suspend 
> > blockers.  (The suspend blocker approach is slightly more efficient, 
> > because it will prevent a suspend from starting if a queue is 
> > non-empty, instead of allowing the suspend to start and then aborting 
> > it partway through.)
> >
> > Maybe I'm missing something here...  No doubt someone will point it out 
> > if I am.
> > 
> 
> Well, from my perspective that would limit changes to the evdev driver
> (well, limited input core plumbing will be needed) but that is using the
> current PM infrastructure. The HW driver changes will be limited to what
> you described "type 2" in your other e-mail.
> 
> Also, not suspending while events are in progress) is probably
> beneficial for platforms other than Android as well. So unless I am
> missing something this sounds like a win.

I agree that simplifying the user API would be an advantage.  Instead 
of the full-blown suspend-blocker interface, we would need only a way 
to initiate an opportunistic suspend.  For example:

	echo opportunistic >/sys/power/state

Alan Stern

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