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

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Message-ID: <Pine.LNX.4.44L0.1005242130290.8440-100000@netrider.rowland.org>
Date:	Mon, 24 May 2010 21:34:54 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Arve Hjønnevåg <arve@...roid.com>
cc:	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 Mon, 24 May 2010, Arve Hjønnevåg wrote:

> > Wakeup events can be lost in at least three different ways:
> >
> >     1. A hardware signal (such as an IRQ) gets ignored.
> >
> >     2. The hardware event occurs, but without effect since the
> >        kernel thread that would handle the event has been frozen.
> >        The event just ends up sitting in a queue somewhere until
> >        something else wakes up the system.
> >
> >     3. The hardware event occurs and the kernel handles it fully,
> >        but the event propagates to userspace for further handling
> >        and the user program is already frozen.
> >
> > 1 is a hardware configuration failure (for example, it might happen as
> > a result of using edge-triggered IRQs instead of level-triggered) and
> > is outside the scope of this discussion.
> >
> > 2 generally represents a failure of the core PM subsystem, or a failure
> > of some other part of the kernel to use the PM core correctly.  In
> > theory we should be able to fix such mistakes.  Right now I'm aware of
> > at least one possible failure scenario that could be fixed fairly
> > easily.
> >
> > 3 is the type of failure that suspend blockers were really meant to
> > handle, particularly the userspace suspend-blocker API.

> I don't see a big difference between 2 and 3. You can use suspend
> blockers to handle either.

You can, but they aren't necessary.  If 2 were the only reason for 
suspend blockers, I would say they shouldn't be merged.

Whereas 3, on the other hand, can _not_ be handled by any existing
mechanism.  3 is perhaps the most important reason for using suspend
blockers.

Alan Stern

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