linux-kernel - Re: [linux-pm] [PATCH 0/8] Suspend block api (version 8)

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Date:	Thu, 27 May 2010 11:47:48 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Thomas Gleixner <tglx@...utronix.de>,
	Arve Hjønnevåg <arve@...roid.com>,
	<Paul@...p1.linux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Florian Mickler <florian@...kler.org>,
	<felipe.balbi@...ia.com>,
	Linux OMAP Mailing List <linux-omap@...r.kernel.org>,
	Linux PM <linux-pm@...ts.linux-foundation.org>
Subject: Re: [linux-pm] [PATCH 0/8] Suspend block api (version 8)

On Thu, 27 May 2010, Peter Zijlstra wrote:

> On Thu, 2010-05-27 at 16:33 +0100, Alan Cox wrote:
> > On Thu, 27 May 2010 17:09:16 +0200
> > Peter Zijlstra <peterz@...radead.org> wrote:
> > 
> > > On Thu, 2010-05-27 at 11:06 -0400, Alan Stern wrote:
> > > > 
> > > >         Opportunistic suspends are okay.
> > > > 
> > > >         The proposed userspace API is too Android-specific.
> > > 
> > > I would argue opportunistic suspends are not ok, and therefore the
> > > proposed API is utterly irrelevant.
> > 
> > Assuming you are happy that opportunistically entering C6 and the like is
> > ok via ACPI can you explain why you have a problem with opportunistic
> > suspend and why it is different to a very deep sleep CPU state such as we
> > have now (and which on many embedded platforms we deal with *is* sleeping
> > external devices too)
> 
> Agreed, but I understood the opportunistic suspend line from Alan Stern
> to mean the echo opportunistic > /sys/power/foo thing.

Yes, that's what I meant.  Why do you think it is any worse than "echo 
mem >/sys/power/state"?  The only difference is that it will block 
until all in-kernel suspend blockers are disabled.

Or do you also think that "echo mem >/sys/power/state" is evil and 
should be removed from the kernel as soon as possible?

And to answer Thomas's question: The whole reason for having in-kernel 
suspend blockers is so that userspace can tell the system to suspend 
without losing wakeup events.

Suppose a key is pressed just as a user program writes "mem" to
/sys/power/state.  The keyboard driver handles the keystroke and queues
an input event.  Then the system suspends and doesn't wake up until
some other event occurs -- even though the keypress was an important
one and it should have prevented the system from suspending.

With in-kernel suspend blockers and opportunistic suspend, this 
scenario is prevented.  That is their raison-d'etre.

Alan Stern

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