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

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Date:	Thu, 27 May 2010 23:00:15 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Alan Stern <stern@...land.harvard.edu>,
	Peter Zijlstra <peterz@...radead.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	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 Thursday 27 May 2010, Thomas Gleixner wrote:
> On Thu, 27 May 2010, Alan Stern wrote:
> > 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.
> 
> I tend to disagree. You are still looking at suspend as some extra
> esoteric mechanism. We should stop doing this and look at suspend (to
> RAM) as an additional deep idle state, which is well defined in terms
> of power savings and wakeup latency.

Well, the ACPI spec doesn't agree with you. :-)

> That's what I think opportunistic suspend is all about. Now if you look at
> our current idle states in x86 the incoming keystroke is never lost. 
> 
> So when suspend does lose the wakeup event then we need to fix that,

With ACPI, we can't, because we have to switch our wakeup sources from
the "runtime" mode to the "system wakeup" mode in a racy fashion.

> but why do we need suspend blockers for this ? Let's take your example:
> 
> > The keyboard driver handles the keystroke and queues an input
> > event. Then the system goes into suspend ....
> 
> Why do we go into suspend at all?

The decision to go to suspend may be made in parallel to the keystroke in
such a way that we may not be able to detect it (that doesn't apply to the
hardware Android currently runs on, though).

> If there is an event queued then something is woken up and got running,
> which is reason enough _not_ to enter suspend. If that's broken in the
> current implementation then we need to fix it,

That's not a matter of implementation (which I admit can be improved).

> but not with a big hammer by adding another
> interface. We have that information already and obviously we do not
> use it, so lets figure out why before adding suspend blockers just
> because they paper over the underlying problem.

I'm not really sure what information you're referring to.

Rafael
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