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

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Date:	Thu, 27 May 2010 18:23:43 +0100
From:	Matthew Garrett <mjg59@...f.ucam.org>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Arve Hjønnevåg <arve@...roid.com>,
	Florian Mickler <florian@...kler.org>,
	Vitaly Wool <vitalywool@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Paul@...p1.linux-foundation.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, May 27, 2010 at 07:15:31PM +0200, Thomas Gleixner wrote:
> On Thu, 27 May 2010, Matthew Garrett wrote:
> > You still need the in-kernel suspend blockers if you want to guarantee 
> > that you can't lose wakeup events. But yes, if you're not concerned 
> > handling badly behaved applications then I believe that you can lose 
> > opportunistic suspend and just use the scheduler.
> 
> No, we do not. We need correctly implemented drivers and a safe
> switchover from normal event delivery to wakeup based.

What is a "Correctly implemented driver" in this case? One that receives 
a wakeup event and then prevents suspend being entered until userspace 
has acknowledged that event? Because that's what an in-kernel suspend 
blocker is.

> > My question was about explicit suspend states, not implicitly handling 
> > an identical state based on scheduler constraints. Suspend-as-a-C-state 
> > isn't usable on x86 - you have to explicitly trigger it based on some 
> 
> And why not ? Just because suspend is not implemented as an ACPI
> C-state ? 
> 
> Nonsense, if we want to push the system into suspend from the idle
> state we can do that. It's just not implemented and we've never tried
> to do it as it requires a non trivial amount of work, but I have done
> it on an ARM two years ago as a prove of concept and it works like a
> charm.

ACPI provides no guarantees about what level of hardware functionality 
remains during S3. You don't have any useful ability to determine which 
events will generate wakeups. And from a purely practical point of view, 
since the latency is in the range of seconds, you'll never have a low 
enough wakeup rate to hit it.

> > policy. And if you want to be able to do that without risking the loss 
> > of wakeup events then you need in-kernel suspend blockers.
> 
> Crap. Stop beating on those lost wakeup events. If we lose them then
> the drivers are broken and do not handle the switch over correctly. Or
> the suspend mechanism is broken as it does not evaluate the system
> state correctly. Blockers are just papering over that w/o tackling the
> real problem.

Ger;kljaserf;kljf;kljer;klj. Suspend blockers are the mechanism for the 
driver to indicate whether the wakeup event has been handled. That's 
what they're there for. The in-kernel ones don't paper over anything.

-- 
Matthew Garrett | mjg59@...f.ucam.org
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