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

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Date:	Thu, 27 May 2010 19:26:33 +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 07:59:02PM +0200, Thomas Gleixner wrote:
> On Thu, 27 May 2010, Matthew Garrett wrote:
> > 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.
> 
> Right, it does not as of today. So we cannot use that on x86
> hardware. Fine. That does not prevent us to implement it for
> architectures which can do it. And if x86 comes to the point where it
> can handle it as well we're going to use it. Where is the problem ? If
> x86 cannot guarantee the wakeup sources it's not going to be used for
> such devices. The kernel just does not provide the service for it, so
> what ?

We were talking about PCs. Suspend-as-c-state is already implemented for 
OMAP.

> So the only thing you are imposing to app writers is to use an
> interface which solves nothing and does not save you any power at
> all. 

It's already been demonstrated that the Android approach saves power.

> Runnable tasks and QoS guarantees are the indicators whether you can
> go to opportunistic suspend or not. Everything else is just window
> dressing.

As I keep saying, this is all much less interesting if you don't care 
about handling suboptimal applications. If you do care about them then 
the Android approach works. Nobody has demonstrated a scheduler-based 
one that does.

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