Message-Id: <201107010055.26952.rjw@sisk.pl>
Date:	Fri, 1 Jul 2011 00:55:26 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Kevin Hilman <khilman@...com>
Cc:	Linux PM mailing list <linux-pm@...ts.linux-foundation.org>,
	"Greg Kroah-Hartman" <gregkh@...e.de>,
	Magnus Damm <magnus.damm@...il.com>,
	Paul Walmsley <paul@...an.com>,
	Alan Stern <stern@...land.harvard.edu>,
	LKML <linux-kernel@...r.kernel.org>, linux-sh@...r.kernel.org,
	Paul Mundt <lethal@...ux-sh.org>
Subject: Re: [PATCH 7/10 v6] PM / Domains: Don't stop wakeup devices during system sleep transitions

On Friday, July 01, 2011, Kevin Hilman wrote:
> "Rafael J. Wysocki" <rjw@...k.pl> writes:
> 
> > On Thursday, June 30, 2011, Kevin Hilman wrote:
> >> "Rafael J. Wysocki" <rjw@...k.pl> writes:
> >> 
> >> > From: Rafael J. Wysocki <rjw@...k.pl>
> >> >
> >> > Devices that are set up to wake up the system from sleep states
> >> > should not be stopped and power should not be removed from them
> >> > when the system goes into a sleep state.  
> >> 
> >> I don't think this belongs in the generic layer since the two
> >> assumptions above are not generally true on embedded systems, and would
> >> result in rather significant power consumption unnecessarily.
> >
> > As to whether or not this belongs to the generic layer, I don't quite agree
> > (see below), but the changelog seems to be a bit inaccurate.
> >
> >> First, whether the device should be stopped on device_may_wakeup():
> >> b
> >> Some IP blocks (at least on OMAP) have "asynchronous" wakeups.  Meaning
> >> that they can generate wakeups even when they're not clocked (a.k.a
> >> stopped).  So in this case, even after a ->stop_device (which clock
> >> gates the IP), it can still generate wakeups.
> >> 
> >> Second, whether the device should be powered off if device_may_wakeup():
> >> 
> >> Embedded SoCs have other ways to wakeup than device-level wakeups.
> >> 
> >> For example, on OMAP, every pad on the SoC can be configured as a wakeup
> >> source So, for example, you could completely power down the UART IP
> >> blocks (and the enclosing power domain), configure the UART RX pad as a
> >> wakeup source, and still wakeup the system on UART activity.  The OMAP
> >> docs call these IO pad wakeups.
> >> 
> >> On OMAP in fact, this is the common, default behavior when we enable
> >> "off-mode" in idle and/or suspend, since most of the IPs are powered off
> >> but can still wake up the system.
> >> 
> >> So in summary, even if device_may_wakeup() is true, many devices (with
> >> additional SoC magic) can still generate wakeups even when stopped and
> >> powered off.
> >
> > Well, on the other hand, on some SoCs there are devices that can't be
> > powered off (or "declocked") if they are supposed to generate wakeups.
> 
> Correct.
> 
> > Also, I'm sure there are cases in which wakeups can be generated for devices
> > with their clocks off, but only if power is present.  
> 
> Yes.
> 
> > So there are multiple
> > cases, but not so many overall.  So, IMO, it makes sense to handle that at
> > the generic level, although not necessarily in such a simplistic way.
> >
> > Now, at this point, I want to do something very simple, which I think is
> > done by this patch. 
> >
> > Is this optimal power comsumption-wise for every potential
> > user of the framework?  
> 
> Well, sub-optimal would be an understatement.  I would consider this a
> major regression since if we were to use this for OMAP, we would never
> hit the full-chip low-power states if *any* device had wakeups enabled,
> whereas today we can.
> 
> > No, but certainly for some it's sufficient.  Is it
> > going to work in general?  I think it is.
> >
> > Of course, there's the question how to handle that more accurately and I have
> > some ideas.  If you have any, please let me know.
> >
> > In the meantime, I'm going to modify the changelog so that it's clear that
> > it's a "first approximation" thing, like in the patch below.
> >
> > Thanks,
> > Rafael
> >
> > ---
> > From: Rafael J. Wysocki <rjw@...k.pl>
> > Subject: PM / Domains: Don't stop wakeup devices during system sleep transitions
> >
> > There is the problem how to handle devices set up to wake up the
> > system from sleep states during system-wide power transitions.
> > In some cases, those devices can be turned off entirely, because the
> > wakeup signals will be generated on their behalf anyway.  In some
> > other cases, they will generate wakeup signals if their clocks are
> > stopped, but only if power is not removed from them.  Finally, in
> > some cases, they can only generate wakeup signals if power is not
> > removed from them and their clocks are enabled.
> 
> That's a good summary.
> 
> > In the future, it will be necessary to take all of the above
> > situations into account, but for starters it is possible to use
> > the observation that if all wakeup devices are treated like the
> > last group (i.e. their clocks are enabled and power in not removed
> > from them during system suspend transitions), they all will be able
> > to generate wakeups, although power consumption in the resulting
> > system sleep state may not be optimal in some cases.
> 
> I'm not opposed to this kind of check happening.  I'm only opposed to it
> happening in this "generic" layer because..., well, it's not generic.
> 
> Not only is it not generic, it would be a major regression in power
> consumption for anyone moving to this layer that has the various
> different wakeup capabilities already described.
> 
> The decision of whether or not to clock gate and/or power gate based on
> wakeup capabilies has to be made somewhere (and in fact is already made
> by existing code.)  But IMO, that decision should only be made where
> wakeup capabilies are known, so that sensible decisions (for power
> management) can be made.
> 
> Until there is a way in the generic code to distinguish between the
> various ways a device can wakeup, this decision should be left up to the
> code that knows how.

OK, so I suppose your suggestion is to drop the patch and let the
.stop_device() and .power_off() PM domain callbacks to hand that, is this
correct?

Rafael
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