Date:	Fri, 15 Apr 2011 01:12:24 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Grant Likely <grant.likely@...retlab.ca>
Cc:	Alan Stern <stern@...land.harvard.edu>,
	Linux PM mailing list <linux-pm@...ts.linux-foundation.org>,
	Kevin Hilman <khilman@...com>,
	LKML <linux-kernel@...r.kernel.org>, Len Brown <lenb@...nel.org>,
	linux-sh@...r.kernel.org, lethal@...ux-sh.org,
	Magnus Damm <magnus.damm@...il.com>
Subject: Re: [RFC][PATCH] PM: Make power domain callbacks take precedence over subsystem ones

On Wednesday, April 13, 2011, Grant Likely wrote:
> On Wed, Apr 13, 2011 at 8:17 AM, Alan Stern <stern@...land.harvard.edu> wrote:
> > On Wed, 13 Apr 2011, Rafael J. Wysocki wrote:
> >
> >> From: Rafael J. Wysocki <rjw@...k.pl>
> >>
> >> Change the PM core's behavior related to power domains in such a way
> >> that, if a power domain is defined for a given device, its callbacks
> >> will be executed instead of and not in addition to the device
> >> subsystem's PM callbacks.
> >>
> >> The idea behind the initial implementation of power domains handling
> >> by the PM core was that power domain callbacks would be executed in
> >> addition to subsystem callbacks, so that it would be possible to
> >> extend the subsystem callbacks by using power domains.  It turns out,
> >> however, that this wouldn't be really convenient in some important
> >> situations.
> >>
> >> For example, there are systems in which power can only be removed
> >> from entire power domains.  On those systems it is not desirable to
> >> execute device drivers' PM callbacks until it is known that power is
> >> going to be removed from the devices in question, which means that
> >> they should be executed by power domain callbacks rather then by
> >> subsystem (e.g. bus type) PM callbacks, because subsystems generally
> >> have no information about what devices belong to which power domain.
> >> Thus, for instance, if the bus type in question is the platform bus
> >> type, its PM callbacks generally should not be called in addition to
> >> power domain callbacks, because they run device drivers' callbacks
> >> unconditionally if defined.
> >
> > What about systems where it makes sense to execute the subsystem
> > callbacks even if power isn't going to be removed from the device?
> > It's quite possible that the subsystem could reduce the device's power
> > consumption even when the device isn't powered down completely.
> 
> The understanding Rafael and I came to was that if a power domain is
> attached to a device, then the power domain becomes the responsible
> party.  Normally this means it will turn around and immediately call
> the bus_type pm ops, but it has the option to not call them if for a
> particular system it knows better, or to defer calling them.
> 
> Basically, if you're using a power domain, it is assumed that the
> power domain has particular knowledge about the system, and it should
> have the option to override the default behaviour.
> 
> >
> > Is the extra overhead of invoking the subsystem callback really all
> > that troublesome?
> 
> It isn't an overhead problem.  It's a control & complexity problem.
> We could try to implement a heuristic or api to control when the bus
> type PM ops should be overridden, but I think it is cleaner to make it
> a rule that if you implement a power domain, then that power domain
> becomes responsible for all PM operations.

Well said. :-)

I'm taking that as an ACK for my patch if you don't mind.

Thanks,
Rafael
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