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Date: Wed, 13 Apr 2011 10:15:16 -0600 From: Grant Likely <grant.likely@...retlab.ca> To: Alan Stern <stern@...land.harvard.edu> Cc: "Rafael J. Wysocki" <rjw@...k.pl>, 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 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. g. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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