On Thu, Apr 14, 2011 at 5:12 PM, Rafael J. Wysocki <rjw@xxxxxxx> wrote: > On Wednesday, April 13, 2011, Grant Likely wrote: >> On Wed, Apr 13, 2011 at 8:17 AM, Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote: >> > On Wed, 13 Apr 2011, Rafael J. Wysocki wrote: >> > >> >> From: Rafael J. Wysocki <rjw@xxxxxxx> >> >> >> >> 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. And so you should. g. _______________________________________________ linux-pm mailing list linux-pm@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/linux-pm
