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. Thanks, Rafael _______________________________________________ linux-pm mailing list linux-pm@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/linux-pm
