On Fri, 15 Apr 2011, Rafael J. Wysocki 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. Grant presented a convincing explanation. I have no objections. Alan Stern _______________________________________________ linux-pm mailing list linux-pm@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/linux-pm
