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    SubjectRe: [RFC][PATCH] PM: Make power domain callbacks take precedence over subsystem ones
    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@rowland.harvard.edu> wrote:
    > > > On Wed, 13 Apr 2011, Rafael J. Wysocki wrote:
    > > >
    > > >> From: Rafael J. Wysocki <rjw@sisk.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.

    Grant presented a convincing explanation. I have no objections.

    Alan Stern



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