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    SubjectRe: [RFC][PATCH] PM: Make power domain callbacks take precedence over subsystem ones
    On Thu, Apr 14, 2011 at 5:12 PM, Rafael J. Wysocki <> wrote:
    > On Wednesday, April 13, 2011, Grant Likely wrote:
    >> On Wed, Apr 13, 2011 at 8:17 AM, Alan Stern <> wrote:
    >> > On Wed, 13 Apr 2011, Rafael J. Wysocki wrote:
    >> >
    >> >> From: Rafael J. Wysocki <>
    >> >>
    >> >> 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.

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