Re: [RFC][PATCH] PM: Make power domain callbacks take precedence oversubsystem ones

[Grant Likely]

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.
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