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

From: Grant Likely
Date: Wed Apr 13 2011 - 12:15:45 EST

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.

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