Re: [RFC][PATCH] Power domains for platform bus type

[Kevin Hilman <khilman@ti.com>]

Grant Likely <grant.likely@secretlab.ca> writes:

> On Mon, Jan 31, 2011 at 11:16:51PM +0100, Rafael J. Wysocki wrote:
>> On Monday, January 31, 2011, Alan Stern wrote:
>> > On Mon, 31 Jan 2011, Rafael J. Wysocki wrote:
>> > 
>> > > On Monday, January 31, 2011, Alan Stern wrote:
>> > > > On Sun, 30 Jan 2011, Rafael J. Wysocki wrote:
>> > > > 
>> > > > > > One thing about this implementation is slightly questionable.  The new
>> > > > > > power_domain callbacks were added to the __weak platform PM routines,
>> > > > > > which means they will have to be included in every overriding routine
>> > > > > > provided by a platform imiplementation.
>> > > > > > 
>> > > > > > Would it be better to separate these things?  Have the power_domain 
>> > > > > > callbacks occur in a static outer function which then calls a public 
>> > > > > > __weak inner function that can be overridden?
>> > > > > 
>> > > > > That certainly is a good idea, but I wasn't sure how to do that.  It looks
>> > > > > like I could keep the __weak functions as they are and modify
>> > > > > platform_dev_pm_ops instead to point to a new set of function that in turn
>> > > > > would call the __weak ones.  For example, the .suspend pointer in
>> > > > > platform_dev_pm_ops might point to a new function, say
>> > > > > platform_pm_full_suspend() that would call the power domain functions and
>> > > > > the "original" platform_pm_suspend().  Is that what you mean?
>> > > > 
>> > > > Yes.  But what about the platform_bus_set_pm_ops() interface?  Should
>> > > > platform-specific replacements for the pm_ops functions also include
>> > > > the power_domain callbacks?
>> > > 
>> > > Well, whoever uses platform_bus_set_pm_ops(), he can simply prevent power
>> > > domains from being used by not defining them in the first place. :-)
>> > 
>> > But what about the case where the user _does_ want to have power
>> > domains?
>> 
>> Ah, OK.  The caller of platform_bus_set_pm_ops() will replace the original
>> platform_dev_pm_ops with his own set of operations, so he will not see the
>> power domains.
>> 
>> > Do you want to make the replacement routines responsible for
>> > invoking the power-domain callbacks, or should the platform core handle
>> > this automatically?
>> 
>> Well, if someone replaces the entire platform_dev_pm_ops object, this means
>> that on his platform power management is substantially different from the
>> generic one.  In that case, IMO, he should be responsible for handling all
>> of the subsystem-level aspects of power management, including power domains.
>
> Part of point of doing something like power_domain is to *get rid* of
> platform_bus_set_pm_ops().  It is a horrid, stop-gap interface that
> doesn't scale.  I don't think much consideration needs to be made for
> users of platform_bus_set_pm_ops() in this regard.

As the author of platform_bus_set_pm_ops(), I humbly agree.  

Also, the __weak functions here were obsoleted by
platform_bus_set_pm_ops().  Once Magnus moves to
platform_bus_set_pm_ops() (or this new interface) the __weak attributes
should be removed (c.f. commit log below[1] where
platform_bus_set_pm_ops() was added.)

Kevin

commit c64a0926710153b9d44c979d2942f4a8648fd74e
Author: Kevin Hilman <khilman@ti.com>
Date:   Wed Aug 25 12:50:00 2010 -0700

    driver core: platform_bus: allow runtime override of dev_pm_ops
    
    Currently, the platform_bus allows customization of several of the
    busses dev_pm_ops methods by using weak symbols so that platform code
    can override them.  The weak-symbol approach is not scalable when
    wanting to support multiple platforms in a single kernel binary.
    
    Instead, provide __init methods for platform code to customize the
    dev_pm_ops methods at runtime.
    
    NOTE: after these dynamic methods are merged, the weak symbols should
          be removed from drivers/base/platform.c.  AFAIK, this will only
          affect SH and sh-mobile which should be converted to use this
          runtime approach instead of the weak symbols.  After SH &
          sh-mobile are converted, the weak symobols could be removed.