Re: [RFC][PATCH 0/14] PM / shmobile: Pass power domain information via DT (was: Re: [RFD] PM: Device tree representation of power domains)

[Arnd Bergmann]

On Tuesday 24 July 2012, Rafael J. Wysocki wrote:
> On Tuesday, July 24, 2012, Arnd Bergmann wrote:
> > On Saturday 21 July 2012, Rafael J. Wysocki wrote:

> > 
> > Sorry for taking so long to reply. I am really not that familiar with the
> > power domain requirements, but I do have two comments on your approach:
> > 
> > * I think when we want to add a generic concept to the device tree such
> >   as power domains, we should always make it specified in a generic way.
> 
> Do we really want that?  I'm a bit skeptical, because apparently nobody
> cares, as the (zero) response to this patchset evidently indicates and
> since nobody cares, it's probably better not to add such "generic" things
> just yet.

Well, the trouble with bindings is that they are much harder to change
later, at least in incompatible ways. 

> >   You have used the "renesas,pmdomain" attribute, which is specific to
> >   one vendor and requires platform specific code to read it.
> >   Is there any reason not to put the code into the generic
> >   drivers/base/power/domain.c file (or near it) and make a binding that
> >   works for everyone?
> 
> Yes, there is.  A couple of them in fact.
> 
> First off, power domains will always need platform-specific code to support
> them, no matter what.  The problem is that they tend to require special
> handling, even within the same SoC, let alone different SoCs, and that handling
> has to be implemented in an SoC-specific way, because it has to know various
> things about the platform (like what to write to what register(s) at what time
> and so on).  Of course, that platform-specific code needs to know which
> domain the given description corresponds to and there doesn't seem to be any
> useful way to specify that through DTs.

We have the same problem for a lot of other subsystems: clock, regulator,
irq, gpio, pinctrl, dma, ...

In each of these cases, we want a driver to be able to associate some
property with a driver (or platform code) from another subsystem.
We try to handle those using generic subsystem code that interprets
regular property names.

> Second, the generic code needed to support such a binding would have to be
> quite complex and I don't see any advantage from adding it.

The generic binding would only need to specify what the property
looks like and how the possible values can be determined. We don't
need to make that code generic right away but could wait until we
have a couple of implementations before we pull out the common bits.

The important part to me is writing the binding in a way that allows
us to do this.

> 
> > * Mark suggested two options (string and phandle), and (you guessed it),
> 
> To me, the Mark's suggestion was to follow the example of TI hwmods, which
> I did.  In fact, the power domains on Renesas platforms are an analogous
> concept, so in my opinion handling them in analogy with hwmods makes a lot of
> sense.

I had never seen the hwmod binding before and if I had I would have
given them the same comments. I definitely don't think we should be
using it as a good example for common code.

> >   IMHO using phandle makes it much easier to do this in a generic way,
> >   without having to document every possible string that this can be
> >   set to in the binding document. Obviously the phandle requires having
> >   a node in the device tree for each domain, but I think having that
> >   node is actually an advantage because it lets you describe the
> >   hierarchy of the domains there.
> 
> I don't quite see how it is better.  I could (and should in fact) represent
> that hierarchy as an array of pairs of strings without adding any special
> parsing code to the kernel (except for a simple routine walking that table
> and calling pm_genpd_add_subdomain_names() for every pair).
> 
> The only disadvantage of the current approach I can see is that whoever
> creates a dts for a board based on the given SoC has to know the names of the
> power domains to use in there.  I don't think this is an overly burdensome
> requirement.

Well, it does mean that we need to maintain a separate binding document
for each soc that has its own set of possible values.

> The other way around, the platform code supposed to handle the power domains
> would need a way to match DT nodes against the domains, which also would
> require some string-based identification and that would have to be documented
> as well.

The part that matches the pm-domain device nodes can and should be
specific to the platform implementing the pm-domain. We can easily
do the same thing we did for regulators, where each regulator can
be either identified through its "reg" property in cases where that
makes sense or through a "regulator-compatible" property in cases
where we need a string representation.

	Arnd
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