Viresh Kumar [off-list ref] writes:

On 24 February 2015 at 04:06, Kevin Hilman [off-list ref] wrote:

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Viresh Kumar [off-list ref] writes:

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+Optional properties:
+- shared-opp: Indicates that device nodes using this OPP descriptor's phandle
+  switch their DVFS state together, i.e. they share clock lines.

... or shared voltage rail?

The point is that they switch their frequencies together. Which means,
sharing voltage rail + PLLs .. How do we write it properly ?

s/they share clock lines/they share clock and/or voltage lines/

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+  Missing property means devices have independent clock lines, but they share OPPs.

huh?  missing 'shared-opp' property means they share OPPs?  -ECONFUSED.

Right. s/OPPs/OPP tables ..

Makes sense now ?

Yes.

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Maybe I missed some of the discussion of why this property is needed,
but I'm left wondering why it's needed explicitly.  With the OPPs as

So that same OPP tables can be shared across CPUs which don't share
voltage rails..  For example, Krait. We only need to define single set
of tables and all CPUs will point to it. But this property would be
missing in that case as CPUs don't change their DVFS state together.

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part of the CPU nodes, shouldnt' the "shared" nature of the OPP be
easily derived from the clock and or regulator (opp-supply) property of
the CPU nodes?  IOW, if two CPU nodes share a clock and/or a regulator,
the framework should know it's "shared".

So you missed all earlier discussions :), there were lots of concerns
around that.  And the best solution we found out is to do it this
way..

- There can be multiple clocks/regulators present in CPU's DT node and
that makes it complex.

- There are cases where immediate clock parents of CPUs are different
but somewhere higher in the hierarchy they have a common ancestor,
which is responsible for rate change. And so it would be difficult to
find out if they share clock/regulator or not..

- People wanted it to be some static data instead of trying to find
with help of some algorithms..

OK, fair enough.  Looks like it's been thought through.  

However, in the end, I don't think it's going to avoid the "help of some
algorithms."  The flag will tell you it's shared, but not how, and that
will likely still need to be determined.

Kevin