Re: [RFC PATCH 1/2] PM / OPP: add support to specify phandle of another node for OPP

[Mark Rutland <mark.rutland@arm.com>]

On Wed, Aug 21, 2013 at 11:48:12PM +0100, Stephen Warren wrote:
> On 08/20/2013 04:00 AM, Sudeep KarkadaNagesha wrote:
> ...
> > Until we get more feedback and agreement on new proposal can we have
> > this simple amendment in this patch to the existing binding ? Since the
> > new proposal[1] is backward compatible(this patch adding support for
> > option#5 to existing option#1), we will have to add support for other
> > binding options in [1] later.
> > 
> > This is needed to support shared OPPs with simple/single OPP profile
> > and also to fix the broken and unused binding
> > @Documentation/devicetree/bindings/cpufreq/arm_big_little_dt.txt
> > 
> > Regards,
> > Sudeep
> > 
> > [1] http://www.spinics.net/lists/cpufreq/msg06563.html
> 
> Presumably the desire for cpu1's node to say "go look at cpu0's node for
> OPP" is because they share OPPs. Don't they share OPPs because they are
> parts of the same device - that device being the CPU complex. As such,
> why not define the OPPs in /cpus rather than in each of /cpus/cpuN?

I'd very much like for it to be possible to factor out common properties
into the /cpus node, but it should follow the ePAPR recommendation fo
being treated as a fallback if not present in a particular /cpus/cpu@N
node -- that way we can handle clusters with differing OPPs. The
property might just be a phandle to a table node, but it should be
possible to make it common.

> 
> Of course, that doesn't help if there are separate CPU and GPU nodes
> that just happen to have the same set of OPPs and you want to share them
> to save DT space. Is that at all likely?

I suspect that the OPPs for CPUs and GPUs are likely to be quite
distict, and they are logically separate regardless. I'm not averse to
sharing of tables if we can handle them in a standard fashion.

> 
> I'd suggest/bike-shed that operating-points-device is not the correct
> property name; it somehow implies that the other device actively defines
> the OPPs for this device, rather than just happening to have the same
> OPPs. Perhaps "operating-points-identical-to"?
> 

I'd rather not have properties that point elsewhere and say "treat me
the same as this node". I'd rather we have common properties as
described above.

Thanks,
Mark.