Re: [PATCH v2 1/5] PM / OPP: extend DT binding to specify phandle of another node for OPP

[Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>]

On Thu, Oct 17, 2013 at 02:22:44PM +0100, Rob Herring wrote:

[...]

> >>> +    cpu3: cpu@101 {
> >>> +            compatible = "arm,cortex-a7";
> >>> +            reg = <101>;
> >>> +            operating-points-phandle = <&cluster1_opp>;
> >>> +    };
> >>> +
> >>> +    opps-table {
> >>> +            cluster0_opp: cluster0_opp {
> >>
> >> Why not use the cpu topology? Then the operating point can apply to
> >> cores based on the position in the topology. You don't even need a
> >> phandle in that case. You can look for OPPs in either a cpu node or in
> >> the topology.
> >>
> > Agreed but few thoughts behind this binding:
> >
> > 1. OPPs are not just cpu specific:
> >    How do we share OPPs for 2 devices in the same clock domain ?
> >    Also moving the OPP into cpu-topo makes parsing specific to cpu.
> >    Currently the OPP library fetches the of_node from the device struct
> >    which is applicable to any devices.
> 
> But an OPP for a cpu is cpu specific, so put it there. For devices, we
> may need something different. Perhaps it should be part of the clock
> binding in some way.
> 
> > 2. Should cpu topology(i.e. cpu-map) just contain the topology info ? and
> >    phandles to these nodes should be used to setup any affinity ?
> 
> Can't the OPP just be in the topology nodes at the appropriate level.
> Then the kernel can look for the OPPs in either place. Perhaps Lorenzo
> has thoughts on this.

The reason we introduced the topology nodes (ie cpu-map) was to provide
a topology description of the system with leaf nodes linked to cpu
nodes; nodes in the topology do not map to HW entities.
Put it differently, IMHO we should attach no HW meaning to nodes in the
cpu-map so we should not add eg OPPs under cluster nodes because those
nodes do not represent HW entities, I would like to keep cpu-map a pure
DT representation of the topology, no more, but debate is open.

Lorenzo