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

[Sudeep KarkadaNagesha <Sudeep.KarkadaNagesha@arm.com>]

On 22/08/13 16:50, Mark Rutland wrote:
> On Thu, Aug 22, 2013 at 04:32:10PM +0100, Sudeep KarkadaNagesha wrote:
>> On 22/08/13 12:59, Mark Rutland wrote:
>>> On Wed, Aug 21, 2013 at 11:48:12PM +0100, Stephen Warren wrote:
[...]
>>>>
>>>> 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.
>>>
>> Agreed, but for platforms with multiple CPU clusters, since we have only
>> one /cpus node, we ned to have table node which is arguable if node has
>> represent a device(as mentioned above)
> 
> I agree that this seems wasteful of space, but I really don't think that
> pointing at another device you want the OPPs of is the best way of
> describing the linkage, and I suspect we'll get all sorts of stupid bugs
> resulting from that style of binding.
> 
> Consider the following (properties trimmed for brevity): 
> 
> cpus {
> 	cpu0: cpu@0 {
> 		operating-points-identical-to = <&cpu1>;
> 	};
> 	cpu1: cpu@1 {
> 		operating-points = <0 100>,
> 				   <23 47>,
> 				   <62 970>;
> 	}
> };
> 
> If we boot a UP kernel on the above, I assume we won't read the info for
> cpu1, and thus we won't get operating points info for cpu0. Worse, what
> if cpu1 has status="disabled"? Does that make its OPP table invalid?
> What if the bootloader drops cpu1?
> 
Another question :), does status property dictate the validity of the
other properties in the node(not specific to above example, in general)?

But I agree that this indirect linkage is broken as boot-loaders can
drop the cpu node holding OPP.

IMO fallback method would be reasonable(and much cleaner compared to a
separate node) if it can even accommodate multiple cpu clusters.

> I really don't like indirecting linkage to a property through an
> arbitrary node, simply because it happened to have the same property. It
> creates an artificial depdendency that will lead to problems.
> 
I totally agree on this.

Regards,
Sudeep