Subject: [RFC PATCH v2 3/4] powerpc: refactor of_get_cpu_node to support other architectures

From: [email-masked] (Sudeep KarkadaNagesha)
Date: Thu, 22 Aug 2013 17:51:50 +0100

On 22/08/13 14:59, Mark Rutland wrote:
> On Mon, Aug 19, 2013 at 02:56:10PM +0100, Sudeep KarkadaNagesha wrote:
>> On 19/08/13 14:02, Rob Herring wrote:
>>> On 08/19/2013 05:19 AM, Mark Rutland wrote:
>>>> On Sat, Aug 17, 2013 at 11:09:36PM +0100, Benjamin Herrenschmidt wrote:
>>>>> On Sat, 2013-08-17 at 12:50 +0200, Tomasz Figa wrote:
>>>>>> I wonder how would this handle uniprocessor ARM (pre-v7) cores, for
>>>>>> which 
>>>>>> the updated bindings[1] define #address-cells = <0> and so no reg 
>>>>>> property.
>>>>>>
>>>>>> [1] - http://thread.gmane.org/gmane.linux.ports.arm.kernel/260795
>>>>>
>>>>> Why did you do that in the binding ? That sounds like looking to create
>>>>> problems ... 
>>>>>
>>>>> Traditionally, UP setups just used "0" as the "reg" property on other
>>>>> architectures, why do differently ?
>>>>
>>>> The decision was taken because we defined our reg property to refer to
>>>> the MPIDR register's Aff{2,1,0} bitfields, and on UP cores before v7
>>>> there's no MPIDR register at all. Given there can only be a single CPU
>>>> in that case, describing a register that wasn't present didn't seem
>>>> necessary or helpful.
>>>
>>> What exactly reg represents is up to the binding definition, but it
>>> still should be present IMO. I don't see any issue with it being
>>> different for pre-v7.
>>>
>> Yes it's better to have 'reg' with value 0 than not having it.
>> Otherwise this generic of_get_cpu_node implementation would need some
>> _hack_ to handle that case.
> 
> I'm not sure that having some code to handle a difference in standard
> between two architectures is a hack. If anything, I'd argue encoding a
> reg of 0 that corresponds to a nonexistent MPIDR value (given that's
> what the reg property is defined to map to on ARM) is more of a hack ;)
> 
Agreed. But I am more confused.

1. This raises another question as how much do we follow from ePAPR
standard. ePAPR marks the reg property in /cpu as required.
Does that mean we must have all the properties marked as required to be
present in DT ? On the contrary timebase/clock-frequency is some thing
that can be found dynamically and need not be present but they are
marked as required too.

> I'm not averse to having a reg value of 0 for this case, but given that
> there are existing devicetrees without it, requiring a reg property will
> break compatibility with them.
> 

2. What exactly does backward compatibility has to cover ? This can't be
considered as breaking of the binding definition. In continuation to the
above argument that reg property is required, do we need to cover this
as it's clearly a case of missing required property(this holds only if
reg is required always).

Regards,
Sudeep