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

On Mon, Aug 19, 2013 at 02:56:10PM +0100, Sudeep KarkadaNagesha wrote:

On 19/08/13 14:02, Rob Herring wrote:

quoted

On 08/19/2013 05:19 AM, Mark Rutland wrote:

quoted

On Sat, Aug 17, 2013 at 11:09:36PM +0100, Benjamin Herrenschmidt wrote:

quoted

On Sat, 2013-08-17 at 12:50 +0200, Tomasz Figa wrote:

quoted

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 ;)

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.

Mark.