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

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

On Wed, Aug 28, 2013 at 08:46:38PM +0100, Grant Likely wrote:
> On Thu, 22 Aug 2013 14:59:30 +0100, Mark Rutland <mark.rutland@arm.com> 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 ;)
> > 
> > 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.
> 
> Then special cases those device trees, but you changing existing
> convention really needs to be avoided. The referenced documentation
> change is brand new, so we're not stuck with it.

I have no problem with changing the bindings and forcing:

#address-cells = <1>;
reg = <0>;

for UP predating v7, my big worry is related to in-kernel dts that we
already patched to follow the #address-cells = <0> rule (and we had to
do it since we got asked that question multiple times on the public
lists).

What do you mean by "special case those device trees" ? I have not
planned to patch them again, unless we really consider that a necessary
evil.

Thanks,
Lorenzo