[RFC PATCH] Documentation: DT: arm: Add topology property to define package boundaries

[lorenzo.pieralisi@arm.com (Lorenzo Pieralisi)]

On Mon, Jan 22, 2018 at 08:45:26PM -0800, Frank Rowand wrote:
> On 01/22/18 09:15, Lorenzo Pieralisi wrote:
> > The current ARM DT topology description provides the operating system
> > with a topological view of the system that is based on leaf nodes
> > representing either cores or threads (in an SMT system) and a
> > hierarchical set of cluster nodes that creates a hierarchical topology
> > view of how those cores and threads are grouped.
> > 
> > As opposed to the ACPI topology description ([1], PPTT table), this
> > hierarchical representation of clusters does not allow to describe what
> > topology level actually represents the physical package boundary, which
> > is a key piece of information to be used by an operating system to
> > optimize resource allocation and scheduling.
> > 
> > Define an optional, backward compatible boolean property for cluster
> > nodes that, by reusing the ACPI nomenclature, add to the ARM DT
> > topological description a binding to define what cluster level
> > represents a physical package boundary.
> > 
> > [1] http://www.uefi.org/sites/default/files/resources/ACPI_6_2.pdf
> > 
> > Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
> > Cc: Rob Herring <robh+dt@kernel.org>
> > Cc: Sudeep Holla <sudeep.holla@arm.com>
> > Cc: Jeremy Linton <jeremy.linton@arm.com>
> > Cc: Morten Rasmussen <morten.rasmussen@arm.com>
> > Cc: Mark Rutland <mark.rutland@arm.com>
> > ---
> >  Documentation/devicetree/bindings/arm/topology.txt | 9 +++++++++
> >  1 file changed, 9 insertions(+)
> > 
> > diff --git a/Documentation/devicetree/bindings/arm/topology.txt b/Documentation/devicetree/bindings/arm/topology.txt
> > index de9eb0486630..8e78d76b0671 100644
> > --- a/Documentation/devicetree/bindings/arm/topology.txt
> > +++ b/Documentation/devicetree/bindings/arm/topology.txt
> > @@ -109,6 +109,15 @@ Bindings for cluster/cpu/thread nodes are defined as follows:
> >  	The cluster node name must be "clusterN" as described in 2.1 above.
> >  	A cluster node can not be a leaf node.
> >  
> > +	Properties for cluster nodes:
> > +
> > +	- physical-package
> > +		Usage: optional
> > +		Value type: <empty>
> > +		Definition: if present the cluster node represents the
> > +			    boundary of a physical package, whether socketed
> > +			    or surface mounted.
> 
> I don't know how to interpret this.  Is the node with this property inside
> or outside the boundary?  If I had to guess, I would guess inside.  A few
> extra words to clarify this please.

The node is neither inside nor outside, it _is_ the boundary. Every node
defines a topology level - the property is there to define which one
corresponds to a package, please let me know if it makes things clearer.

Thanks,
Lorenzo

> > +
> >  	A cluster node's child nodes must be:
> >  
> >  	- one or more cluster nodes; or
> > 
>