[PATCH v2] Documentation: DT: arm: Add topology property to define package boundaries

[Rob Herring]

On Fri, Feb 09, 2018 at 12:00:34PM +0000, 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.

Don't the NUMA related bindings already provide this?

The only thing I could see this being needed for is servicing (e.g. to 
replace a bad socket).

> 
> 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 at arm.com>
> Cc: Rob Herring <robh+dt at kernel.org>
> Cc: Sudeep Holla <sudeep.holla at arm.com>
> Cc: Jeremy Linton <jeremy.linton at arm.com>
> Cc: Morten Rasmussen <morten.rasmussen at arm.com>
> Cc: Frank Rowand <frowand.list at gmail.com>
> Cc: Mark Rutland <mark.rutland at arm.com>
> ---
> v1->v2:
> 
> 	- Added dual-package example
> 	- Improved physical-package property description according to review
> 	- Dropped RFC tag
> 
> v1: https://marc.info/?l=linux-arm-kernel&m=151664137216555&w=2
> 
>  Documentation/devicetree/bindings/arm/topology.txt | 432 +++++++++++++++++++++
>  1 file changed, 432 insertions(+)
> 
> diff --git a/Documentation/devicetree/bindings/arm/topology.txt b/Documentation/devicetree/bindings/arm/topology.txt
> index de9eb0486630..09b3b22e57c1 100644
> --- a/Documentation/devicetree/bindings/arm/topology.txt
> +++ b/Documentation/devicetree/bindings/arm/topology.txt
> @@ -109,6 +109,21 @@ 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.
> +			    The cluster node itself and all its children nodes
> +			    represent a single distinct physical-package unit.
> +			    The cluster node parent and sibling cluster nodes
> +			    (if any) must therefore be considered part of
> +			    separate physical package units in multi
> +			    physical-package systems.
> +
>  	A cluster node's child nodes must be:
>  
>  	- one or more cluster nodes; or
> @@ -470,6 +485,423 @@ cpus {
>  	};
>  };
>  
> +Example 3 (ARM 64-bit, 32-cpu system, 2 packages):

Can't you expand an existing example. If you have the complex cases, you 
can derive simple ones.

> +
> +cpus {
> +	#size-cells = <0>;
> +	#address-cells = <2>;
> +
> +	cpu-map {
> +		cluster0 {
> +			physical-package;

Why not just make this part of the node name? .../cluster0/cluster0/... 
isn't really the best naming IMO.

> +
> +			cluster0 {
> +				core0 {
> +					cpu = <&CPU0>;
> +				};