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

[sudeep.holla@arm.com (Sudeep Holla)]

On 22/01/18 23:25, Jeremy Linton wrote:
> Hi,
> 
> On 01/22/2018 11:15 AM, 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.
>> +
>> ????? A cluster node's child nodes must be:
>> ? ????? - one or more cluster nodes; or
>>
> 
> Looks good so far, but I would worry about it being optional. 

We have no choice, it has to be optional for compatibility reasons as
stated in the commit message.

> That seemed like a mistake on the ACPI side.

Not necessarily. It's firmware choice as long as it knows how to deal
with the default(absence in case of DT)

> 
> Assuming it is optional, it might be worth a sentence explaining what
> the fallback behavior is if the property is missing.

That would be helpful.

-- 
Regards,
Sudeep