Currently, there is an assumption that the performance domains as provided by the SCMI protocol should be mirroring the exact implementation in hardware, for example, the clock domains, which are a typical type of performance domains. By design, an SCMI performance domain defines the granularity of performance controls, it does not describe any underlying hardware dependencies (although they may match in many cases). As a consequence, in platforms where hardware may have the ability to control cpu performance at different granularity and choose to describe fine-grained performance control through SCMI performance domains, there is currently no way for OSPM to discover the actual cpu hardware dependencies. Inevitably, software components that rely on this missing description will cease to work. Thus, there is a need for a new description of hardware dependencies where the performance level is coordinated by hardware (or firmware) since these dependency domains might be larger than the SCMI performance domains. The example given here is referring specifically to SCMI in order to provide a clear and direct example of a scenario. However, the proposed binding is not SCMI specific. It allows DT to describe a generic hardware property, CPU performance dependency, which may not be available through other sources. This new optional binding will provide visibility to OSPM on any hardware or firmware coordination of performance requests and enable more accurate assumptions about performance and performance side-effects of requesting performance level changers. This is essential information for OSPM thermal and energy management frameworks. There are two main reasons to support this new addition: 1) Per-cpu control & SCMI performance domains Same as explained above. Some platforms would like to make aggregation decisions in firmware and want to describe themselves as having per-cpu control. In order to continue to make sane decisions in the OSPM layer, we need to know about the underlying connections. With this optional binding, we provide performance dependencies information to OSPM for sets of CPUs while the h/w coordinates the performance level for each cpu. 2) ACPI vs dt With respect to performance, ACPI describes two main types of coordination that may take place in system when logical processors are required to transition to a different power/performance state. These two types are software coordination (SW) and hardware coordination (HW). In the first one, OSPM is in charge of handling such transitions while preserving the integrity of the entire system. In the latter case, the h/w is responsible for ensuring correct operations. In the HW coordination, OSPM can control each processor as if they were all independent each other. However, platforms can use ACPI defined interfaces to group CPUs to create so called "dependency domain". Such interface is the _PSD method. Users in kernel that need to know dependencies among cores, can retrieve such information via _PSD [1]. If the same system needs to work with dt + SCMI, we will have all the controls, but we are missing the information performance level coordination in hardware/firmware, This new dt binding provides the missing bits. [1]ACPI Specification, version 6.3 - 8.3 Power, Performance, and Throttling State Dependencies Signed-off-by: Nicola Mazzucato <nicola.mazzucato@xxxxxxx> --- .../bindings/arm/cpu-perf-dependencies.yaml | 48 +++++++++++++++++++ 1 file changed, 48 insertions(+) create mode 100644 Documentation/devicetree/bindings/arm/cpu-perf-dependencies.yaml diff --git a/Documentation/devicetree/bindings/arm/cpu-perf-dependencies.yaml b/Documentation/devicetree/bindings/arm/cpu-perf-dependencies.yaml new file mode 100644 index 000000000000..c7a577236cd6 --- /dev/null +++ b/Documentation/devicetree/bindings/arm/cpu-perf-dependencies.yaml @@ -0,0 +1,48 @@ +# SPDX-License-Identifier: GPL-2.0-only or BSD-2-Clause +%YAML 1.2 +--- +$id: http://devicetree.org/schemas/arm/cpu-perf-dependencies.yaml# +$schema: http://devicetree.org/meta-schemas/core.yaml# + +title: CPU Performance Dependencies + +maintainers: + - Nicola Mazzucato <nicola.mazzucato@xxxxxxx> + +description: |+ + This optional node provides information to OSPM of cpu performance + dependencies. + Each list represents a set of CPUs which have performance level + dependencies and can assumed to be roughly at the same performance + level coordinated by hardware and/or firmware. + Example: Describing CPUs in the same clock domain. + +properties: + compatible: + items: + - enum: + - arm,cpu-perf-dependencies + + cpu-perf-affinity: + $ref: '/schemas/types.yaml#/definitions/phandle' + description: | + Specifies a list of phandles to CPU nodes corresponding to a set of CPUs + which have performance affinity. + +additionalProperties: false + +examples: + - | + cpu-performance-dependencies { + compatible = "arm,cpu-perf-dependencies"; + cpu-perf-dep0 { + cpu-perf-affinity = <&CPU0>, <&CPU1>, <&CPU2>, <&CPU3>; + }; + cpu-perf-dep1 { + cpu-perf-affinity = <&CPU4>, <&CPU5>, <&CPU6>; + }; + cpu-perf-dep2 { + cpu-perf-affinity = <&CPU7>; + }; + }; +... -- 2.27.0
