linux-kernel - Re: [PATCH v3 1/2] dt-bindings: cpufreq: add bindings for virtual cpufreq

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Date:   Sat, 5 Aug 2023 21:38:34 +0200
From:   Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
To:     David Dai <davidai@...gle.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Conor Dooley <conor+dt@...nel.org>,
        Sudeep Holla <sudeep.holla@....com>,
        Saravana Kannan <saravanak@...gle.com>
Cc:     Quentin Perret <qperret@...gle.com>,
        Masami Hiramatsu <mhiramat@...gle.com>,
        Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Marc Zyngier <maz@...nel.org>,
        Oliver Upton <oliver.upton@...ux.dev>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Pavan Kondeti <quic_pkondeti@...cinc.com>,
        Gupta Pankaj <pankaj.gupta@....com>,
        Mel Gorman <mgorman@...e.de>, kernel-team@...roid.com,
        linux-pm@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/2] dt-bindings: cpufreq: add bindings for virtual
 cpufreq

On 31/07/2023 19:46, David Dai wrote:
> Adding bindings to represent a virtual cpufreq device.
> 
> Virtual machines may expose MMIO regions for a virtual cpufreq device for
> guests to read frequency information or to request frequency selection. The
> virtual cpufreq device has an individual controller for each CPU.

A nit, subject: drop second/last, redundant "bindings for". The
"dt-bindings" prefix is already stating that these are bindings.

> 
> Co-developed-by: Saravana Kannan <saravanak@...gle.com>
> Signed-off-by: Saravana Kannan <saravanak@...gle.com>
> Signed-off-by: David Dai <davidai@...gle.com>
> ---
>  .../bindings/cpufreq/cpufreq-virtual.yaml     | 89 +++++++++++++++++++
>  1 file changed, 89 insertions(+)
>  create mode 100644 Documentation/devicetree/bindings/cpufreq/cpufreq-virtual.yaml
> 
> diff --git a/Documentation/devicetree/bindings/cpufreq/cpufreq-virtual.yaml b/Documentation/devicetree/bindings/cpufreq/cpufreq-virtual.yaml
> new file mode 100644
> index 000000000000..f377cfc972ca
> --- /dev/null
> +++ b/Documentation/devicetree/bindings/cpufreq/cpufreq-virtual.yaml
> @@ -0,0 +1,89 @@
> +# SPDX-License-Identifier: GPL-2.0-only OR BSD-2-Clause
> +%YAML 1.2
> +---
> +$id: http://devicetree.org/schemas/cpufreq/cpufreq-virtual.yaml#
> +$schema: http://devicetree.org/meta-schemas/core.yamll#
> +
> +title: Virtual CPUFreq
> +
> +maintainers:
> +  - David Dai <davidai@...gle.com>
> +  - Saravana Kannan <saravanak@...gle.com>
> +
> +description:
> +  Virtual CPUFreq is a virtualized driver in guest kernels that sends frequency
> +  selection of its vCPUs as a hint to the host through MMIO regions. The host
> +  uses the hint to schedule vCPU threads and select physical CPU frequency. It
> +  enables accurate Per-Entity Load Tracking for tasks running in the guest by
> +  querying host CPU frequency unless a virtualized FIE (ex. AMU) exists.

Why do you need DT for this? You control hypervisor, thus control the
interface to the guest. I think Rob made it pretty clear that
discoverable usecases (which is yours) are not for DT.

Incomplete style-review follows:

> +
> +properties:
> +  compatible:
> +    const: virtual,cpufreq

Missing blank line.

> +  reg:
> +    maxItems: 1
> +
> +required:
> +  - compatible
> +  - reg
> +
> +additionalProperties: false
> +
> +examples:
> +  - |
> +    cpus {
> +      #address-cells = <1>;
> +      #size-cells = <0>;
> +
> +      cpu@0 {
> +        compatible = "arm,arm-v8";
> +        device_type = "cpu";
> +        reg = <0x0>;
> +        operating-points-v2 = <&opp_table0>;
> +      };
> +
> +      cpu@1 {
> +        compatible = "arm,arm-v8";
> +        device_type = "cpu";
> +        reg = <0x0>;
> +        operating-points-v2 = <&opp_table1>;
> +      };
> +    };
> +
> +    opp_table0: opp-table-0 {
> +      compatible = "operating-points-v2";
> +
> +      opp1098000000 {
> +        opp-hz = /bits/ 64 <1098000000>;
> +        opp-level = <1>;
> +      };
> +
> +      opp1197000000 {
> +        opp-hz = /bits/ 64 <1197000000>;
> +        opp-level = <2>;
> +      };
> +    };
> +
> +    opp_table1: opp-table-1 {
> +      compatible = "operating-points-v2";
> +
> +      opp1106000000 {
> +        opp-hz = /bits/ 64 <1106000000>;
> +        opp-level = <1>;
> +      };
> +
> +      opp1277000000 {
> +        opp-hz = /bits/ 64 <1277000000>;
> +        opp-level = <2>;
> +      };
> +    };
> +
> +    soc {
> +      #address-cells = <1>;
> +      #size-cells = <1>;
> +
> +      cpufreq {

Missing unit address

> +        reg = <0x1040000 0x10>;
> +        compatible = "virtual,cpufreq";

compatible is always the first property.

Also, you did not test it...


Best regards,
Krzysztof