linux-kernel - Re: [PATCH v5 1/2] dt-bindings: cpufreq: add virtual cpufreq device

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Date: Wed, 31 Jan 2024 11:06:08 -0600
From: Rob Herring <robh@...nel.org>
To: David Dai <davidai@...gle.com>
Cc: "Rafael J. Wysocki" <rafael@...nel.org>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
	Conor Dooley <conor+dt@...nel.org>,
	Sudeep Holla <sudeep.holla@....com>,
	Saravana Kannan <saravanak@...gle.com>,
	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 v5 1/2] dt-bindings: cpufreq: add virtual cpufreq device

On Fri, Jan 26, 2024 at 04:43:15PM -0800, 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 frequency domain. Performance points for a given domain can be
> normalized across all domains for ease of allowing for virtual machines
> to migrate between hosts.
> 
> Co-developed-by: Saravana Kannan <saravanak@...gle.com>
> Signed-off-by: Saravana Kannan <saravanak@...gle.com>
> Signed-off-by: David Dai <davidai@...gle.com>
> ---
>  .../cpufreq/qemu,cpufreq-virtual.yaml         | 110 ++++++++++++++++++

> +    const: qemu,virtual-cpufreq

Well, the filename almost matches the compatible.

> +
> +  reg:
> +    maxItems: 1
> +    description:
> +      Address and size of region containing frequency controls for each of the
> +      frequency domains. Regions for each frequency domain is placed
> +      contiguously and contain registers for controlling DVFS(Dynamic Frequency
> +      and Voltage) characteristics. The size of the region is proportional to
> +      total number of frequency domains. This device also needs the CPUs to
> +      list their OPPs using operating-points-v2 tables. The OPP tables for the
> +      CPUs should use normalized "frequency" values where the OPP with the
> +      highest performance among all the vCPUs is listed as 1024 KHz. The rest
> +      of the frequencies of all the vCPUs should be normalized based on their
> +      performance relative to that 1024 KHz OPP. This makes it much easier to
> +      migrate the VM across systems which might have different physical CPU
> +      OPPs.
> +
> +required:
> +  - compatible
> +  - reg
> +
> +additionalProperties: false
> +
> +examples:
> +  - |
> +    // This example shows a two CPU configuration with a frequency domain
> +    // for each CPU showing normalized performance points.
> +    cpus {
> +      #address-cells = <1>;
> +      #size-cells = <0>;
> +
> +      cpu@0 {
> +        compatible = "arm,armv8";
> +        device_type = "cpu";
> +        reg = <0x0>;
> +        operating-points-v2 = <&opp_table0>;
> +      };
> +
> +      cpu@1 {
> +        compatible = "arm,armv8";
> +        device_type = "cpu";
> +        reg = <0x0>;
> +        operating-points-v2 = <&opp_table1>;
> +      };
> +    };
> +
> +    opp_table0: opp-table-0 {
> +      compatible = "operating-points-v2";
> +
> +      opp64000 { opp-hz = /bits/ 64 <64000>; };

opp-64000 is the preferred form.

> +      opp128000 { opp-hz = /bits/ 64 <128000>; };
> +      opp192000 { opp-hz = /bits/ 64 <192000>; };
> +      opp256000 { opp-hz = /bits/ 64 <256000>; };
> +      opp320000 { opp-hz = /bits/ 64 <320000>; };
> +      opp384000 { opp-hz = /bits/ 64 <384000>; };
> +      opp425000 { opp-hz = /bits/ 64 <425000>; };
> +    };
> +
> +    opp_table1: opp-table-1 {
> +      compatible = "operating-points-v2";
> +
> +      opp64000 { opp-hz = /bits/ 64 <64000>; };
> +      opp128000 { opp-hz = /bits/ 64 <128000>; };
> +      opp192000 { opp-hz = /bits/ 64 <192000>; };
> +      opp256000 { opp-hz = /bits/ 64 <256000>; };
> +      opp320000 { opp-hz = /bits/ 64 <320000>; };
> +      opp384000 { opp-hz = /bits/ 64 <384000>; };
> +      opp448000 { opp-hz = /bits/ 64 <448000>; };
> +      opp512000 { opp-hz = /bits/ 64 <512000>; };
> +      opp576000 { opp-hz = /bits/ 64 <576000>; };
> +      opp640000 { opp-hz = /bits/ 64 <640000>; };
> +      opp704000 { opp-hz = /bits/ 64 <704000>; };
> +      opp768000 { opp-hz = /bits/ 64 <768000>; };
> +      opp832000 { opp-hz = /bits/ 64 <832000>; };
> +      opp896000 { opp-hz = /bits/ 64 <896000>; };
> +      opp960000 { opp-hz = /bits/ 64 <960000>; };
> +      opp1024000 { opp-hz = /bits/ 64 <1024000>; };
> +
> +    };

I don't recall your prior versions having an OPP table. Maybe it was 
incomplete. You are designing the "h/w" interface. Why don't you make it 
discoverable or implicit (fixed for the h/w)? Do you really need it if 
the frequency is normalized?

Also, we have "opp-level" for opaque values that aren't Hz.

Rob