linux-kernel - Re: [PATCH V4 2/2] base/drivers/arch_topology: Default dmips-mhz if they are not set in DT

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Date:   Tue, 27 Nov 2018 09:27:35 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Daniel Lezcano <daniel.lezcano@...aro.org>
Cc:     rjw@...ysocki.net, linux-kernel@...r.kernel.org,
        Chris Redpath <chris.redpath@...aro.org>,
        Quentin Perret <quentin.perret@...aro.org>,
        Amit Kucheria <amit.kucheria@...aro.org>,
        Nicolas Dechesne <nicolas.dechesne@...aro.org>,
        Niklas Cassel <niklas.cassel@...aro.org>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Sudeep Holla <sudeep.holla@....com>,
        Li Yang <leoyang.li@....com>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>
Subject: Re: [PATCH V4 2/2] base/drivers/arch_topology: Default dmips-mhz if
 they are not set in DT

On 26-11-18, 13:20, Daniel Lezcano wrote:
> diff --git a/Documentation/devicetree/bindings/arm/cpu-capacity.txt b/Documentation/devicetree/bindings/arm/cpu-capacity.txt
> index 84262cd..f53a3c9 100644
> --- a/Documentation/devicetree/bindings/arm/cpu-capacity.txt
> +++ b/Documentation/devicetree/bindings/arm/cpu-capacity.txt
> @@ -54,6 +54,12 @@ fall back to the default capacity value for every CPU. If cpufreq is not
>  available, final capacities are calculated by directly using capacity-dmips-
>  mhz values (normalized w.r.t. the highest value found while parsing the DT).
>  
> +If capacity-dmips-mhz is not specified or if the parsing fails, the
> +default capacity value will be computed against the highest frequency.
> +When all CPUs have the same OPP, they will have the same capacity
> +value otherwise the capacity will be scaled down for CPUs having lower
> +frequencies.

I know you added this based on Quentin's feedback, but I wonder if this is
really required and if it is improving anything at all. This is what the
documentation says currently without this patch:

"
capacity-dmips-mhz is an optional cpu node [1] property: u32 value
representing CPU capacity expressed in normalized DMIPS/MHz. At boot time, the
maximum frequency available to the cpu is then used to calculate the capacity
value internally used by the kernel.

capacity-dmips-mhz property is all-or-nothing: if it is specified for a cpu
node, it has to be specified for every other cpu nodes, or the system will
fall back to the default capacity value for every CPU. If cpufreq is not
available, final capacities are calculated by directly using capacity-dmips-
mhz values (normalized w.r.t. the highest value found while parsing the DT).
"

So it already clearly says two things:
- If all CPUs don't have this property, we fallback to default capacity for
  every CPU.
- And the OS may also normalize the capacity based on the maximum frequency.

What more do we want to add here ?

-- 
viresh