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 09:31:49 +0100
From:   Daniel Lezcano <daniel.lezcano@...aro.org>
To:     Viresh Kumar <viresh.kumar@...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 27/11/2018 04:57, Viresh Kumar wrote:
> 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 ?

I think what is new is the silver-gold platform. I agree the description
above gives us the information but in a condensed way. With this extra
paragraph we elaborate a bit and make it more clear for SMP/AMP systems.

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