linux-kernel - Re: [PATCH] dt-bindings: arm: Fix cpu capacity mismatch in example

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Message-ID: <1297c984-789f-0298-c3b8-8668f5b898de@linaro.org>
Date:   Mon, 29 Oct 2018 10:26:57 +0100
From:   Daniel Lezcano <daniel.lezcano@...aro.org>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     catalin.marinas@....com, Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        linux-arm-kernel@...ts.infradead.org,
        Vincent Guittot <vincent.guittot@...aro.org>,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] dt-bindings: arm: Fix cpu capacity mismatch in example

On 29/10/2018 07:34, Viresh Kumar wrote:
> On 26-10-18, 10:30, Daniel Lezcano wrote:
>> On 26/10/2018 06:11, Viresh Kumar wrote:
>>> On 25-10-18, 14:04, Daniel Lezcano wrote:
>>>> I think it is actually correct. The example is confusing on what the
>>>> numbers are. IIUC, it is:
>>>>
>>>> (after normalizing)
>>>>
>>>> dhrystone result on big CPU is 1024 at 1100MHz
>>>> dhrystone result on little CPU is 446 at 850MHz
>>>>
>>>> We have to scale the result of the little for 1100MHz in order to compare.
>>>>
>>>> 1100/850 = 1.294
>>>>
>>>> dhrystone result on little CPU is 446 * 1.294 = 577 at 1100MHz
>>>>
>>>> So we put the normalized values 1024 and 577. The arch_topology will
>>>> scale 577 back to 446 as it will compute the max capacity based on the
>>>> max freq which is 850MHz.
>>>>
>>>> The *final* capacities are 1024 for cluster0 and 446 for cluster1 (read
>>>> the cpu max capacity, the ones showed in the sysfs).
>>>>
>>>> Did I miss something ?
>>>
>>> No. What about making it more clear in the example to save the next idiot (like
>>> me) from wasting time :)
>>>
>>> diff --git a/Documentation/devicetree/bindings/arm/cpu-capacity.txt b/Documentation/devicetree/bindings/arm/cpu-capacity.txt
>>> index 9b5685a1d15d..d061e6575bde 100644
>>> --- a/Documentation/devicetree/bindings/arm/cpu-capacity.txt
>>> +++ b/Documentation/devicetree/bindings/arm/cpu-capacity.txt
>>> @@ -61,7 +61,10 @@ mhz values (normalized w.r.t. the highest value found while parsing the DT).
>>>  Example 1 (ARM 64-bit, 6-cpu system, two clusters):
>>>  capacities-dmips-mhz are scaled w.r.t. 1024 (cpu@0 and cpu@1)
>>>  supposing cluster0@...-freq=1100 and custer1@...-freq=850,
>>> -final capacities are 1024 for cluster0 and 446 for cluster1
>>> +final capacities are 1024 for cluster0 and 446 for cluster1.
>>> +Note that the values mentioned below in the example (1024 and 578)
>>> +aren't normalized based on max frequency of each cluster and that is
>>> +left for the operating system to do.
>>
>> Yes, it will help but if you want to make things even more clear, I
>> suggest to elaborate the text a bit and give the numbers above to
>> explain (1100/850=1.294, 446 * 1.294 = 577, ...)
> 
> I didn't wanted to explain way too much, but how about this..
> 
> diff --git a/Documentation/devicetree/bindings/arm/cpu-capacity.txt b/Documentation/devicetree/bindings/arm/cpu-capacity.txt
> index 9b5685a1d15d..84262cdb8d29 100644
> --- a/Documentation/devicetree/bindings/arm/cpu-capacity.txt
> +++ b/Documentation/devicetree/bindings/arm/cpu-capacity.txt
> @@ -59,9 +59,11 @@ mhz values (normalized w.r.t. the highest value found while parsing the DT).
>  ===========================================
>  
>  Example 1 (ARM 64-bit, 6-cpu system, two clusters):
> -capacities-dmips-mhz are scaled w.r.t. 1024 (cpu@0 and cpu@1)
> -supposing cluster0@...-freq=1100 and custer1@...-freq=850,
> -final capacities are 1024 for cluster0 and 446 for cluster1
> +The capacities-dmips-mhz or DMIPS/MHz values (scaled to 1024)
> +are 1024 and 578 for cluster0 and cluster1. Further normalization
> +is done by the operating system based on cluster0@...-freq=1100 and
> +custer1@...-freq=850, final capacities are 1024 for cluster0 and
> +446 for cluster1 (576*850/1100).

Acked-by: Daniel Lezcano <daniel.lezcano@...aro.org>


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