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

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Date:   Fri, 26 Oct 2018 09:41:07 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Daniel Lezcano <daniel.lezcano@...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 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.
 
 cpus {
        #address-cells = <2>;

-- 
viresh