linux-kernel - Re: [PATCH v3 4/5] cpufreq: apple-soc: Add new driver to control Apple SoC CPU P-states

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Message-ID: <367270ad-114a-96ca-a92a-a7f54a5f3318@marcan.st>
Date:   Mon, 14 Nov 2022 15:57:12 +0900
From:   Hector Martin <marcan@...can.st>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Matthias Brugger <matthias.bgg@...il.com>,
        Sven Peter <sven@...npeter.dev>,
        Alyssa Rosenzweig <alyssa@...enzweig.io>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Stephen Boyd <sboyd@...nel.org>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Marc Zyngier <maz@...nel.org>,
        Mark Kettenis <mark.kettenis@...all.nl>, asahi@...ts.linux.dev,
        linux-arm-kernel@...ts.infradead.org, linux-pm@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 4/5] cpufreq: apple-soc: Add new driver to control
 Apple SoC CPU P-states

On 14/11/2022 15.51, Viresh Kumar wrote:
> On 09-11-22, 21:36, Hector Martin wrote:
>> On 02/11/2022 15.18, Viresh Kumar wrote:
>>>> +	ret = dev_pm_opp_set_sharing_cpus(cpu_dev, policy->cpus);
>>>
>>> Why do you need this ? The OPP core should be able to find this
>>> information by itself in your case AFAIU. The OPP core will refer
>>> "operating-points-v2 = <&pcluster_opp>" and find that the cores are
>>> related.
>>
>> We have multiple clusters sharing an OPP table (e.g. the M1 Ultra has 2
>> e-cluster and 4 p-clusters, and duplicating OPP tables seems very
>> silly), so this is necessary to tell it about the subset of cores
>> sharing a table that are actually one domain.
> 
> The cluster sharing information is already part of the OPP tables, "opp-shared"
> property. Platforms like scpi needed this because they didn't have the OPP table
> in DT and so no way to find out the relation of the CPUs.
> 
> See how drivers/cpufreq/mediatek-cpufreq.c has done this.
> dev_pm_opp_of_get_sharing_cpus() followed by dev_pm_opp_of_cpumask_add_table().

I don't think you understood me. We have multiple identical clusters.
All those clusters share an OPP table but are *not* the same cpufreq
domain. So we can have 8 CPUs which are two 4-CPU cluster using one OPP
table.

There is no way to express this relationship with OPP tables without
duplicating the tables themselves.

- Hector