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

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Date:   Wed, 2 Nov 2022 03:17:17 +0900
From:   Hector Martin <marcan@...can.st>
To:     Ulf Hansson <ulf.hansson@...aro.org>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Viresh Kumar <viresh.kumar@...aro.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>, 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 02/11/2022 00.16, Ulf Hansson wrote:
> On Mon, 24 Oct 2022 at 06:40, Hector Martin <marcan@...can.st> wrote:
>>
>> This driver implements CPU frequency scaling for Apple Silicon SoCs,
>> including M1 (t8103), M1 Max/Pro/Ultra (t600x), and M2 (t8112).
>>
>> Each CPU cluster has its own register set, and frequency management is
>> fully automated by the hardware; the driver only has to write one
>> register. There is boost frequency support, but the hardware will only
>> allow their use if only a subset of cores in a cluster are in
>> non-deep-idle. Since we don't support deep idle yet, these frequencies
>> are not achievable, but the driver supports them. They will remain
>> disabled in the device tree until deep idle is implemented, to avoid
>> confusing users.
> 
> Out of curiosity, may I ask if this implies the need of a
> synchronization mechanism on the Linux side? Or is the boost frequency
> dynamically managed solely by HW/FW?

It's managed by hardware - Linux gets to request whatever frequency it
wants, and the hardware will limit it to what is achievable given the
current idle states within the cluster (and it will change automatically
with them). So if Linux asks for 3.2 GHz but there are no deep idle
cores in the cluster, you get 3.0. If there's one deep idle core, you
get 3.1 (I think). Three, 3.2. So this driver doesn't have to do
anything (and will report the correct current-frequency as long as the
per-SoC compatible is matched; without that this feature is disabled and
it just reports the requested frequency). We could enable the boost
states today just fine, it's just that they would never actually be
reached by the hardware.

- Hector