linux-kernel - Re: [PATCH 4/4] arm64: dts: qcom: sm8350: Fixup the cpufreq node

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Date:   Wed, 4 Aug 2021 16:58:38 -0700
From:   Matthias Kaehlcke <mka@...omium.org>
To:     Bjorn Andersson <bjorn.andersson@...aro.org>
Cc:     Sibi Sankar <sibis@...eaurora.org>, sboyd@...nel.org,
        robh+dt@...nel.org, viresh.kumar@...aro.org, agross@...nel.org,
        rjw@...ysocki.net, linux-arm-msm@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-pm@...r.kernel.org, dianders@...omium.org,
        tdas@...eaurora.org
Subject: Re: [PATCH 4/4] arm64: dts: qcom: sm8350: Fixup the cpufreq node

On Wed, Aug 04, 2021 at 05:59:04PM -0500, Bjorn Andersson wrote:
> On Thu 29 Jul 13:04 CDT 2021, Sibi Sankar wrote:
> 
> > Fixup the register regions used by the cpufreq node on SM8350 SoC to
> > support per core L3 DCVS.
> > 
> 
> That sounds good, but why are you dropping the platform-specific
> compatible?
> 

I also stared at this and the patch that changes the code for a while.

My understanding is that removing the platform-specific compatible is part
of not breaking 'old' DTBs. Old DTBs for SM8350 contain the larger register
regions and must be paired with 'epss_sm8250_soc_data' (driver code) which
has the 'old' 'reg_perf_state' offset. New SM8350 DTs only have the
'qcom,cpufreq-epss' compatible, which pairs their smaller register regions
with 'epss_soc_data' with the new 'reg_perf_state' offset.

It is super-confusing that the platform-specific compatible string is
missing. The binding should probably mention that the two
platform-specific compatible strings are for backwards compatibility
only and should not be added to new or existing DT files that don't
have them already. Maybe a 'qcom,sm8350-cpufreq-epss-v2' or similar
should be added to avoid/reduce possible confusion and have to option
to add SM8350 specific code later.