linux-kernel - Re: [PATCH 4/5] i2c: qcom-cci: Add OPP table support and enforce FAST

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Message-ID: <f508bf92-a513-467a-a946-17c41e1d72d1@oss.qualcomm.com>
Date: Mon, 8 Sep 2025 10:43:50 +0200
From: Konrad Dybcio <konrad.dybcio@....qualcomm.com>
To: Stephan Gerhold <stephan.gerhold@...aro.org>,
        Konrad Dybcio <konradybcio@...nel.org>
Cc: Bjorn Andersson <andersson@...nel.org>, Rob Herring <robh@...nel.org>,
        Krzysztof Kozlowski <krzk+dt@...nel.org>,
        Conor Dooley
 <conor+dt@...nel.org>,
        Bryan O'Donoghue <bryan.odonoghue@...aro.org>,
        Loic Poulain <loic.poulain@....qualcomm.com>,
        Robert Foss
 <rfoss@...nel.org>, Andi Shyti <andi.shyti@...nel.org>,
        Marijn Suijten <marijn.suijten@...ainline.org>,
        linux-arm-msm@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-i2c@...r.kernel.org
Subject: Re: [PATCH 4/5] i2c: qcom-cci: Add OPP table support and enforce
 FAST_PLUS requirements

On 9/8/25 10:36 AM, Stephan Gerhold wrote:
> On Thu, Sep 04, 2025 at 04:31:23PM +0200, Konrad Dybcio wrote:
>> From: Konrad Dybcio <konrad.dybcio@....qualcomm.com>
>>
>> The CCI clock has voltage requirements, which need to be described
>> through an OPP table.
>>
>> The 1 MHz FAST_PLUS mode requires the CCI core clock runs at 37,5 MHz
>> (which is a value common across all SoCs), since it's not possible to
>> reach the required timings with the default 19.2 MHz rate.
>>
>> Address both issues by introducing an OPP table and using it to vote
>> for the faster rate.
>>
>> Signed-off-by: Konrad Dybcio <konrad.dybcio@....qualcomm.com>
> 
> Using an OPP table for a single static rate that remains the same over
> the whole lifetime of the driver feels like overkill to me. Couldn't you
> just put the "required-opps" directly into the device node so that it is
> automatically applied when the device goes in/out of runtime suspend?
> 
> And since you need to make DT additions anyway, couldn't you just use
> "assigned-clock-rates" to avoid the need for a driver patch entirely? We
> use that for e.g. USB clocks as well.

This is futureproofing, in case someone invents FastMode++ with a higher
dvfs requirement or for when the driver adds presets for a 19.2 MHz CCI
clock which would (marginally) decrease power consumption

Konrad