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

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Message-ID: <aL6X-RiCyPVbHlYN@linaro.org>
Date: Mon, 8 Sep 2025 10:46:49 +0200
From: Stephan Gerhold <stephan.gerhold@...aro.org>
To: Konrad Dybcio <konrad.dybcio@....qualcomm.com>
Cc: Konrad Dybcio <konradybcio@...nel.org>,
	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 Mon, Sep 08, 2025 at 10:43:50AM +0200, Konrad Dybcio wrote:
> 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
> 

If 19.2 MHz CCI clock is feasible and has lower voltage requirements,
then I would expect a separate entry for 19.2 MHz in the OPP table of
PATCH 5/5? The DT is unrelated to what functionality you implement in
the driver, and that would make the OPP table look less useless. :-)

Thanks,
Stephan