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

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Message-ID: <5178a6b1-1b5a-40d9-af40-68ee13975509@oss.qualcomm.com>
Date: Mon, 8 Sep 2025 11:49:52 +0200
From: Konrad Dybcio <konrad.dybcio@....qualcomm.com>
To: Stephan Gerhold <stephan.gerhold@...aro.org>
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 9/8/25 10:46 AM, Stephan Gerhold wrote:
> 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. :-)

The frequency plan for 8280 does not recommend any rate != 37.5 MHz

For x1e80100 however, the lovsvs_d1 corner is recommended to be 30
(yes, thirty) MHz, sourced from CAM_PLL8 for $reasons

Konrad