Re: [PATCH 4/5] i2c: qcom-cci: Add OPP table support and enforce FAST_PLUS requirements

[Stephan Gerhold <stephan.gerhold@linaro.org>]

On Mon, Sep 08, 2025 at 11:49:52AM +0200, Konrad Dybcio wrote:
> 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@oss.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@oss.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
> 

The 37.5 MHz rate still exists on X1E I presume, or are you saying we
need more changes to support those odd 30 MHz?

Personally, I'm not fully convinced there is ever going to be a use case
of someone using a "non-standard" frequency. Even if "FastMode++" is
invented most devices will probably want to use it. And the voltage
requirements we're currently talking about here like "low svs" during
camera use cases are kind of negligible compared to others too.

But I'm fine with either solution, just wanted to mention it. :D

Thanks,
Stephan