Re: [PATCH v4 2/2] arm64: dts: sc7180: Add required-opps for i2c

[Stephen Boyd <swboyd@chromium.org>]

Quoting Bjorn Andersson (2021-07-16 13:52:12)
> On Fri 16 Jul 15:21 CDT 2021, Stephen Boyd wrote:
>
> > Quoting Bjorn Andersson (2021-07-16 13:18:56)
> > > On Fri 16 Jul 05:00 CDT 2021, Rajendra Nayak wrote:
> > >
> > > > qup-i2c devices on sc7180 are clocked with a fixed clock (19.2 MHz)
> > > > Though qup-i2c does not support DVFS, it still needs to vote for a
> > > > performance state on 'CX' to satisfy the 19.2 Mhz clock frequency
> > > > requirement.
> > > >
> > >
> > > Sounds good, but...
> > >
> > > > Use 'required-opps' to pass this information from
> > > > device tree, and also add the power-domains property to specify
> > > > the CX power-domain.
> > > >
> > >
> > > ..is the required-opps really needed with my rpmhpd patch in place?
> > >
> >
> > Yes? Because rpmhpd_opp_low_svs is not the lowest performance state for
> > CX.
>
> On e.g. sm8250 the first available non-zero corner presented in cmd-db
> is low_svs.

Indeed. On sc7180 it's not the first non-zero corner. I suppose
retention for CX isn't actually used when the SoC is awake so your
rpmhpd patch is putting in a vote for something that doesn't do anything
at runtime for CX? I imagine that rpmh only sets the aggregate corner to
retention when the whole SoC is suspended/sleeping, otherwise things
wouldn't go very well. Similarly, min_svs may be VDD minimization? If
so, those first two states are basically states that shouldn't be used
at runtime, almost like sleep states.

>
> And if this (which?) clock requires a higher corner than the lowest
> possible in order to tick at this "lowest" frequency, I'm certainly
> interested in some more details.
>