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

[Bjorn Andersson <bjorn.andersson@linaro.org>]

On Fri 16 Jul 16:49 CDT 2021, Stephen Boyd wrote:

> 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.
> 

But if that's the case, I don't think it's appropriate for the "enabled
state" of the domain to use any of those corners.

As this means that anyone who needs any of the rpmhpd domains active
also needs to specify required-opps, which wouldn't be needed for any
other power domain provider.

And more importantly it means that a device sitting in a GDSC, which
would be parented by a rpmhpd domain has no way to specify the GDSC and
trickle the minimum-vote up to the rpmhpd domain. (And I know that we
don't describe the parentship of the GDSCs today, but this patch
tells me that it's around the corner - for more than MMCX)

Regards,
Bjorn

> >
> > 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.
> >