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

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

On Mon 19 Jul 04:37 CDT 2021, Rajendra Nayak wrote:

> 
> 
> On 7/17/2021 3:29 AM, Bjorn Andersson wrote:
> > 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.
> 
> what rail is this? the mmcx? Perhaps it does not support RET.
> cx usually supports both collapse state and RET.
> 

That was the one I was specifically looking at for the MDSS_GDSC->MMCX
issue, so it's likely I didn't look elsewhere.

> > > 
> > > 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.
> 
> I rechecked the downstream kernels where all this voting happens from within
> the clock drivers, and I do see votes to min_svs for some clocks, but Stephen is
> right that RET is not something that's voted on while in active state.
> 
> But always going with something just above the ret level while active will also
> not work for all devices, for instance for i2c on 7180, it needs a cx vote of
> low svs while the rail (cx) does support something lower than that which is min svs.
> (why can't it just work with min svs?, I don't know, these values and recommendations
> come in from the voltage plans published by HW teams for every SoC and we just end up
> using them in SW, perhaps something to dig further and understand which I will try and
> do but these are the values in voltage plans and downstream kernels which work for now)
> 

So to some degree this invalidates my argumentation about the
enabled_corner in rpmhpd, given that "enabled" means a different corner
for each rail - not just the one with lowest non-zero value.

So perhaps instead of introducing the enabled_corner we need to
introduce your patch and slap a WARN_ON(corner == 0) in
rpmhpd_power_on() - to ensure that all clients that uses a rpmhpd domain
actually do vote for a high enough corner?

Regards,
Bjorn

> > 
> > 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.
> > > > 
> 
> -- 
> QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a member
> of Code Aurora Forum, hosted by The Linux Foundation