Date:   Tue, 27 Jul 2021 22:46:42 -0500
From:   Bjorn Andersson <bjorn.andersson@...aro.org>
To:     Rajendra Nayak <rnayak@...eaurora.org>
Cc:     Stephen Boyd <swboyd@...omium.org>, ulf.hansson@...aro.org,
        viresh.kumar@...aro.org, linux-pm@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-msm@...r.kernel.org, rojay@...eaurora.org,
        stephan@...hold.net
Subject: Re: [PATCH v4 2/2] arm64: dts: sc7180: Add required-opps for i2c

On Tue 27 Jul 02:35 CDT 2021, Rajendra Nayak wrote:

> 
> On 7/25/2021 10:31 PM, Bjorn Andersson wrote:
> > On Mon 19 Jul 23:29 CDT 2021, Rajendra Nayak wrote:
> > 
> > > 
> > > 
> > > On 7/20/2021 12:49 AM, Bjorn Andersson wrote:
> > > > 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.
> > > 
> > > Right, it might work in some cases but might not work for all.
> > > 
> > 
> > Which makes it way less desirable.
> > 
> > The enable state for rpmhpd power domains doesn't meet my expectations
> > for how a power domain should behave,
> 
> Right and that's perhaps because these are not the usual power-domains,
> which have one "on/active" state and one or more "off/inactive" states (off/ret/clock-stop)
> Rpmhpd has multiple "on/active" states, and whats "on/active" for one consumer
> might not be "on/active" for another, so this information is hard to be managed
> at a generic level and these requests in some way or the other need to come
> in explicitly from the resp. consumers.
> 

I think it's fine if we just acknowledge that this is how the rpmhpd
domains works.

But I am worried about how we're going to handle the case where the
consumer is indirectly referencing one of these power-domains using a
subdomain (gdsc).

And the open question is if a solution to that problem will solve this
problem as well, or if we need to have this and some mechanism to
describe the "on state" for the parent of a subdomain.

Regards,
Bjorn

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