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Subject: Re: [PATCH v4 2/2] arm64: dts: sc7180: Add required-opps for i2c
To:     Bjorn Andersson <bjorn.andersson@linaro.org>
Cc:     Stephen Boyd <swboyd@chromium.org>, ulf.hansson@linaro.org,
        viresh.kumar@linaro.org, linux-pm@vger.kernel.org,
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        linux-arm-msm@vger.kernel.org, rojay@codeaurora.org,
        stephan@gerhold.net
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From:   Rajendra Nayak <rnayak@codeaurora.org>
Message-ID: <b6b315ae-ee68-6ba8-7453-f28dace3d8d2@codeaurora.org>
Date:   Tue, 27 Jul 2021 13:05:52 +0530
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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.

> but we should at least be
> consistent across all consumers of it then...
> 
> 
> But the original issue remains, that when a device is powered by
> MDSS_GDSC, which is a subdomain of MMCX we still need to ensure that
> "on" for MMCX is actually "on" - which just happens to be the first
> non-0 corner.
> 
> But I presume we will end up having to do the same with &gcc's GDSCs,
> which are subdomains of CX and MX where this isn't true.
> 
>>>
>>> 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?
>>
>> So this would mean the expectation is that the clients set the perf state/corner
>> before they call power_on? I don;t think that's the case today with most clients,
>> infact its the opposite, we power on first and then make a call to set the perf
>> state of the domain.
>>
> 
> You're right, it's pretty much always the opposite, given that genpd
> will always enable the domain during attach.
> 
> Regards,
> Bjorn
> 
>>>
>>> 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
>>
>> -- 
>> QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a member
>> of Code Aurora Forum, hosted by The Linux Foundation

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