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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,
        devicetree@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-arm-msm@vger.kernel.org, rojay@codeaurora.org,
        stephan@gerhold.net
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From:   Rajendra Nayak <rnayak@codeaurora.org>
Message-ID: <bc79ea3e-4981-8f9b-f9a7-59cb972047a7@codeaurora.org>
Date:   Tue, 20 Jul 2021 09:59:09 +0530
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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.

> 
> 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.

> 
> 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