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

[Rajendra Nayak <rnayak@codeaurora.org>]

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.

>>
>> 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)

> 
> 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.
>>>

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