linux-kernel - Re: [PATCH v2 0/9] Exynos Adaptive Supply Voltage support

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Date:   Tue, 20 Aug 2019 11:03:09 +0200
From:   Sylwester Nawrocki <s.nawrocki@...sung.com>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     Marek Szyprowski <m.szyprowski@...sung.com>, krzk@...nel.org,
        robh+dt@...nel.org, vireshk@...nel.org, devicetree@...r.kernel.org,
        kgene@...nel.org, pankaj.dubey@...sung.com,
        linux-samsung-soc@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-pm@...r.kernel.org, b.zolnierkie@...sung.com
Subject: Re: [PATCH v2 0/9] Exynos Adaptive Supply Voltage support

On 8/20/19 05:01, Viresh Kumar wrote:
> On 19-08-19, 15:39, Sylwester Nawrocki wrote:
>> Unfortunately not, the patch set as I see it is another way of updating 
>> an OPP after it was parsed from DT.  OPP remove/add could work equally 
>> well in our use case.
> 
> Adding OPPs dynamically has limitations, you can't set many values which are
> otherwise possible with DT. And removing/adding is not the right thing to do
> technically.

Thanks for explanation, I was not aware of that.

>> The problem is that we have the information on how to translate the 
>> common OPP voltage to a voltage specific to given silicon encoded jointly 
>> in the ASV tables and the CHIPID registers (efuse/OTP memory). 
>> Additionally, algorithm of selecting ASV data (OPP voltage) based on 
>> the "key" data from registers is not generic, it is usually different 
>> per each SoC type.
>>
>> I tried to identify some patterns in those tables in order to simplify 
>> possible DT binding, but that was not really successful. I ended up just 
>> keeping whole tables.
> 
> Sorry but I am unable to understand the difficulty you are facing now. So what I
> suggest is something like this.

The difficulty was about representing data from tables asv_{arm,kfc}_table[][]
added in patch 3/9 of the series in devicetree.  If you have no objections
about keeping those tables in the driver then I can't see any difficulties. 
 
> - Use DT to get a frequency and voltage for each frequency.

Yes, this is what happens now, we have common OPPs in DT that work for each SoC
revision. 

> - At runtime, based on SoC, registers, efuses, etc, update the voltage of the
>   OPPs.
> - This algo can be different for each SoC, no one is stopping you from doing
>   that.
> 
> Am I missing something ?

Not really, this is basically what happens in the $subject patch series. 

Then IIUC what I would need to change is to modify exynos_asv_update_cpu_opps() 
function in patch 3/9 to use dev_pm_opp_adjust_voltage() rather than 
dev_pm_opp_remove(), dev_pm_opp_add().

-- 
Thanks,
Sylwester