linux-kernel - Re: [RFC PATCH v2 01/17] OPP: Allow to request stub voltage regulators

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Date:   Sun, 28 Oct 2018 15:58:25 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Lucas Stach <l.stach@...gutronix.de>,
        Viresh Kumar <viresh.kumar@...aro.org>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Rob Herring <robh+dt@...nel.org>,
        Thierry Reding <thierry.reding@...il.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Nishanth Menon <nm@...com>, Stephen Boyd <sboyd@...nel.org>,
        Marcel Ziswiler <marcel.ziswiler@...adex.com>,
        linux-tegra@...r.kernel.org, linux-pm@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v2 01/17] OPP: Allow to request stub voltage
 regulators

On 10/26/18 6:37 PM, Lucas Stach wrote:
> Am Freitag, den 26.10.2018, 15:03 +0300 schrieb Dmitry Osipenko:
> [...]
>>> On the other hand, the tegra20 cpufreq driver is common across a lot of boards.
>>> What will happen if the DT for some of the boards isn't correct and missed the
>>> necessary regulator node ?
>>
>> AFAIK, there is assumption that bootloader should setup regulators in
>> a way that kernel could work properly at max clock rates. Otherwise
>> things won't work.
> 
> This isn't true. The assumption is that the bootloader sets up the
> regulators such that stable operation at the CPU speed used by the
> bootloader is guaranteed.
> 
> Often the bootloader doesn't know about specific SKUs, so drives things
> at a rate/voltage that is safe across all SKUs [1], in which case the
> bootloader is totally unaware of the voltage needed to run the CPU at
> highest possible clock frequency.
> 
> Regards,
> Lucas
> 
> [1] http://git.denx.de/?p=u-boot.git;a=commit;h=2364e151e432b4ccf32dc9e6147121253d4ff86d
> 

Oh, okay. Thank you for pointing at [1], though the assumption is true for T20. 

I agree that it's better not to rely on configuration left from bootloader in general.