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

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Date:   Fri, 26 Oct 2018 15:03:39 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     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 10/24/18 9:41 AM, Viresh Kumar wrote:
> On 22-10-18, 15:12, Dmitry Osipenko wrote:
>> Because there is one Tegra20 board (tegra20-trimslice) that doesn't declare
>> necessary regulators, but we want to have CPU frequency scaling. I couldn't
>> find board schematics and so don't know if CPU / CORE voltages are fixed on
>> Trim-Slice or it is just preferable not to have DVFS for that board, it is an
>> outlet-powered device [0]. Hence tegra20-cpufreq driver will request a dummy
>> regulators when appropriate. 
> 
> We have been using the regulator_get_optional() variant until now in the OPP
> core to make sure that we don't do DVFS for the CPU without the mandatory
> regulators being present, as that may make things unstable and cause harm to the
> SoC if we try to take CPU to frequency range over the currently programmed
> regulator can support.
> 
> Now coming back to tegra-20 SoC, which actually requires a regulator normally by
> design. On one of the boards (which is outlet powered), you aren't sure if there
> is a programmable regulator or not, or if DVFS should really be done or not.
> Isn't it worth checking the same from Tegra maintainers, or whomsoever has
> information on that board ?

I'll try to find out more detailed information for the next revision of the patchset.

 What would happen if there actually was a regulator
> and its default settings aren't good enough for high end frequencies ?

Usually this causes kernel/applications crashes and/or machine hang.

> 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.

 And because you are moving to regulator_get() API for
> the entire SoC (i.e. its cpufreq driver), people will never find the missing
> regulator.

Regulators core prints info message when dummy regulator is being used.

> If we can do it safely for all tegra20 boards, why not migrate to using
> regulator_get() instead of regulator_get_optional() in the OPP core API itself
> for everyone ?
> 

This should be a platform-specific decision. For Tegra we know that regulators should be in a good state at kernel boot time, I don't think that this applies to other platforms.