Date:   Mon, 29 Oct 2018 12:23:28 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Dmitry Osipenko <digetx@...il.com>
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 26-10-18, 15:03, Dmitry Osipenko wrote:
> 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.

Thanks Dmitry.

>  What would happen if there actually was a regulator

Please preserve the '>' from previous replies at the beginning of the
lines. Otherwise it looks as if you have written the above line. :)

> > and its default settings aren't good enough for high end frequencies ?
> 
> Usually this causes kernel/applications crashes and/or machine hang.

Sure. I also do remember from some guys (maybe TI), where they
mentioned that such scenarios can harm the hardware as well sometimes.
Don't remember the details though.

>  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.

Sure, but they are easy to miss and they are only seen by developers
not regular users of a machine.

> > 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.

Based on the other discussion with Lucas on this thread, I don't think
this is correct any more ?

Don't get me wrong, I am all good for changes and another API change
doesn't matter much to me. I am just wondering if it would be the
right approach to fix the issue. Why not rather call
dev_pm_opp_set_regulators() conditionally and avoid calling it for the
specific tegra20 board.

-- 
viresh

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