linux-kernel - Re: [PATCH v3 2/2] dt-bindings: cpufreq: Document allwinner,cpu-operating-points-v2

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Date:   Tue, 30 Apr 2019 10:12:45 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Rob Herring <robh@...nel.org>
Cc:     Frank Lee <tiny.windzz@...il.com>,
        Viresh Kumar <vireshk@...nel.org>, Nishanth Menon <nm@...com>,
        Stephen Boyd <sboyd@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Maxime Ripard <maxime.ripard@...tlin.com>,
        Chen-Yu Tsai <wens@...e.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        David Miller <davem@...emloft.net>,
        Mauro Carvalho Chehab <mchehab+samsung@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Nicolas Ferre <nicolas.ferre@...rochip.com>,
        Linux PM <linux-pm@...r.kernel.org>,
        devicetree@...r.kernel.org,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 2/2] dt-bindings: cpufreq: Document
 allwinner,cpu-operating-points-v2

On 29-04-19, 11:18, Rob Herring wrote:
> On Sun, Apr 28, 2019 at 4:53 AM Frank Lee <tiny.windzz@...il.com> wrote:
> >
> > On Sat, Apr 27, 2019 at 5:15 AM Rob Herring <robh@...nel.org> wrote:
> > >
> > > On Wed, Apr 10, 2019 at 01:41:39PM -0400, Yangtao Li wrote:
> > > > Allwinner Process Voltage Scaling Tables defines the voltage and
> > > > frequency value based on the speedbin blown in the efuse combination.
> > > > The sunxi-cpufreq-nvmem driver reads the efuse value from the SoC to
> > > > provide the OPP framework with required information.
> > > > This is used to determine the voltage and frequency value for each
> > > > OPP of operating-points-v2 table when it is parsed by the OPP framework.
> > > >
> > > > The "allwinner,cpu-operating-points-v2" DT extends the "operating-points-v2"
> > > > with following parameters:
> > > > - nvmem-cells (NVMEM area containig the speedbin information)
> > > > - opp-microvolt-<name>: voltage in micro Volts.
> > > >   At runtime, the platform can pick a <name> and matching
> > > >   opp-microvolt-<name> property.
> > > >                       HW:             <name>:
> > > >                       sun50iw-h6      speed0 speed1 speed2
> > >
> > > We already have at least one way to support speed bins with QC kryo
> > > binding. Why do we need a different way?
> >
> > For some SOCs, for some reason (making the CPU have approximate performance),
> > they use the same frequency but different voltage. In the case where
> > this speed bin
> > is not a lot and opp uses the same frequency, too many repeated opp
> > nodes are a bit
> > redundant and not intuitive enough.
> >
> > So, I think it's worth the new method.
> 
> Well, I don't.
> 
> We can't have every SoC vendor doing their own thing just because they
> want to. If there are technical reasons why existing bindings don't
> work, then maybe we need to do something different. But I haven't
> heard any reasons.

Well there is a good reason for attempting the new bindings and I wasn't sure if
updating the earlier bindings or adding another one for platform is correct. As
we aren't really adding new bindings, but just documentation around it.

So there are two ways OPP core support this thing:

- opp-supported-hw: This is a better fit if we have a smaller group of
  frequencies to select from a bigger group, so we disable non-required OPPs
  completely. This is what Qcom did as they wanted to select different
  frequencies all together.

- opp-microvolt-<name>: This is a better fit if the frequencies remain same and
  only few of the properties like voltage/current have a different value. So we
  don't disable any OPPs but just select the right voltage/current for those
  frequencies. This avoids unnecessary duplication of the OPPs in DT and that's
  what allwinner guys want.

The kryo nvmem bindings currently supports opp-supported-hw, maybe we can add
mention support for second one in the same file and rename it well.

-- 
viresh