linux-kernel - Re: [linux-sunxi] [PATCH 10/10] ARM: dts: sun8i: Add SY8106A regulator to Orange Pi PC

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Date:   Wed, 26 Jul 2017 13:44:20 +0200
From:   Maxime Ripard <maxime.ripard@...e-electrons.com>
To:     Ondřej Jirman <megous@...ous.com>
Cc:     icenowy@...c.io, wens@...e.org,
        Liam Girdwood <lgirdwood@...il.com>,
        Mark Brown <broonie@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        devicetree <devicetree@...r.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        linux-clk <linux-clk@...r.kernel.org>,
        "open list:THERMAL" <linux-pm@...r.kernel.org>,
        linux-sunxi <linux-sunxi@...glegroups.com>
Subject: Re: [linux-sunxi] [PATCH 10/10] ARM: dts: sun8i: Add SY8106A
 regulator to Orange Pi PC

Hi,

On Wed, Jul 26, 2017 at 12:23:48PM +0200, Ondřej Jirman wrote:
> Hi,
> 
> icenowy@...c.io píše v St 26. 07. 2017 v 15:36 +0800:
> > 
> > > > > 
> > > > > Otherwse
> > > > > 
> > > > > > +               regulator-max-microvolt = <1400000>;
> > > > > > +               regulator-ramp-delay = <200>;
> > > > > 
> > > > > Is this an actual constraint of the SoC? Or is it a characteristic
> > > > > of the regulator? If it is the latter, it belongs in the driver.
> > > > > AFAIK the regulator supports varying the ramp delay (slew rate).
> > 
> > I don't know...
> > 
> > Maybe I should ask Ondrej?
> 
> It is probably neither.
> 
> It is used to calculate a delay inserted by the kernel between setting
> a new target voltage over I2C and changing the frequency of the CPU.
> The actual delay is calculated by the difference between previous and
> the new voltage.
> 
> I don't remember seeing anything in the datasheet of the regulator.
> This is just some low value that works.
> 
> It would probably be dependent on the capacitance on the output of the
> regulator, actual load (which varies), etc. So it is a board specific
> value. One could measure it with an oscilloscope if there's a need to
> optimize this.

If this is a reasonable default, then this should be in the
driver. You can't expect anyone to properly calculate a ramp delay and
have access to both a scope and the CPU power lines.

Maxime

-- 
Maxime Ripard, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com

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