linux-kernel - Re: [PATCH 1/2] dt-bindings: pinctrl: renesas,r9a09g077: Document pin configuration properties

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Message-ID: <CAD++jL=rp=_J7vN4E9hUqu0Fa4H+1E1EhMFAe79Tc8jMtNHTcA@mail.gmail.com>
Date: Thu, 11 Dec 2025 01:00:43 +0100
From: Linus Walleij <linusw@...nel.org>
To: Conor Dooley <conor@...nel.org>
Cc: "Lad, Prabhakar" <prabhakar.csengg@...il.com>, 
	Geert Uytterhoeven <geert+renesas@...der.be>, Linus Walleij <linus.walleij@...aro.org>, 
	Rob Herring <robh@...nel.org>, Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley <conor+dt@...nel.org>, 
	Magnus Damm <magnus.damm@...il.com>, linux-renesas-soc@...r.kernel.org, 
	linux-gpio@...r.kernel.org, devicetree@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Biju Das <biju.das.jz@...renesas.com>, 
	Fabrizio Castro <fabrizio.castro.jz@...esas.com>, 
	Lad Prabhakar <prabhakar.mahadev-lad.rj@...renesas.com>
Subject: Re: [PATCH 1/2] dt-bindings: pinctrl: renesas,r9a09g077: Document pin
 configuration properties

Hi Lad,

thanks for your patch!

On Mon, Dec 8, 2025 at 7:01 PM Conor Dooley <conor@...nel.org> wrote:
> On Mon, Dec 08, 2025 at 10:36:04AM +0000, Lad, Prabhakar wrote:

> > > > > +      slew-rate:
> > > > > +        enum: [0, 1]
> > > >
> > > > What are the meanings of "0" and "1" for slew rate? Why isn't this given
> > > I'll add a description for it (0 = slow, 1 = fast) and the same values
> > > are programmed in the register to configure the slew rate.
> > >
> > > > as the actual rates? The docs surely give more detail than just "slow"
> > > > and "fast".
> > > You mean to represent slew-rate in some sort of a unit?
> > >
> > Based on the comments from the HW team, there is no numerical
> > definition to represent slow/fast It only defines a relative
> > relationship.

Then describe relative to what, so we can understand when to use
which setting?

> > The current value is determined by the load on the external circuit
> > and is not affected by the choice of drive strength.
(...)
> Remember, drive strength is the current that can be delivered through a
> pin, not how much it is delivering at a given point in time.

This seems to be the core of the misunderstanding here.

The setting defines the cap. How much current *can* be delivered.

If the pin controller had a fuse that would bust if we delivered too
much current, this would be the grading of that fuse.

It's the current where the driver stage(s) stop pushing in more
electrons, it's a very real thing and does not depend on what the
circuit look like.

Pins usually have protected driver stages, so connecting an
amperemeter directly to ground and driving the line high would
actually give this value.

Yours,
Linus Walleij