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

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Message-ID: <CA+V-a8un1cF=acNjG=79_v7oaR8gzBQ+3z1As8AqrJnOnk-OUw@mail.gmail.com>
Date: Fri, 17 Oct 2025 16:33:56 +0100
From: "Lad, Prabhakar" <prabhakar.csengg@...il.com>
To: Conor Dooley <conor@...nel.org>
Cc: 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 Conor,

Thank you for the review.

On Thu, Oct 16, 2025 at 5:41 PM Conor Dooley <conor@...nel.org> wrote:
>
> On Tue, Oct 14, 2025 at 08:11:20PM +0100, Prabhakar wrote:
> > From: Lad Prabhakar <prabhakar.mahadev-lad.rj@...renesas.com>
> >
> > Document the pin configuration properties supported by the RZ/T2H pinctrl
> > driver. The RZ/T2H SoC supports configuring various electrical properties
> > through the DRCTLm (I/O Buffer Function Switching) registers.
> >
> > Add documentation for the following standard properties:
> > - bias-disable, bias-pull-up, bias-pull-down: Control internal
> >   pull-up/pull-down resistors (3 options: no pull, pull-up, pull-down)
> > - input-schmitt-enable, input-schmitt-disable: Control Schmitt trigger
> >   input
> > - slew-rate: Control output slew rate (2 options: slow/fast)
> >
> > Add documentation for the custom property:
> > - renesas,drive-strength: Control output drive strength using discrete
> >   levels (0-3) representing low, medium, high, and ultra high strength.
> >   This custom property is needed because the hardware uses fixed discrete
> >   levels rather than configurable milliamp values.
> >
> > Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@...renesas.com>
> > ---
> >  .../bindings/pinctrl/renesas,r9a09g077-pinctrl.yaml | 13 +++++++++++++
> >  1 file changed, 13 insertions(+)
> >
> > diff --git a/Documentation/devicetree/bindings/pinctrl/renesas,r9a09g077-pinctrl.yaml b/Documentation/devicetree/bindings/pinctrl/renesas,r9a09g077-pinctrl.yaml
> > index 36d665971484..9085d5cfb1c8 100644
> > --- a/Documentation/devicetree/bindings/pinctrl/renesas,r9a09g077-pinctrl.yaml
> > +++ b/Documentation/devicetree/bindings/pinctrl/renesas,r9a09g077-pinctrl.yaml
> > @@ -72,6 +72,19 @@ definitions:
> >        input: true
> >        input-enable: true
> >        output-enable: true
> > +      bias-disable: true
> > +      bias-pull-down: true
> > +      bias-pull-up: true
> > +      input-schmitt-enable: true
> > +      input-schmitt-disable: true
> > +      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?

>
> > +      renesas,drive-strength:
> > +        description:
> > +          Drive strength configuration value. Valid values are 0 to 3, representing
> > +          increasing drive strength from low, medium, high and ultra high.
>
> I see what you wrote in the commit message, but I don't really get why
> you need a custom property. I would imagine most devices only have some
> some small set of "fixed discrete levels", yet manage with milli- or
> micro-amps fine. Converting from mA to register values in a driver is
> not difficult, and I figure the documentation for the device probably
> doesn't just give vague strengths like "medium" or "ultra high", but
> probably provides currents?
>
> I dunno, I am just confused by the need to shove register values into
> these properties, rather than using the actual units.
>
I'm checking this with the HW team. I'll get back on this once I have
some feedback.

Cheers,
Prabhakar