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

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Message-ID: <20251016-dimmed-affidavit-90bae7e162aa@spud>
Date: Thu, 16 Oct 2025 17:41:19 +0100
From: Conor Dooley <conor@...nel.org>
To: Prabhakar <prabhakar.csengg@...il.com>
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

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
as the actual rates? The docs surely give more detail than just "slow"
and "fast".

> +      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.

> +        $ref: /schemas/types.yaml#/definitions/uint32
> +        enum: [0, 1, 2, 3]
>      oneOf:
>        - required: [pinmux]
>        - required: [pins]
> -- 
> 2.43.0
> 

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