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

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Message-ID: <CAD++jL=J2UpxQSNrZhCMw2fJ0umM8NRtNys2zMBouAFBoK0m2A@mail.gmail.com>
Date: Mon, 19 Jan 2026 01:09:54 +0100
From: Linus Walleij <linusw@...nel.org>
To: "Lad, Prabhakar" <prabhakar.csengg@...il.com>
Cc: Conor Dooley <conor@...nel.org>, Linus Walleij <linus.walleij@...aro.org>, 
	Geert Uytterhoeven <geert+renesas@...der.be>, 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,

I think this back-and-forth must be a bit stressful. Sorry about that.

On Wed, Jan 14, 2026 at 9:53 PM Lad, Prabhakar
<prabhakar.csengg@...il.com> wrote:

> > > > > > +      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 got the feedback from the HW team "The RZ/T2H drive strength
> (driving ability) is expressed using abstract levels such as Low,
> Middle, and High. These values do not correspond directly to specific
> mA units.

But they do correspond to *something* electrical inside the
silicon do they not? Then what is that?

I think it is just 1, 2, 3 or 4 driver stages.

> To determine how much current the pin can actually drive,
> the engineer must refer to the electrical characteristics table.
> Therefore, the drive strength in RZ/T2H is a parameter that switches
> the internal output transistor mode rather than directly representing
> a physical drive current.
>
> Consequently, expressing RZ/T2H drive strength in milli- or
> micro-amps, as suggested by the reviewer, is inappropriate. To
> accurately reflect the SoC's hardware specification, introducing a
> custom property is essential."

This is past my point. What is the maximum current dissapation?

I am asking for the current flowing through the pin if it connected
to ground and 1 second after the driver is turned on, or however
long time it takes for any dynamic transients to die out.

> To elaborate more on this [0] has the tables which are extracted from
> the HW manual [1] (which needs login). For example, considering SDHI
> referring to table 58.39 in [0] the drive strength can be calculated
> for SD using  I = C × (delta V / deltaT).

This is so clearly about dynamic currents, I am asking for
*static* *maximum* for each setting. In SI unit, milliamperes.

The document says:

  drive-strength-microamp:
    description: sink or source at most X uA

So *at most*.
And low, middle, high isn't any SI unit.

If you *really* want to make this complicated and related to
an equation using capacitance, voltage and time, then you are
perhaps looking for the slew-rate property:

  slew-rate:
    $ref: /schemas/types.yaml#/definitions/uint32
    description: set the slew rate

This we haven't defined, because it has exactly the dynamic
properties you are talking about. It can be low/medium/high
and depend on the load. It reflects something like
V/s (voltage change per time unit) in the end.

Yours,
Linus Walleij