[PATCH v3 02/10] dt-bindings: pincfg-node: Add properties 'skew-delay-{in,out}put'

[Linus Walleij]

On Tue, Oct 14, 2025 at 8:04 PM Conor Dooley <conor at kernel.org> wrote:
> On Tue, Oct 14, 2025 at 04:04:43PM +0200, Antonio Borneo wrote:

> > +  skew-delay-input:
> > +    $ref: /schemas/types.yaml#/definitions/uint32
> > +    description:
> > +      this affects the expected clock skew on input pins.
> > +      Typically indicates how many double-inverters are used to
> > +      delay the signal.
>
> This property seems to be temporal, I would expect to see a unit of time
> mentioned here, otherwise it'll totally inconsistent in use between
> devices, and also a standard unit suffix in the property name.
> pw-bot: changes-requested

Don't blame the messenger, the existing property skew-delay
says:

  skew-delay:
    $ref: /schemas/types.yaml#/definitions/uint32
    description:
      this affects the expected clock skew on input pins
      and the delay before latching a value to an output
      pin. Typically indicates how many double-inverters are
      used to delay the signal.

This in turn comes from the original
Documentation/devicetree/bindings/pinctrl/pinctrl-bindings.txt
document, which in turn comes from this commit:

commit e0e1e39de490a2d9b8a173363ccf2415ddada871
Author: Linus Walleij <linus.walleij at linaro.org>
Date:   Sat Oct 28 15:37:17 2017 +0200

    pinctrl: Add skew-delay pin config and bindings

    Some pin controllers (such as the Gemini) can control the
    expected clock skew and output delay on certain pins with a
    sub-nanosecond granularity. This is typically done by shunting
    in a number of double inverters in front of or behind the pin.
    Make it possible to configure this with a generic binding.

    Cc: devicetree at vger.kernel.org
    Acked-by: Rob Herring <robh at kernel.org>
    Acked-by: Hans Ulli Kroll <ulli.kroll at googlemail.com>
    Signed-off-by: Linus Walleij <linus.walleij at linaro.org>

So by legacy skew-delay is a custom format, usually the number of
(double) inverters.

I don't recall the reason for this way of defining things, but one reason
could be that the skew-delay incurred by two inverters is very
dependent on the production node of the silicon, and can be
nanoseconds or picoseconds, these days mostly picoseconds.
Example: Documentation/devicetree/bindings/net/adi,adin.yaml

Antonio, what do you say? Do you have the actual skew picosecond
values for the different settings so we could define this as

skew-delay-input-ps:
skew-delay-output-ps:

and translate it to the right register values in the driver?

If we have the proper data this could be a good time to add this
ISO unit to these two props.

Yours,
Linus Walleij