linux-kernel - Re: [PATCH v2 1/2] dt-bindings: gpio: add gpio-aggregator binding

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Message-ID: <CAMuHMdX8=oJMmiB4KaJQUpAO5gGHUkiWVPJ3A0X=ggz_QaNCRQ@mail.gmail.com>
Date: Wed, 11 Feb 2026 14:30:42 +0100
From: Geert Uytterhoeven <geert@...ux-m68k.org>
To: Herve Codina <herve.codina@...tlin.com>
Cc: James Hilliard <james.hilliard1@...il.com>, linux-gpio@...r.kernel.org, 
	Linus Walleij <linusw@...nel.org>, Bartosz Golaszewski <brgl@...nel.org>, Rob Herring <robh@...nel.org>, 
	Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley <conor+dt@...nel.org>, 
	Alexander Stein <linux@...tq-group.com>, devicetree@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] dt-bindings: gpio: add gpio-aggregator binding

Hi Hervé,

On Wed, 11 Feb 2026 at 11:58, Herve Codina <herve.codina@...tlin.com> wrote:
> On Wed, 11 Feb 2026 11:01:24 +0100
> Geert Uytterhoeven <geert@...ux-m68k.org> wrote:
> > On Wed, 11 Feb 2026 at 10:57, Herve Codina <herve.codina@...tlin.com> wrote:
> > > On Wed, 11 Feb 2026 10:47:53 +0100
> > > Geert Uytterhoeven <geert@...ux-m68k.org> wrote:
> > > > On Wed, 11 Feb 2026 at 09:35, James Hilliard <james.hilliard1@...il.com> wrote:
> > > > > On Wed, Feb 11, 2026 at 1:26 AM Geert Uytterhoeven <geert@...ux-m68k.org> wrote:
> > > > > > On Wed, 11 Feb 2026 at 09:14, James Hilliard <james.hilliard1@...il.com> wrote:
> > > > > > > Document the gpio-aggregator virtual GPIO controller with a dedicated
> > > > > > > schema and compatible string.
> > > > > > >
> > > > > > > Also extend the GPIO AGGREGATOR MAINTAINERS entry to cover the new
> > > > > > > binding file.
> > > > > > >
> > > > > > > Signed-off-by: James Hilliard <james.hilliard1@...il.com>
> > > > > >
> > > > > > > --- /dev/null
> > > > > > > +++ b/Documentation/devicetree/bindings/gpio/gpio-aggregator.yaml
> > > > > > > @@ -0,0 +1,54 @@
> > > > > > > +# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
> > > > > > > +%YAML 1.2
> > > > > > > +---
> > > > > > > +$id: http://devicetree.org/schemas/gpio/gpio-aggregator.yaml#
> > > > > > > +$schema: http://devicetree.org/meta-schemas/core.yaml#
> > > > > > > +
> > > > > > > +title: GPIO aggregator controller
> > > > > > > +
> > > > > > > +maintainers:
> > > > > > > +  - Alexander Stein <linux@...tq-group.com>
> > > > > > > +
> > > > > > > +description:
> > > > > > > +  GPIO aggregator forwards selected GPIO lines from one or more GPIO
> > > > > > > +  controllers and exposes them as a virtual GPIO controller.
> > > > > >
> > > > > > > +examples:
> > > > > > > +  - |
> > > > > > > +    #include <dt-bindings/gpio/gpio.h>
> > > > > > > +
> > > > > > > +    gpio_agg: gpio-aggregator {
> > > > > > > +        compatible = "gpio-aggregator";
> > > > > > > +        #gpio-cells = <2>;
> > > > > > > +        gpio-controller;
> > > > > > > +        gpios = <&gpio0 3 GPIO_ACTIVE_LOW>,
> > > > > > > +                <&gpio3 1 GPIO_ACTIVE_HIGH>;
> > > > > > > +        gpio-line-names = "modem-reset", "modem-enable";
> > > > > > > +    };
> > > > > >
> > > > > > Looking at the example, it seems you intend to use the gpio-aggregator
> > > > > > as a "Generic GPIO Driver", like in the example in the documentation[1].
> > > > > > Hence I think you should not introduce and abuse the "gpio-aggregator"
> > > > > > compatible value for this, but instead:
> > > > > >   1. Use a proper compatible value that matches your device,
> > > > > >   2. Write proper DT bindings for the device,
> > > > > >   3. Add the proper device's compatible value to the gpio-aggregator
> > > > > >      driver's match table.
> > > > > > The above is very similar to how spidev is handled, which also forbids
> > > > > > using the spidev compatible value in DTS.
> > > > >
> > > > > Isn't this gpio-aggregator driver supposed to be non-hardware
> > > > > specific?
> > > > >
> > > > > I'm trying to use it as described here, I noticed the compatible
> > > > > in the blog post was missing and just needed adding to the
> > > > > driver: https://bootlin.com/blog/gpio-aggregator-a-virtual-gpio-chip/
> > > >
> > > > Let's kick Hervé to fix that ;-)
> > >
> > > Where is it missing ?
> >
> > In the driver ;-)
> >
> > DTS must never use 'compatible = "gpio-aggregator"'.
>
> Why?
>
> gpio-aggregator can be used as a virtual device to aggregate GPIOs for
> user-space.

It can be used for that, but that is not hardware description.

> I think we should have 'compatible = "gpio-aggregator"' in dts.

Proper hardware description requires a proper compatible value.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds