Re: [PATCH 1/2] dt-bindings: gpio-mmio: Add ngpios property

[Linus Walleij <linus.walleij@xxxxxxxxxx>]

On Wed, Oct 16, 2024 at 5:47 PM Rob Herring <robh@xxxxxxxxxx> wrote:
> On Wed, Oct 16, 2024 at 2:21 AM Linus Walleij <linus.walleij@xxxxxxxxxx> wrote:
> >
> > This adds the ngpios property to MMIO GPIO. We restrict the
> > property to 1..63 since there is no point in 0 GPIO lines and
> > we support up to 64bits wide registers for now.
>
> Why does it need to be restricted? Is something bad going to happen if
> for some reason someone tries to control a non-existent GPIO?

Unlikely. But the biggest inconvenience is that non-existing GPIO lines
gets exposed to userspace which causes confusion. It's a bit like
saying you have 32 harddisks on your system just because the register
has 32 bits.

> If there
> is, maybe there should be a specific compatible as the h/w is not so
> generic.

The gpio-mmio is quite generic, it's the most generic GPIO
driver we have.

The ngpios property is also generic, it is in:
Documentation/devicetree/bindings/gpio/gpio.txt
(commit aacaffd1d9a6f8e2c7369d83c21d41c3b53e2edc)

At the time (2015) I just documented the already widespread use
of this property.

It is also reflected in several of the new yaml bindings, a git grep
ngpios will show you.

Yours,
Linus Walleij