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From:   Linus Walleij <linus.walleij@linaro.org>
Date:   Sat, 4 Jan 2020 01:38:38 +0100
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Subject: Re: [PATCH v3 5/7] gpio: Add GPIO Aggregator/Repeater driver
To:     Geert Uytterhoeven <geert@linux-m68k.org>
Cc:     Geert Uytterhoeven <geert+renesas@glider.be>,
        Bartosz Golaszewski <bgolaszewski@baylibre.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Rob Herring <robh+dt@kernel.org>,
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        Eugeniu Rosca <erosca@de.adit-jv.com>,
        Alexander Graf <graf@amazon.com>,
        Peter Maydell <peter.maydell@linaro.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Phil Reid <preid@electromag.com.au>,
        Marc Zyngier <marc.zyngier@arm.com>,
        Christoffer Dall <christoffer.dall@arm.com>,
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        "open list:GPIO SUBSYSTEM" <linux-gpio@vger.kernel.org>,
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Sorry for slowness... christmas.

On Thu, Dec 12, 2019 at 4:24 PM Geert Uytterhoeven <geert@linux-m68k.org> wrote:
> On Thu, Dec 12, 2019 at 3:34 PM Linus Walleij <linus.walleij@linaro.org> wrote:

> > > +         This can serve the following purposes:
> > > +           1. Assign a collection of GPIOs to a user, or export them to a
> > > +              virtual machine,
> >
> > This is ambiguous. What is a "user"? A process calling from
> > userspace? A device tree node?
>
> A user is an entity with a UID, typically listed in /etc/passwd.
> This is similar to letting some, not all, people on the machine access
> the CD-ROM drive.

Ah I get it. Maybe we can say "assign permissions for a collection
of GPIOs to a user".

> > I would write "assign a collection of GPIO lines from any lines on
> > existing physical GPIO chips to form a new virtual GPIO chip"
> >
> > That should be to the point, right?
>
> Yes, that's WHAT it does. The WHY is the granular access control.

So I guess we can write both?

> > > +           3. Provide a generic driver for a GPIO-operated device, to be
> > > +               controlled from userspace using the GPIO chardev interface.
> >
> > I don't understand this, it needs to be elaborated. What is meant
> > by a "GPIO-operated device" in this context? Example?
>
> E.g. a motor. Or a door opener.
>
>         door-opener {
>                 compatible = "mydoor,opener";
>
>                 gpios = <&gpio2 19 GPIO_ACTIVE_HIGH>;
>         };
>
> You don't need a full-featured kernel driver for that, so just bind the
> gpio-aggregator to the door-opener, and control it through libgpiod.

Yep it's a perfect industrial control example, I get it.

Maybe we should blurb something about industrial control?

The rest I think we cleared out else I will see it when I review again.

Yours,
Linus Walleij