linux-kernel - Re: [PATCH v3 5/7] gpio: Add GPIO Aggregator/Repeater driver

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Date:   Sat, 4 Jan 2020 01:38:38 +0100
From:   Linus Walleij <linus.walleij@...aro.org>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Geert Uytterhoeven <geert+renesas@...der.be>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        Jonathan Corbet <corbet@....net>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Harish Jenny K N <harish_kandiga@...tor.com>,
        Eugeniu Rosca <erosca@...adit-jv.com>,
        Alexander Graf <graf@...zon.com>,
        Peter Maydell <peter.maydell@...aro.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Phil Reid <preid@...ctromag.com.au>,
        Marc Zyngier <marc.zyngier@....com>,
        Christoffer Dall <christoffer.dall@....com>,
        Magnus Damm <magnus.damm@...il.com>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        Linux Doc Mailing List <linux-doc@...r.kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        Linux-Renesas <linux-renesas-soc@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        QEMU Developers <qemu-devel@...gnu.org>
Subject: Re: [PATCH v3 5/7] gpio: Add GPIO Aggregator/Repeater driver

Sorry for slowness... christmas.

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