linux-kernel - Re: [PATCH] gpiolib: TODO: add an item about GPIO safe-state

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Message-ID: <CAL_JsqKxdX_Pya-N9n+w8a9ZqCa3AdFaRUXu0E31Joyb0psk_w@mail.gmail.com>
Date:   Fri, 16 Sep 2022 08:47:26 -0500
From:   Rob Herring <robh+dt@...nel.org>
To:     Linus Walleij <linus.walleij@...aro.org>
Cc:     Bartosz Golaszewski <brgl@...ev.pl>,
        Kent Gibson <warthog618@...il.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Drew Fustini <dfustini@...libre.com>,
        Marek Vasut <marek.vasut@...il.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] gpiolib: TODO: add an item about GPIO safe-state

On Fri, Sep 16, 2022 at 8:12 AM Linus Walleij <linus.walleij@...aro.org> wrote:
>
> On Fri, Sep 16, 2022 at 9:11 AM Bartosz Golaszewski <brgl@...ev.pl> wrote:
>
> > Of course we were not the first ones to think about it...
>
> :D
>
> > I'll read through it later. Was there any particular reason why this
> > never made its way into the kernel?
>
> Inability to drive through a DT binding that was acceptable for the
> DT binding maintainers.

AFAICT, this came up briefly in 2015, then 2017, then 2019. (You're a
year late this time.) A nice regular pattern to not get something
upstream...

> A good idea to get it passed I think would be to ask Rob (with some
> examples) how he thinks it should look and finalize the bindings
> before coding.

My issue was more that by the time the kernel or even bootloader runs,
quite a bit of time has passed given all the firmware that runs
nowadays. Doesn't a safe state need to be set as early as possible?
Like probably before anything using DT?

A node per GPIO could end up being a lot of nodes and I can certainly
see folks just initializing every GPIO. That would be a lot of bloat.
As I see it we need 4 bits per line: direction, state(high/low), pull
up/down/none.

Finally, don't non-GPIO pins need the same thing? You don't want a
default output driving what needs to be an input. Of course, a good
h/w designer wouldn't design such a thing.

Rob