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

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Date:   Fri, 16 Sep 2022 16:00:33 +0200
From:   Linus Walleij <linus.walleij@...aro.org>
To:     Rob Herring <robh+dt@...nel.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 3:47 PM Rob Herring <robh+dt@...nel.org> wrote:

> > 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...

:D

> > 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?

So that is less related to the bindings and more related to the
implementation, right?

I think this has always been seen as "default state at the point
when the operating system boots" but possibly we should just add
a flag or something to the bindings to say how early-ish the
particular default safe state config applies?

> 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.

I don't know exactly which use cases people have here, but yeah
definitely biasing for example, so a certain line can go out of reset
thanks to getting pulled up.

I imagined something like a ngpios long array:

/* Initial states */
gpio-init-states = <GPIO_OUT_LOW, GPIO_OUT_HIGH,
  GPIO_NO_CHANGE, GPIO_NO_CHANGE, GPIO_OUT_HIGH ...>;

these defines does not exist in include/dt-bindings/gpio/gpio.h,
it's just my idea of it.

> Finally, don't non-GPIO pins need the same thing? You don't want a
> default output driving what needs to be an input.

The pin control subsystem has something like this, in the "default"
and "init" states the pins are set to a default mux/pin config.
So for SoC GPIOs in many cases this problem is already solved
by using pin control states and hogs, as pin control is often used
as a back-end for GPIO. e.g. all Qualcomm platforms TLMM.

GPIO lines however can also be on a I2C or USB expander or
something where pin control is not applicable at all, so this would be
for initializing lines on those controllers.

Yours,
Linus Walleij