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From:   Arnd Bergmann <arnd@arndb.de>
Date:   Thu, 18 Aug 2022 14:46:23 +0200
Message-ID: <CAK8P3a0uuJ_z8wmNmQTW_qPNqzz7XoxZdHgqbzmK+ydtjraeHg@mail.gmail.com>
Subject: Re: [PATCH] gpio: Allow user to customise maximum number of GPIOs
To:     Linus Walleij <linus.walleij@linaro.org>
Cc:     Arnd Bergmann <arnd@arndb.de>,
        Alexandre Courbot <gnurou@gmail.com>,
        Alexandre Courbot <acourbot@nvidia.com>,
        Bartosz Golaszewski <brgl@bgdev.pl>,
        Christophe Leroy <christophe.leroy@csgroup.eu>,
        Jonathan Corbet <corbet@lwn.net>,
        Russell King <linux@armlinux.org.uk>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@vger.kernel.org>,
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        open list <linux-kernel@vger.kernel.org>,
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On Thu, Aug 18, 2022 at 2:25 PM Linus Walleij <linus.walleij@linaro.org> wrote:
> On Thu, Aug 18, 2022 at 1:33 PM Arnd Bergmann <arnd@arndb.de> wrote:
> > On Thu, Aug 18, 2022 at 1:13 PM Linus Walleij <linus.walleij@linaro.org> wrote:
> > > I think there may be systems and users that still depend on GPIO base
> > > numbers being assigned from ARCH_NR_GPIOS and
> > > downwards (userspace GPIO numbers in sysfs will also change...)
> > > otherwise we could assign from 0 and up.
> >
> > Is it possible to find in-kernel users that depend on well-known
> > numbers for dynamically assigned gpios? I would argue
> > that those are always broken.
>
> Most in-kernel users hard-code the base to something like
> 0 etc it's only the ones that code -1 into .base that are in
> trouble because that will activate dynamic assignment for the
> base.
>
> git grep 'base = -1' yields these suspects:
>
> arch/arm/common/sa1111.c:       sachip->gc.base = -1;
> arch/arm/common/scoop.c:        devptr->gpio.base = -1;
> arch/powerpc/platforms/52xx/mpc52xx_gpt.c:      gpt->gc.base = -1;
> arch/powerpc/platforms/83xx/mcu_mpc8349emitx.c: gc->base = -1;
>
> That's all! We could just calculate these to 512-ngpios and
> hardcode that instead.

How do the consumers find the numbers for these four?

> > > Right now the safest would be:
> > > Assign from 512 and downwards until we hit 0 then assign
> > > from something high, like U32_MAX and downward.
> > >
> > > That requires dropping gpio_is_valid() everywhere.
> > >
> > > If we wanna be bold, just delete gpio_is_valid() and assign
> > > bases from 0 and see what happens. But I think that will
> > > lead to regressions.
> >
> > I'm still unsure how removing gpio_is_valid() would help.
>
> If we allow GPIO base all the way to U32_MAX
> this function becomes:
>
> static inline bool gpio_is_valid(int number)
> {
>         return number >= 0 && number < U32_MAX;
> }
>
> and we can then just
>
> #define gpio_is_valid true
>
> and in that case it is better to delete the use of this function
> altogether since it can not fail.

S32_MAX might be a better upper bound. That allows to
just have no number assigned to a gpio chip. Any driver
code calling desc_to_gpio() could then get back -1
or a negative error code.

Making the ones that are invalid today valid sounds like
a step backwards to me if the goal is to stop using
gpio numbers and most consumers no longer need them.

          Arnd