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    SubjectRe: [PATCH] gpio: Allow user to customise maximum number of GPIOs
    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:

    > > static inline bool gpio_is_valid(int number)
    > > {
    > > return number >= 0 && number < ARCH_NR_GPIOS;
    > > }
    > >
    > > ?
    > >
    > > If using GPIO descriptors, any descriptor != NULL is valid,
    > > this one is just used with legacy GPIOs. Maybe we should just
    > > delete gpio_is_valid() everywhere and then drop the cap?
    >
    > I think it makes sense to keep gpio_is_valid() for as long as we
    > support the numbers.

    Hmmm....

    > > 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.

    > Even for the sysfs interface, it is questionable to rely on
    > specific numbers because at least in an arm multiplatform
    > kernel the top number changes based on kernel configuration.

    Yeah :/ still these users tend to angrily report any breakage
    due to expected (fragile) behaviour.

    > > 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.

    > What I could imagine as a next step would be to mark all
    > consumer drivers and the sysfs interface that use gpio
    > numbers as 'depends on GPIO_LEGACY' and then only
    > provide the corresponding drivers if that option is set.

    Hm I wonder what Bartosz and Alexandre Courbot and thinks
    about a GPIO_LEGACY symbol to phase out the global
    GPIO numberspace. I kind of like the idea.

    I made the sysfs depend on CONFIG_EXPERT to at least make it less
    accessible and not provide users with guns to shoot themselves
    in the foot.

    Yours,
    Linus Walleij

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