linux-kernel - Re: [PATCH] gpio: Allow user to customise maximum number of GPIOs

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Date:   Thu, 18 Aug 2022 13:11:16 +0000
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Arnd Bergmann <arnd@...db.de>,
        Linus Walleij <linus.walleij@...aro.org>
CC:     Alexandre Courbot <gnurou@...il.com>,
        Alexandre Courbot <acourbot@...dia.com>,
        Bartosz Golaszewski <brgl@...ev.pl>,
        Jonathan Corbet <corbet@....net>,
        Russell King <linux@...linux.org.uk>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        "moderated list:ARM PORT" <linux-arm-kernel@...ts.infradead.org>,
        "open list:GENERIC INCLUDE/ASM HEADER FILES" 
        <linux-arch@...r.kernel.org>
Subject: Re: [PATCH] gpio: Allow user to customise maximum number of GPIOs



Le 18/08/2022 à 14:46, Arnd Bergmann a écrit :
> On Thu, Aug 18, 2022 at 2:25 PM Linus Walleij <linus.walleij@...aro.org> wrote:
>> On Thu, Aug 18, 2022 at 1:33 PM Arnd Bergmann <arnd@...db.de> wrote:
>>> On Thu, Aug 18, 2022 at 1:13 PM Linus Walleij <linus.walleij@...aro.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.
> 

What about GPIO AGGREGATOR, drivers/gpio/gpio-aggregator.c

	bitmap = bitmap_alloc(ARCH_NR_GPIOS, GFP_KERNEL);


Christophe