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

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Date:   Sun, 28 Aug 2022 09:06:35 +0000
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Linus Walleij <linus.walleij@...aro.org>
CC:     Arnd Bergmann <arnd@...db.de>,
        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 26/08/2022 à 23:54, Linus Walleij a écrit :
> On Fri, Aug 26, 2022 at 5:08 PM Christophe Leroy
> <christophe.leroy@...roup.eu> wrote:
>> Le 26/08/2022 à 15:49, Linus Walleij a écrit :
>>> On Thu, Aug 25, 2022 at 4:00 PM Christophe Leroy
>>> <christophe.leroy@...roup.eu> wrote:
>>>
>>>>> Christophe? Will you take a stab at it?
>>>>
>>>> Which patch should I write ?
>>>
>>> One that removes CONFIG_ARCH_HAS_NR_GPIO entirely, then
>>> allocate bases for new GPIO chips from 0 and upward instead.
>>> And then see what happens.
>>>
>>
>> Ok, I can give it a try.
> 
> Nice!
> 
>> But what do I do with:
>>
>> drivers/gpio/gpio-aggregator.c: bitmap = bitmap_alloc(ARCH_NR_GPIOS,
>> GFP_KERNEL);
> 
> That's just used locally in that driver to loop over the arguments to the
> aggregator (from the file in sysfs). I would set some arbitrary root
> like
> #define AGGREGATOR_MAX_GPIOS 512
> and just search/replace with that.
> 

And what about gsta_gpio_setup() that requests base 0 with the following 
comment:

	/*
	 * ARCH_NR_GPIOS is currently 256 and dynamic allocation starts
	 * from the end. However, for compatibility, we need the first
	 * ConneXt device to start from gpio 0: it's the main chipset
	 * on most boards so documents and drivers assume gpio0..gpio127
	 */


And I guess there might be other drivers like that (I found that one 
because of its comment mentionning ARCH_NR_GPIOS.

Another solution could be to leave first GPIOs for static allocation, 
and allocate dynamic ones from 256 or from 512 ?

Maybe in two steps:
- First step: Allocate dynamic from 256 upwards and add a pr_warn() for 
all static allocations.
- Second step later: Allocate dynamic from 0 and forbid static allocation.

Any opinion ?

Christophe