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

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Date:   Thu, 18 Aug 2022 11:33:01 +0200
From:   Linus Walleij <linus.walleij@...aro.org>
To:     Christophe Leroy <christophe.leroy@...roup.eu>
Cc:     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>, Arnd Bergmann <arnd@...db.de>,
        "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

On Tue, Aug 9, 2022 at 12:41 PM Christophe Leroy
<christophe.leroy@...roup.eu> wrote:

> At the time being, the default maximum number of GPIOs is set to 512
> and can only get customised via an architecture specific
> CONFIG_ARCH_NR_GPIO.
>
> The maximum number of GPIOs might be dependent on the number of
> interface boards and is somewhat independent of architecture.
>
> Allow the user to select that maximum number outside of any
> architecture configuration. To enable that, re-define a
> core CONFIG_ARCH_NR_GPIO for architectures which don't already
> define one. Guard it with a new hidden CONFIG_ARCH_HAS_NR_GPIO.
>
> Only two architectures will need CONFIG_ARCH_HAS_NR_GPIO: x86 and arm.
>
> On arm, do like x86 and set 512 as the default instead of 0, that
> allows simplifying the logic in asm-generic/gpio.h
>
> Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>

I see what you're trying to do and this might be a possible interim solution.

I would like some comments sprinkled into the patched sites that this is
supposed to go away.

The GPIO descriptor refactoring which has been ongoing for a few
years, see drivers/gpio/TODO (please read), has the end goal of making
descriptor allocation fully dynamic. Once we free ourselves from
the fixed GPIO numberspace, there is nothing preventing us from
just kmalloc() ing a new descriptor whenever one is needed.

Help with rooting out the remaining fixed GPIO number clients
is much appreciated!

The per-arch GPIO number only exist for one reason: embedded
GPIOs (think SoC:s) that refer to fixed numbers in numberspace in
the board support code. This makes it necessary to allocate
descriptors up front in some compiled-in GPIO chips.

Yours,
Linus Walleij