Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
MIME-Version: 1.0
References: <20220730003846.1730265-1-balsup@cisco.com>
In-Reply-To: <20220730003846.1730265-1-balsup@cisco.com>
From:   Arnd Bergmann <arnd@arndb.de>
Date:   Sat, 30 Jul 2022 13:54:18 +0200
Message-ID: <CAK8P3a0Bn9xKRX9z369zVCy6CuguTvNbtXOOPbf_OfS697cDyQ@mail.gmail.com>
Subject: Re: [PATCH] Allow configuration of ARCH_NR_GPIO
To:     Billie Alsup <balsup@cisco.com>
Cc:     linux@armlinux.org.uk, tglx@linutronix.de, mingo@redhat.com,
        bp@alien8.de, dave.hansen@linux.intel.com, x86@kernel.org,
        hpa@zytor.com, arnd@arndb.de, linus.walleij@linaro.org,
        ardb@kernel.org, rmk+kernel@armlinux.org.uk, rostedt@goodmis.org,
        nick.hawkins@hpe.com, john@phrozen.org,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Sat, Jul 30, 2022 at 2:38 AM Billie Alsup <balsup@cisco.com> wrote:
>
> From: Billie R Alsup <balsup@cisco.com>
>
> Problem: Some systems support a high number of GPIO pins.  Allow
> the kernel builder to configure the maximum number of pins, rather
> than forcing a large value on everyone.
>
> Impact: Once a .config is generated, the ARCH_NR_GPIO setting
> will persist.  To return to a default setting, the CONFIG_ARCH_NR_GPIO
> must be removed from .config file first.
>
> Trade-offs: It is possible to achieve similar via command line
> parameters, e.g.
>
>     make KBUILD_CFLAGS_KERNEL=-DARCH_NR_GPIOS=16384
>
> to the build. This is problematic because the setting does not
> show up in /proc/config.gz.  It is also problematic for out-of-tree
> module builds, which require similar if they invoke the API
> gpio_is_valid().  In both cases, one could envision one system
> working normally, and another failing, even though they both have
> the same kernel version and /proc/config.gz.  Therefore, it is
> better to have the setting available in .config
>
> Signed-off-by: Billie R Alsup <balsup@cisco.com>

What is the use case for manually setting this rather than deriving
it from the selected platforms?

Have you tried to use both a platform-specific option for the minimum
number of this setting, and then restictricting the CONFIG_ARCH_NR_GPIO
setting with a 'range' statement?

         Arnd