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

From: "H. Peter Anvin" <hpa@zytor.com>
To: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: Christophe Leroy <christophe.leroy@csgroup.eu>,
	Linus Walleij <linus.walleij@linaro.org>,
	Bartosz Golaszewski <brgl@bgdev.pl>,
	Jonathan Corbet <corbet@lwn.net>,
	Russell King <linux@armlinux.org.uk>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	"maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)"
	<x86@kernel.org>, Arnd Bergmann <arnd@arndb.de>,
	"open list:GPIO SUBSYSTEM" <linux-gpio@vger.kernel.org>,
	Linux Documentation List <linux-doc@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	linux-arm Mailing List <linux-arm-kernel@lists.infradead.org>,
	Linux-Arch <linux-arch@vger.kernel.org>
Subject: Re: [PATCH] gpio: Allow user to customise maximum number of GPIOs
Date: Fri, 12 Aug 2022 16:21:36 -0700	[thread overview]
Message-ID: <B4A6C65C-1480-4D20-9A0C-D97BEABC838F@zytor.com> (raw)
In-Reply-To: <CAHp75VfFQe3Ce-Si1sax8CCG1-rq+Y=8JhwH=82d3XgytCAmOQ@mail.gmail.com>

On August 12, 2022 2:58:36 PM PDT, Andy Shevchenko <andy.shevchenko@gmail.com> wrote:
>On Thu, Aug 11, 2022 at 11:12 PM H. Peter Anvin <hpa@zytor.com> wrote:
>>
>> On August 9, 2022 3:40:38 AM PDT, Christophe Leroy <christophe.leroy@csgroup.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
>
>...
>
>> This seems very odd to me. GPIOs can be, and often are, attached to peripheral buses which means that the *same system* can have anything from none to thousands of gpios ..
>
>Basically this setting should give us a *minimum* GPIO lines that are
>present on the system. And that is perfectly SoC dependent. The real
>issue is that the GPIO framework has these global arrays that (still?)
>can't be initialized from the heap due to too early initialization (is
>it the true reason?).
>

Ok that makes more sense... but in that case, it would also be good to reclaim excess storage that turns out to not be needed.

I am a bit skeptical, though – we get basic memory allocation very early.