linux-kernel - Re: [PATCH v2 1/2] gpio: Increase ARCH_NR

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Date:	Tue, 16 Sep 2014 12:34:31 +0300
From:	Mika Westerberg <mika.westerberg@...ux.intel.com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Linus Walleij <linus.walleij@...aro.org>,
	Alexandre Courbot <gnurou@...il.com>,
	Alan Cox <alan@...ux.intel.com>, Ning Li <ning.li@...el.com>,
	linux-gpio@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] gpio: Increase ARCH_NR_GPIOs to 512

On Mon, Sep 15, 2014 at 06:50:34PM +0200, Arnd Bergmann wrote:
> On Monday 15 September 2014, Mika Westerberg wrote:
> > Some newer Intel SoCs, like Braswell already have more than 256 GPIOs
> > available so the default limit is exceeded. Instead of adding more
> > architecture specific gpio.h files with custom ARCH_NR_GPIOs we increase
> > the gpiolib default limit to be twice the current.
> > 
> > Signed-off-by: Mika Westerberg <mika.westerberg@...ux.intel.com>
> > ---
> > Changes to previous version is that now we increase the common limit
> > instead of adding x86 specific gpio.h
> 
> Can you please include the reasoning for this decision in the
> changeset description? I'm sure you have your reasons, but from
> the text above, it sounds like a rather bad idea. What other
> architectures are impacted by this, and what is the kernel size
> cost for it on those architectures?

How about,

Current generic ARCH_NR_GPIOS limit is 256 which starts to be too small
for newer Intel SoCs like Braswell. In order to support GPIO controllers
on these SoCs we increase ARCH_NR_GPIOS to be 512 which should be
sufficient for now.

The kernel size increases a bit with this change. Below is an example of
x86_64 kernel image.

ARCH_NR_GPIOS=256
 text     data    bss     dec      hex    filename
 11476173 1971328 1265664 14713165 e0814d vmlinux

ARCH_NR_GPIOS=512
 text     data    bss     dec      hex    filename
 11476173 1971328 1269760 14717261 e0914d vmlinux

So the BSS size and this the kernel image size increases by 4k.
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