linux-kernel - Re: [PATCH RFC 0/9] gpio: improve support for shared GPIOs

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Message-ID: <CACRpkda-ZvrAC4bNLnA+ao0Y8-Nd_-b89N6HU10hhEdaOUYAjw@mail.gmail.com>
Date: Wed, 1 Oct 2025 12:53:07 +0200
From: Linus Walleij <linus.walleij@...aro.org>
To: Bartosz Golaszewski <brgl@...ev.pl>
Cc: Kees Cook <kees@...nel.org>, Mika Westerberg <westeri@...nel.org>, 
	Dmitry Torokhov <dmitry.torokhov@...il.com>, Andrew Morton <akpm@...ux-foundation.org>, 
	Manivannan Sadhasivam <mani@...nel.org>, Rob Herring <robh@...nel.org>, 
	Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley <conor+dt@...nel.org>, 
	Saravana Kannan <saravanak@...gle.com>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
	Andy Shevchenko <andy@...nel.org>, Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>, 
	Srinivas Kandagatla <srini@...nel.org>, Liam Girdwood <lgirdwood@...il.com>, Mark Brown <broonie@...nel.org>, 
	Jaroslav Kysela <perex@...ex.cz>, Takashi Iwai <tiwai@...e.com>, linux-hardening@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-gpio@...r.kernel.org, 
	linux-arm-kernel@...ts.infradead.org, linux-sound@...r.kernel.org, 
	linux-arm-msm@...r.kernel.org, 
	Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
Subject: Re: [PATCH RFC 0/9] gpio: improve support for shared GPIOs

Replying to self so Bartosz don't have to tell me off...

On Wed, Oct 1, 2025 at 10:49 AM Linus Walleij <linus.walleij@...aro.org> wrote:

> and every GPIO access on every system will be proxied
> and then this better be fast.

What about I read the code before I talk :/

Inspecting patch 4/9 it is clear that only GPIOs that actually
need to be proxied are proxied.

> Two things come to mind, and I bet you have thought of
> them already:
>
> 1. Footprint: all systems using regulators will now have
>    to compile in all this code as well.

This still holds. It could be a concern if it's a lot of code.

> 2. Performance, I didn't quite get it if every GPIO on the
>   system will be proxied through a layer of indirection
>   if you select HAVE_SHARED_GPIOS
>   but that would not be good, since some users are in
>   fastpath such as IRQ handlers, and the old way of
>   sharing GPIOs would just affect pins that are actually
>   shared.

It is clear from patch 4/9 that this only affects GPIOs
that are actually shared, and those tend to not be
performance-critical so this concern is moot.

Yours,
Linus Walleij