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

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Message-ID: <CACRpkdYcVtJjRHRJ8GgeU7rZDuyaJKu0vgcknb7DsHPjZGKGuA@mail.gmail.com>
Date: Wed, 1 Oct 2025 10:49:09 +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

Hi Bartosz,

I see the big picture of this plan!

One quick comment:

On Wed, Sep 24, 2025 at 4:51 PM Bartosz Golaszewski <brgl@...ev.pl> wrote:

> I'm Cc'ing some people that may help with reviewing/be interested in
> this: OF maintainers (because the main target are OF systems initially),
> Mark Brown because most users of GPIOD_FLAGS_BIT_NONEXCLUSIVE live
> in audio or regulator drivers and one of the goals of this series is
> dropping the hand-crafted GPIO enable counting via struct
> regulator_enable_gpio in regulator core),

...and that is what I thought as well, so:

>       arm64: select HAVE_SHARED_GPIOS for ARCH_QCOM

why would we be selecting this per-subarch?

What will happen is that CONFIG_REGULATOR will select
it and since everyone and their dog is using regulator, what
will happen is that every system will have this enabled,
and every GPIO access on every system will be proxied
and then this better be fast.

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.

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.

I don't know of a good generic solution for (2) to be honest,
last resort would be something like runtime patching of
calls when a GPIO becomes shared and that is really
advanced but maybe necessary to get a performant and
generic solution.

Yours,
Linus Walleij