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

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Message-ID: <CAMRc=Mdb_cUG+hKq8GyfUP1SYBh0p19J+4dFG7G3JSuZTr4n8Q@mail.gmail.com>
Date: Wed, 1 Oct 2025 06:04:51 -0700
From: Bartosz Golaszewski <brgl@...ev.pl>
To: Linus Walleij <linus.walleij@...aro.org>
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>, Bartosz Golaszewski <brgl@...ev.pl>
Subject: Re: [PATCH RFC 0/9] gpio: improve support for shared GPIOs

On Wed, 1 Oct 2025 12:53:07 +0200, Linus Walleij
<linus.walleij@...aro.org> said:
> 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.

It depends on how we implement this. If we just rip out the enable counting
from regulator core entirely, then it would be transparent from the
regulator's point of view and each platform could still select the new option
as required.

However there's the issue of regulator consumers who need to know when
something changes on a regulator and to that end subscribe to the regulator
notifer. Regulator core knows then it actually changes the GPIO so it emits
the event. There are several ways to approach it but the best one seems to
be: allow to subscribe for a per-descriptor event notifier (implementation
details may include: only actually creating the notifier for shared GPIOs),
and be notified about an actual change in value and then propagate it to
regulator users. This would still be transparent and allow us to select
HAVE_SHARED_GPIOS on a per-arch basis.

Bartosz

>
>> 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
>