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

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Message-ID: <zk4ea5cibrkp4vttuy4evrqybf76b3nop5lnyck4ws4nyf2yc4@ghj2eyswsoow>
Date: Mon, 6 Oct 2025 08:43:47 -0700
From: Manivannan Sadhasivam <mani@...nel.org>
To: Dmitry Torokhov <dmitry.torokhov@...il.com>
Cc: Bartosz Golaszewski <brgl@...ev.pl>, Kees Cook <kees@...nel.org>, 
	Mika Westerberg <westeri@...nel.org>, Andrew Morton <akpm@...ux-foundation.org>, 
	Linus Walleij <linus.walleij@...aro.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

On Wed, Sep 24, 2025 at 11:25:12AM -0700, Dmitry Torokhov wrote:
> Hi Bartosz,
> 
> On Wed, Sep 24, 2025 at 04:51:28PM +0200, Bartosz Golaszewski wrote:
> > Here's a functional RFC for improving the handling of shared GPIOs in
> > linux.
> > 
> > Problem statement: GPIOs are implemented as a strictly exclusive
> > resource in the kernel but there are lots of platforms on which single
> > pin is shared by multiple devices which don't communicate so need some
> > way of properly sharing access to a GPIO. What we have now is the
> > GPIOD_FLAGS_BIT_NONEXCLUSIVE flag which was introduced as a hack and
> > doesn't do any locking or arbitration of access - it literally just hand
> > the same GPIO descriptor to all interested users.
> > 
> > The proposed solution is composed of three major parts: the high-level,
> > shared GPIO proxy driver that arbitrates access to the shared pin and
> > exposes a regular GPIO chip interface to consumers, a low-level shared
> > GPIOLIB module that scans firmware nodes and creates auxiliary devices
> > that attach to the proxy driver and finally a set of core GPIOLIB
> > changes that plug the former into the GPIO lookup path.
> > 
> > The changes are implemented in a way that allows to seamlessly compile
> > out any code related to sharing GPIOs for systems that don't need it.
> > 
> > The practical use-case for this are the powerdown GPIOs shared by
> > speakers on Qualcomm db845c platform, however I have also extensively
> > tested it using gpio-virtuser on arm64 qemu with various DT
> > configurations.
> 
> How is this different from the existing gpio-backed regulator/supply?
> IMO GPIOs are naturally exclusive-use resources (in cases when you need
> to control them, not simply read their state), and when there is a need
> to share them there are more appropriate abstractions that are built on
> top of GPIOs...
> 

Not always... For something like shared reset line, consumers request the line
as GPIO and expect gpiolib to do resource manangement.

- Mani

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