Message-ID: <0b402bba-0399-4f93-873e-890a78570ff7@kernel.org>
Date: Sat, 4 Oct 2025 14:31:16 +0100
From: Srinivas Kandagatla <srini@...nel.org>
To: Bartosz Golaszewski <brgl@...ev.pl>, Kees Cook <kees@...nel.org>,
 Mika Westerberg <westeri@...nel.org>,
 Dmitry Torokhov <dmitry.torokhov@...il.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Linus Walleij <linus.walleij@...aro.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>
Cc: 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 9/24/25 3:51 PM, 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.

Isn't the main issue here is about not using a correct framework around
to the gpios that driver uses. ex: the codec usecase that you are
refering in this is using gpio to reset the line, instead of using a
proper gpio-reset control. same with some of the gpio-muxes. the problem
is fixed once such direct users of gpio are move their correct frameworks.

Am not sure adding a abstraction with-in gpio framework is right
solution, But I do agree that NONEXCLUSIVE flags should disappear and
users that are using this should be moved to correct frameworks where
they belong.

--srini

> 
> 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.
> 
> 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), Andy and Mika because I'd like
> to also cover ACPI (even though I don't know about any ACPI platform that
> would need this at the moment, I think it makes sense to make the
> solution complete), Dmitry (same thing but for software nodes), Mani
> (because you have a somewhat related use-case for the PERST# signal and
> I'd like to hear your input on whether this is something you can use or
> maybe it needs a separate, implicit gpio-perst driver similar to what
> Krzysztof did for reset-gpios) and Greg (because I mentioned this to you
> last week in person and I also use the auxiliary bus for the proxy
> devices).
> 
> First patch in the series is a bugfix targetting stable, I'm surprised
> nobody noticed the lockdep splat yet. The second adds a library function
> I use in a later patch. All remaining patches implement or use the
> shared GPIO support.
> 
> Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
> ---
> Bartosz Golaszewski (9):
>       gpio: wcd934x: mark the GPIO controller as sleeping
>       string: provide strends()
>       gpiolib: define GPIOD_FLAG_SHARED
>       gpiolib: implement low-level, shared GPIO support
>       gpio: shared-proxy: implement the shared GPIO proxy driver
>       gpiolib: support shared GPIOs in core subsystem code
>       arm64: select HAVE_SHARED_GPIOS for ARCH_QCOM
>       ASoC: wsa881x: drop GPIOD_FLAGS_BIT_NONEXCLUSIVE flag from GPIO lookup
>       ASoC: wsa883x: drop GPIOD_FLAGS_BIT_NONEXCLUSIVE flag from GPIO lookup
> 
>  arch/arm64/Kconfig.platforms     |   1 +
>  drivers/gpio/Kconfig             |  17 ++
>  drivers/gpio/Makefile            |   2 +
>  drivers/gpio/gpio-shared-proxy.c | 328 ++++++++++++++++++++++++++
>  drivers/gpio/gpio-wcd934x.c      |   2 +-
>  drivers/gpio/gpiolib-shared.c    | 481 +++++++++++++++++++++++++++++++++++++++
>  drivers/gpio/gpiolib-shared.h    |  71 ++++++
>  drivers/gpio/gpiolib.c           |  50 +++-
>  drivers/gpio/gpiolib.h           |   1 +
>  include/linux/string.h           |   2 +
>  lib/string.c                     |  19 ++
>  lib/tests/string_kunit.c         |  13 ++
>  sound/soc/codecs/wsa881x.c       |   3 +-
>  sound/soc/codecs/wsa883x.c       |   7 +-
>  14 files changed, 980 insertions(+), 17 deletions(-)
> ---
> base-commit: b46f7370d4a0f0b55f05b854e73b2a90dff41e1b
> change-id: 20250908-gpio-shared-67ec352884b6
> 
> Best regards,

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