gpio: improve support for shared GPIOs

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.

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@linaro.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,
-- 
Bartosz Golaszewski <bartosz.golaszewski@linaro.org>