linux-kernel - [RFC PATCH v1 0/2] add support for splitting GPIOs

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Message-ID: <20251009223501.570949-1-jelonek.jonas@gmail.com>
Date: Thu,  9 Oct 2025 22:34:59 +0000
From: Jonas Jelonek <jelonek.jonas@...il.com>
To: Linus Walleij <linus.walleij@...aro.org>,
	Bartosz Golaszewski <brgl@...ev.pl>,
	Rob Herring <robh@...nel.org>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>
Cc: linux-gpio@...r.kernel.org,
	devicetree@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Jonas Jelonek <jelonek.jonas@...il.com>
Subject: [RFC PATCH v1 0/2] add support for splitting GPIOs

Hereby I propose a new type of virtual GPIO controller and corresponding
driver for splitting GPIOs using a multiplexer for different usecases.
Existing drivers apparently do not serve the purpose for what I need.

I came across an issue with a switch device from Zyxel which has two
SFP+ cages. Most similar switches either wire up the SFP signals
(RX_LOS, MOD_ABS, TX_FAULT, TX_DISABLE) directly to the SoC (if it has
enough GPIOs) or two a GPIO expander (for which a driver usually
exists). However, Zyxel decided to do it differently in the following
way:
  The signals RX_LOS, MOD_ABS and TX_FAULT share a single GPIO line to
  the SoC. Which one is actually connected to that GPIO line at a time
  is controlled by a separate multiplexer, a GPIO multiplexer in this
  case (which uses two other GPIOs). Only the TX_DISABLE is separate.

The SFP core/driver doesn't seem to support such a usecase for now, for
each signal one needs to specify a separate GPIO like:

  los-gpio = <&gpio0 0 GPIO_ACTIVE_HIGH>;
  mod-def0-gpio = <&gpio0 1 GPIO_ACTIVE_LOW>;
  ...

But for my device, I actually need to directly specify multiplexing
behavior in the SFP node or provide a mux-controller with 'mux-control'.

To fill this gap, I created a dt-schema and a working driver which
exactly does what is needed. It takes a phandle to a mux-controller and
the 'shared' gpio, and provides several virtual GPIOs based on the
children nodes defined for the gpio-split controller, each with a
mux-state which is used on access.

This virtual gpio-controller can then be referenced in the '-gpio'
properties of the SFP node (or other nodes depending on the usecase) as
usual and do not require any modification to the SFP core/driver.

--
Jonas Jelonek (2):
  dt-bindings: gpio: add gpio-split controller
  gpio: add gpio-split driver

 .../devicetree/bindings/gpio/gpio-split.yaml  |  77 +++++++
 MAINTAINERS                                   |   6 +
 drivers/gpio/Kconfig                          |   8 +
 drivers/gpio/Makefile                         |   1 +
 drivers/gpio/gpio-split.c                     | 210 ++++++++++++++++++
 5 files changed, 302 insertions(+)
 create mode 100644 Documentation/devicetree/bindings/gpio/gpio-split.yaml
 create mode 100644 drivers/gpio/gpio-split.c

base-commit: bc061143637532c08d9fc657eec93fdc2588068e
-- 
2.48.1