This proposes a new type of virtual GPIO controller and corresponding
driver to provide a 1-to-many mapping between virtual GPIOs and a single
real GPIO in combination with a multiplexer. 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-controls'.

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
gpio-line-mux-states property.

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.

---
Changelog:

v5: - renamed "shared" to "muxed" to avoid confusion with Bartosz' work
    - dropped Reviewed-by of Krzysztof due to binding change
    - use GPIOD_IN in devm_gpiod_get instead of calling
      gpiod_direction_input explicitly afterwards

Link to v4:
https://lore.kernel.org/linux-gpio/20251105103607.393353-1-jelonek.jonas@gmail.com/

v4: - dropped useless cast (as suggested by Thomas)
    - dropped unneeded locking (as suggested by Peter)
    - fixed wording in commit message
    - included Reviewed-by of Krzysztof

Link to v3:
https://lore.kernel.org/linux-gpio/20251104210021.247476-1-jelonek.jonas@gmail.com/

v3: - fixed dt_binding_check errors in DT schema
    - as requested by Rob (for DT schema):
      - removed example from gpio-mux.yaml
      - added '|' to preserve formatting
      - 'shared-gpio' --> 'shared-gpios'
    - general fixes to DT schema
    - use mux_control_select_delay (as suggested by Peter) with
      hopefully reasonable delay of 100us
    - gpiochip ops implementation changes:
      - drop '.set' implementation (as suggested by Peter)
      - new '.set' implementation just returning -EOPNOTSUPP
      - '.direction_output' and '.direction_input' dropped
      - '.get_direction' returns fixed value for 'input'
    - direction of shared gpio set to input during probe
    - as suggested by Thomas
      - usage of dev_err_probe
      - further simplifications

    Since the consensus was that this should be input-only,
    '.direction_output' and '.direction_input' have been dropped
    completely, as suggested in the docs of struct gpio_chip. '.set' is
    kept but returns -ENOTSUPP.

    The shared GPIO is set to input during probe, thus '.direction_input'
    doesn't need to be implemented. '.get_direction' is kept (as
    suggested in docs of struct gpio_chip) but always returns
    GPIO_LINE_DIRECTION_IN.

Link to v2:
https://lore.kernel.org/linux-gpio/20251026231754.2368904-1-jelonek.jonas@gmail.com/

v2: - as requested by Linus:
      - renamed from 'gpio-split' to 'gpio-line-mux'
      - added better description and examples to DT bindings
    - simplified driver
    - added missing parts to DT bindings
    - dropped RFC tag
    - renamed patchset

Link to v1 (in case it isn't linked properly due to changed title):
https://lore.kernel.org/linux-gpio/20251009223501.570949-1-jelonek.jonas@gmail.com/

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

 .../bindings/gpio/gpio-line-mux.yaml          | 109 +++++++++++++++
 MAINTAINERS                                   |   6 +
 drivers/gpio/Kconfig                          |   9 ++
 drivers/gpio/Makefile                         |   1 +
 drivers/gpio/gpio-line-mux.c                  | 125 ++++++++++++++++++
 5 files changed, 250 insertions(+)
 create mode 100644 Documentation/devicetree/bindings/gpio/gpio-line-mux.yaml
 create mode 100644 drivers/gpio/gpio-line-mux.c


base-commit: bac88be0d2a83daf761129828e7ae3c79cc260c2
-- 
2.48.1