[RFC 0/4] gpio: add SCMI pinctrl based driver

[AKASHI Takahiro <takahiro.akashi@xxxxxxxxxx>]

I'm currently working on implementing SCMI pinctrl/gpio drivers
on U-Boot[1]. Although the pinctrl driver for the kernel[2] was submitted
by EPAM, it doesn't contain the gpio driver and I believe that we should
discuss a couple of points on the kernel side to finalize my design for
U-Boot. 

So this RFC is intended for reviews, especially to raise some issues.
(Please note that I have *never* tested the code because I don't have
any real hardware to test SCMI on it.)

1) how to obtain a value on an input pin
   All the existing gpio drivers are set to obtain a value on an input
   pin by accessing the hardware directly. In SCMI case, however, this is
   just impossible in its nature and must be supported via a protocol
   using "Input-value" configuration type. (See the spec[3], table-23.)

   The current pinconf framework is missing the feature (the pinconf
   parameter and a helper function). See patch#1 and #2.

   Please note that there is an issue around the pin configuration in
   EPAM's current pinctrl driver as I commented[4].

2) DT bindings
   I would like to propose a generic binding for SCMI pinctrl based
   gpio driver. This allows a "consumer" driver to handle gpio input pins
   like as other normal gpio controllers provide. (patch#4)

3) generic GPIO driver
   Based on (2), I tried to prototype a generic driver in patch#3.
   As you can see, there is no SCMI-specific line of code as a set of
   existing helper functions, except (1), seem to be enough to implement
   required interfaces.
   So I'm not sure whether the driver should has a "compatibles" property
   of "arm,scmi-gpio-generic".

I will appreciate any comments.

-Takahiro Akashi

[1] https://lists.denx.de/pipermail/u-boot/2023-September/529765.html
[2] https://lkml.iu.edu/hypermail/linux/kernel/2308.1/01082.html
[3] https://developer.arm.com/documentation/den0056/
[4] https://lkml.iu.edu/hypermail/linux/kernel/2308.2/07483.html

AKASHI Takahiro (4):
  pinctrl: define PIN_CONFIG_INPUT
  pinctrl: add pinctrl_gpio_get_config()
  gpio: scmi: add SCMI pinctrl based gpio driver
  dt-bindings: gpio: Add bindings for SCMI pinctrl based gpio

 .../bindings/gpio/arm,scmi-gpio.yaml          |  71 ++++++++
 drivers/gpio/Kconfig                          |   8 +
 drivers/gpio/Makefile                         |   1 +
 drivers/gpio/gpio-scmi.c                      | 154 ++++++++++++++++++
 drivers/pinctrl/core.c                        |  19 +++
 include/linux/pinctrl/consumer.h              |   8 +
 include/linux/pinctrl/pinconf-generic.h       |   3 +
 7 files changed, 264 insertions(+)
 create mode 100644 Documentation/devicetree/bindings/gpio/arm,scmi-gpio.yaml
 create mode 100644 drivers/gpio/gpio-scmi.c

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2.34.1