[RFC v2 0/5] gpio: add pinctrl based generic gpio driver

[AKASHI Takahiro <takahiro.akashi@xxxxxxxxxx>]

This is a revised version of my previous RFC[1]. Although I modified
the commits to make them look SCMI-independent, they are still posted
as RFC because I have never tested them on real hardware.

(background)
I'm currently working on implementing SCMI pinctrl/gpio drivers
on U-Boot[2]. Although the pinctrl driver for the kernel[3] 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.

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[4], table-23.)

   The current pinconf framework is missing the feature (the pinconf
   parameter and a helper function). See patch#1, #2 and #3.

   Please note that there is an issue around the pin configuration in
   EPAM's current pinctrl driver as I commented[5].

2) DT bindings
   I would like to propose a generic binding for pinctrl based gpio driver.
   This allows a "consumer" driver to handle gpio pins like as other
   normal gpio controllers support. (patch#5)

3) generic GPIO driver
   Based on (2), I tried to prototype a generic driver in patch#4.
   Thanks to a set of existing pinctrl_gpio helper functions, except (1),
   It seems that the driver can be implemented not relying on pin controller
   specific code, at least for SCMI pinctrl.

I will appreciate any comments.

-Takahiro Akashi

[1] https://lkml.iu.edu//hypermail/linux/kernel/2310.0/00362.html
[2] https://lists.denx.de/pipermail/u-boot/2023-September/529765.html
[3] https://lkml.iu.edu/hypermail/linux/kernel/2308.1/01082.html
[4] https://developer.arm.com/documentation/den0056/
[5] https://lkml.iu.edu/hypermail/linux/kernel/2308.2/07483.html

AKASHI Takahiro (5):
  pinctrl: define PIN_CONFIG_INPUT
  pinctrl: always export pin_config_get_for_pin()
  pinctrl: add pinctrl_gpio_get_config()
  gpio: add pinctrl based generic gpio driver
  dt-bindings: gpio: Add bindings for pinctrl based generic gpio driver

 .../bindings/gpio/pin-control-gpio.yaml       |  55 ++++++
 drivers/gpio/Kconfig                          |   7 +
 drivers/gpio/Makefile                         |   1 +
 drivers/gpio/gpio-by-pinctrl.c                | 165 ++++++++++++++++++
 drivers/pinctrl/core.c                        |  19 ++
 drivers/pinctrl/pinconf.h                     |  10 +-
 include/linux/pinctrl/consumer.h              |   8 +
 include/linux/pinctrl/pinconf-generic.h       |   5 +
 8 files changed, 268 insertions(+), 2 deletions(-)
 create mode 100644 Documentation/devicetree/bindings/gpio/pin-control-gpio.yaml
 create mode 100644 drivers/gpio/gpio-by-pinctrl.c

-- 
2.34.1