[QUERY] RZ/V2H pinctrl implementation

["Lad, Prabhakar" <prabhakar.csengg@xxxxxxxxx>]

Hello,

Currently I am implementing a pinctrl driver for the Renesas RZ/V2H
SoC. I will be reusing the RZ/G2L pinctrl driver for this.

On the RZ/V2H SoC there are a couple of settings (like
output-impedance) which vary depending on what power rail it's
connected to.

For example, for the output impedance there are 4 groups of pins.
- Group1:
      Impedance is
          150Ω / 75Ω / 38Ω / 25Ω (at 3.3 V)
          130Ω / 65Ω / 33Ω / 22Ω (at 1.8 V)

- Group 2:
     Impedance is
          50Ω / 40Ω / 33Ω / 25Ω (at 1.8V)

- Group 3:
     Impedance is
         150Ω / 75Ω / 37.5Ω / 25Ω (at 3.3 V)
         130Ω / 65Ω / 33Ω / 22Ω (at 1.8 V)

- Group 4:
      Impedance is
          110Ω / 55Ω / 30Ω / 20Ω (at 1.8 V)
          150Ω / 75Ω / 38Ω / 25Ω (at 1.2 V)

The power rails connected to these pin groups will be connected to PMIC.

Below are the options have been explored,

Option#1
- Passing the power rail information from the PMIC to PFC (pinctrl
driver) so that pinctrl driver can read the voltage level and set the
values accordingly. Here we will be using the
PIN_CONFIG_OUTPUT_IMPEDANCE_OHMS to get/set values
Pros:
  • Debugfs can show the value in ohms
Cons:
  • Race condition at boot between pfc, i2c, and pmic
  • Late time of probing
  • Impossible to validate dt-bindings correctly
  • Manual doesn't say that pfc has access to the power rails, this
could be a challenge
With option #1 I am currently using fixed regulators but I see an
issue when we add a PMIC driver with regulators for example if i2c
pinmux (to which pmic is connected) that itself requires
output-impedance setting.

Option#2
- Specify the voltage in the pinmux/pins child node alongside the
output impedance (using power-source property)
Pros:
  • both driver and bindings can validate the settings
Cons:
  • the figure of the voltage supplied will have been replicated, as
it would be listed in the corresponding power regulator, but also in
the definition of the pin

Option#3
- Have an IP specific compatible ("renesas,v2h-output-impedance") with
value 1, 2, 4 or 6 (which indicates x1, x2, x4, x6 strength)
Pros:
  • Very simple to support and validate.
  • The user cannot really get this wrong
Cons:
  • new proprietary property
  • we would not be using the output impedance property offered by the subsystem

Please share your thoughts what could be the best approach to add
pinctrl support for RZ/V2H

Cheers,
Prabhakar