Representing an unusual I2C controller

[Chris Packham]

Hi I2C Enthusiasts,

I'm starting to look at supporting the I2C controller on the Realtek 
RTL9300 and I'm struggling a little bit to figure out how to represent 
it. It's a bit odd in that there are basically two I2C controllers which 
have a SCL pin each but then there are 8 common SDA pins that can be 
split between the two controllers to give a level of muxing and/or 
concurrency.

Basically I can have: SDA[0:7] + SCL8 or SDA[0:7] + SCL17 or any of the 
possible permutations. I could be accessing SDA0+SCL8 and SCL1+SDA17 
concurrently but if the hardware were connected as SDA0+SCL8 and 
SCL1+SDA8 I'd have to make those mutually exclusive.

I think it might just make sense to represent the two entities that own 
the SCL pins as separate controllers but then it'd be hard to enforce 
the fact that the individual SDA pins can only belong to one controller. 
Complicating things a bit further there is a common register that says 
whether the pins are being used for SDA or as GPIO (that can probably 
just be handled with pinctrl).

So I was thinking something like

i2c@36c {
  reg = <0x36c 0x14>
  compatible = "realtek,rtl9300-i2c";
  scl-pin = <8>;

  i2c@0 {
      sda-pin = <0>; // could use reg instead of scl-pin
  }
  i2c@1 {
      sda-pin = <1>;
  }
};

i2c@388 {
  reg = <0x388 0x14>
  scl-pin = <17>;
  compatible = "realtek,rtl9300-i2c";

  i2c@2 {
      sda-pin = <2>;
  }
  i2c@3 {
      sda-pin = <3>;
  }
};

Does that make sense? I'm not entirely sure how I can have those root 
controllers as something the I2C framework knows about without them 
being adapters in their own right (they can't have any devices attached 
directly that'd need to be done on one of the channels).

There are drivers for this in openwrt[1][2] but they ignore the fact 
that there are multiple controllers and the muxing is done separately. I 
was planning on using them for a bit of inspiration.

[1] - 
https://git.openwrt.org/?p=openwrt/openwrt.git;a=blob;f=target/linux/realtek/files-5.15/drivers/i2c/busses/i2c-rtl9300.c
[2] - 
https://git.openwrt.org/?p=openwrt/openwrt.git;a=blob;f=target/linux/realtek/files-5.15/drivers/i2c/muxes/i2c-mux-rtl9300.c

Thanks,
Chris