Devicetree binding/description for pinmux on centralized pin func SoCs

[galak]

This issue is to discuss ideas on a generic / template for describing pinmux/cfg information for SoCs that handle pincfg as part of the GPIO block. Examples of such SoCs are STM, NXP, etc.

In such systems typically the following information is related to a pin:

• port (which GPIO block the pin is on - (GPIO A, GPIO B, GPIO C, PORT A, PORT B, PORT C, PIO 0, PIO 1, etc.)
• pin (typically a 0..31) number for the pin on the port
• function - how the pin is configured (used as GPIO, analog, I2C_SCL, etc)
• attributes/flags - additional params of the pin (pull-up/pull-down, speed/slew-rate, etc).

some how all this information needs to be encoded in the devicetree.

---

Some general assumptions/references:

• We try and conform the basic concepts that already exist for pinctrl as part of the "upstream" linux bindings.
• We keep things simple for MCUs and avoid group concepts, and have a node per pin+func combination
• upstream pinctrl binding:
  https://git.kernel.org/pub/scm/linux/kernel/git/devicetree/devicetree-rebasing.git/tree/Bindings/pinctrl/pinctrl-bindings.txt
  https://git.kernel.org/pub/scm/linux/kernel/git/devicetree/devicetree-rebasing.git/tree/Bindings/pinctrl/pinmux-node.yaml
• common pin attribute/flags binding:
  https://git.kernel.org/pub/scm/linux/kernel/git/devicetree/devicetree-rebasing.git/tree/Bindings/pinctrl/pincfg-node.yaml

[galak]

PORT:

Option 1 - encode via hierarchy - place pincfg nodes under the port device that contains the registers to configure that pin.

porta: pinmux@40049000 {
        compatible = "nxp,kinetis-pinmux";
        reg = < 0x40049000 0xd0 >;
        pinctrl {
           foo {
              pin = <3 ...>;
           };
        };
};

portb: pinmux@4004a000 {
        compatible = "nxp,kinetis-pinmux";
        reg = < 0x4004a000 0xd0 >;
        pinctrl {
           bar {
              pin = <6 ...>;
           };
        };
};

Option 2 - place all pinctrl in one block and provide references per pin to the port (in example below encoding is via phandle reference, could be encoded in some other means)

        pinctrl {
           foo {
              pin = <&porta 3 ...>;
           };
           bar {
              pin = <&portb 6 ...>;
           };
        };

[galak]

FLAGS/ATTRIBUTES:

Utilize common properties as per:

https://git.kernel.org/pub/scm/linux/kernel/git/devicetree/devicetree-rebasing.git/tree/Bindings/pinctrl/pincfg-node.yaml

Than per SoC bindings can add/remove/modify additional flags/attributes as it makes sense.