overlays prototype

conf.nix

  hardware.deviceTree.overlays = [
    { name = "spi"; dtsFile = ./dt/spi.dts; }
    { name = "pps"; 
      dtsText = ''
        /dts-v1/;
        /plugin/;
        
        / {
                compatible = "raspberrypi";
                fragment@0 {
                        target-path = "/soc";
                        __overlay__ {
                                pps {
                                        compatible = "pps-gpio";
                                        status = "okay";
                                        pinctl-name = "default";
                                        pinctrl-0 = <&pps_pins>;
                                        gpios = <&gpio 18 0>;
                                        assert-falling-edge;
                                };
                        };
                };
                fragment@1 {
                        target = <&gpio>;
                        __overlay__ {
                                pps_pins: pps_pins {
                                        brcm,pins = <18>;
                                        brcm,function = <0>; // input
                                        brcm,pull = <0>;    // off
                                };
                        };
                };
        };
      '';
    }
  ];

spi.dts

/dts-v1/;
/plugin/;

/ {
        compatible = "raspberrypi";
        fragment@0 {
                target = <&spi>;
                __overlay__ {
                        cs-gpios = <&gpio 8 1>, <&gpio 7 1>;
                        status = "okay";
                        pinctrl-names = "default";
                        pinctrl-0 = <&spi0_pins &spi0_cs_pins>;
                        #address-cells = <1>;
                        #size-cells = <0>;
                        spidev@0 {
                                reg = <0>;      // CE0
                                spi-max-frequency = <500000>;
                                compatible = "spidev";
                        };

                        spidev@1 {
                                reg = <1>;      // CE1
                                spi-max-frequency = <500000>;
                                compatible = "spidev";
                        };
                };
        };
        fragment@1 {
                target = <&alt0>;
                __overlay__ {
                        // Drop GPIO 7, SPI 8-11 
                        brcm,pins = <4 5>;
                };
        };

        fragment@2 {
                target = <&gpio>;
                __overlay__ {
                        spi0_pins: spi0_pins {
                                brcm,pins = <9 10 11>;
                                brcm,function = <4>; // alt0
                        };
                        spi0_cs_pins: spi0_cs_pins {
                                brcm,pins = <8 7>;
                                brcm,function = <1>; // out
                        };
                };
        };
};