modules/hardware/ads7846.nix


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{ ... }:

{
  # calibration for touchscreen
  services =
    let
      calibrationPoints = "3900 200 200 3900";
      calibrationMatrix = "-1 0 1 0 1 0 0 0 1";
    in
    {
      libinput.enable = true;

      # xorg configuration
      xserver.inputClassSections = [
        ''
          Identifier	"ads7846 calibration"
          MatchProduct	"ADS7846 Touchscreen"
          Option	"Calibration"	"${calibrationPoints}"
          Option	"SwapAxes"	"0"
          Driver  "libinput"
        ''
      ];

      # wayland configuration
      udev.extraRules = ''
        ATTRS{name}=="ADS7846 Touchscreen", ENV{LIBINPUT_CALIBRATION_MATRIX}="${calibrationMatrix}"
      '';
    };

  hardware.deviceTree =
    let
      # todo: parameter-ize this more.
      penirq = 25;
    in
    {
      overlays = [
        {
          name = "ads7846-overlay";
          # taken from raspberry-pi kernel tree:
          # https://github.com/raspberrypi/linux/blob/rpi-6.6.y/arch/arm/boot/dts/overlays/ads7846-overlay.dts
          dtsText = ''
            /*
            * Generic Device Tree overlay for the ADS7846 touch controller
            *
            */

            /dts-v1/;
            /plugin/;

            / {
                    compatible = "brcm,bcm2711";

                    fragment@0 {
                            target = <&spi0>;
                            __overlay__ {
                                    status = "okay";
                            };
                    };

                    fragment@1 {
                            target = <&spidev0>;
                            __overlay__ {
                                    status = "disabled";
                            };
                    };

                    fragment@2 {
                            target = <&spidev1>;
                            __overlay__ {
                                    status = "disabled";
                            };
                    };

                    fragment@3 {
                            target = <&gpio>;
                            __overlay__ {
                                    ads7846_pins: ads7846_pins {
                                            brcm,pins = <${toString penirq}>; /* mel: previously illegal value of 255 */
                                            brcm,function = <0>; /* in */
                                            brcm,pull = <0>; /* none */
                                    };
                            };
                    };

                    fragment@4 {
                            target = <&spi0>;
                            __overlay__ {
                                    /* needed to avoid dtc warning */
                                    #address-cells = <1>;
                                    #size-cells = <0>;

                                    ads7846: ads7846@1 {
                                            compatible = "ti,ads7846";
                                            reg = <1>;
                                            pinctrl-names = "default";
                                            pinctrl-0 = <&ads7846_pins>;

                                            spi-max-frequency = <2000000>;
                                            interrupts = <${toString penirq} 2>; /* high-to-low edge triggered */
                                            interrupt-parent = <&gpio>;
                                            pendown-gpio = <&gpio ${toString penirq} 1>;

                                            /* driver defaults */
                                            ti,x-min = /bits/ 16 <0>;
                                            ti,y-min = /bits/ 16 <0>;
                                            ti,x-max = /bits/ 16 <0x0FFF>;
                                            ti,y-max = /bits/ 16 <0x0FFF>;
                                            ti,pressure-min = /bits/ 16 <0>;
                                            ti,pressure-max = /bits/ 16 <0xFFFF>;
                                            ti,x-plate-ohms = /bits/ 16 <400>;
                                    };
                            };
                    };
                    __overrides__ {
                            cs =     <&ads7846>,"reg:0";
                            speed =  <&ads7846>,"spi-max-frequency:0";
                            penirq = <&ads7846_pins>,"brcm,pins:0", /* REQUIRED */
                                    <&ads7846>,"interrupts:0",
                                    <&ads7846>,"pendown-gpio:4";
                            penirq_pull = <&ads7846_pins>,"brcm,pull:0";
                            swapxy = <&ads7846>,"ti,swap-xy?";
                            xmin =   <&ads7846>,"ti,x-min;0";
                            ymin =   <&ads7846>,"ti,y-min;0";
                            xmax =   <&ads7846>,"ti,x-max;0";
                            ymax =   <&ads7846>,"ti,y-max;0";
                            pmin =   <&ads7846>,"ti,pressure-min;0";
                            pmax =   <&ads7846>,"ti,pressure-max;0";
                            xohms =  <&ads7846>,"ti,x-plate-ohms;0";
                    };
            };
          '';
        }
      ];
    };
}