Nix-Vibe/modules/hardware/hp-battery-limit.nix

# HP battery charge limiter (HP ENVY x360 / similar, board 81AD).
#
# HP firmware exposes no percentage-based charge threshold (no
# charge_control_end_threshold sysfs on this machine). Instead the DSDT
# exposes ACPI charge-mode methods in the root scope (\):
#
#   \SBCC <n>  -> miscellaneous battery charge control register writes
#   \SBCO <n>  -> set battery charge operation:
#                   0x0500 = inhibit charge (stop charging / hold level)
#                   0x0000 = auto (normal charging)  <-- restores on 81AD
#                   0x0200 = force discharge (drain while on AC)
#   \GBCC       -> read back charge control
#   \GBCO       -> read back charge operation mode
#
# These are driven from userspace through the `acpi_call` kernel module
# (/proc/acpi/call). The "limit to 80%" is implemented as a policy on top:
# a root systemd poller inhibits charge once capacity >= 80% and re-enables
# auto charging below ~75%. This mirrors what the HP kernel-driver RFC
# (platform/x86: hp-wmi charge behaviour support) describes leaving to
# userspace while only the mode toggles live in the firmware.
#
# Control from the shell / CLI:
#   battery-charge-limit on|off|status
# `on`/`off` persist the choice to /var/lib/hp-battery-charge-limit/state
# and are applied immediately; the systemd service re-applies the policy
# on a poll tick. petere has NOPASSWD sudo so the QuickShell cog can call
# `sudo battery-charge-limit on/off` directly.
{
  config,
  lib,
  pkgs,
  ...
}:

let
  cfg = config.my.hardware.hpBatteryLimit;

  stateDir = "/var/lib/hp-battery-charge-limit";
  stateFile = "${stateDir}/state";

  # charge mode constants read from Arg0 by SBCO (high byte).
  acpiInhibit = ''\\SBCO 0x0500'';
  acpiAuto = ''\\SBCO 0x0000'';

  controlScript = pkgs.writeShellScriptBin "battery-charge-limit" ''
    set -eu

    state_dir=${stateDir}
    state_file=${stateFile}
    battery=/sys/class/power_supply/BAT0
    acpi_proc=/proc/acpi/call

    stop_at=${toString cfg.stopAt}
    resume_at=${toString cfg.resumeAt}

    acpi() {
      # $1 = ACPI method + args, e.g. "\SBCO 0x0500"
      if [ ! -e "$acpi_proc" ]; then
        echo "acpi_call module not loaded" >&2
        return 1
      fi
      printf '%s\n' "$1" > "$acpi_proc" || return 1
      cat "$acpi_proc" || true
    }

    read_capacity() {
      cat "$battery/capacity" 2>/dev/null | tr -d '\n' || echo 0
    }

    ensure_state() {
      mkdir -p "$state_dir"
      if [ ! -f "$state_file" ]; then
        echo "${if cfg.defaultOn then "on" else "off"}" > "$state_file"
      fi
    }

    get_state() {
      ensure_state
      cat "$state_file"
    }

    apply_policy() {
      local state capacity
      state=$(get_state)
      capacity=$(read_capacity)
      case "$state" in
        on)
          if [ "$capacity" -ge "$stop_at" ]; then
            acpi "${acpiInhibit}" || true
          elif [ "$capacity" -le "$resume_at" ]; then
            acpi "${acpiAuto}" || true
          fi
          ;;
        *)
          # off (or unknown) -> normal charging
          acpi "${acpiAuto}" || true
          ;;
      esac
    }

    case "$1" in
      on)
        ensure_state
        echo on > "$state_file"
        apply_policy
        echo on
        ;;
      off)
        ensure_state
        echo off > "$state_file"
        apply_policy
        echo off
        ;;
      status)
        get_state
        ;;
      apply)
        apply_policy
        ;;
      *)
        echo "usage: battery-charge-limit {on|off|status|apply}" >&2
        exit 2
        ;;
    esac
  '';
in
{
  options.my.hardware.hpBatteryLimit = {
    enable = lib.mkEnableOption "HP battery charge limiter (stop charging at stopAt%, re-enable below resumeAt).";
    stopAt = lib.mkOption {
      type = lib.types.int;
      default = 80;
      description = "Charge capacity % at which charging is inhibited.";
    };
    resumeAt = lib.mkOption {
      type = lib.types.int;
      default = 75;
      description = "Charge capacity % below which auto charging is re-enabled.";
    };
    defaultOn = lib.mkOption {
      type = lib.types.bool;
      default = false;
      description = "Start with limiting active on first boot (state file not present yet).";
    };
    pollInterval = lib.mkOption {
      type = lib.types.int;
      default = 120;
      description = "Seconds between policy re-applications by the systemd poller.";
    };
  };

  config = lib.mkIf cfg.enable {
    # acpi_call -> /proc/acpi/call so userspace can invoke SBCC/SBCO.
    boot.extraModulePackages = [ config.boot.kernelPackages.acpi_call ];
    boot.kernelModules = [ "acpi_call" ];

    environment.systemPackages = [
      controlScript
      # Compatibility alias: older QuickShell configs referenced the
      # hp-prefixed binary name; keep it working until all hosts redeploy.
      (pkgs.writeShellScriptBin "hp-battery-charge-limit" ''
        exec ${controlScript}/bin/battery-charge-limit "$@"
      '')
    ];

    systemd.services.battery-charge-limit = {
      description = "HP battery charge limiter (${toString cfg.stopAt}% cap)";
      wantedBy = [ "multi-user.target" ];
      after = [ "systemd-modules-load.service" ];
      wants = [ "systemd-modules-load.service" ];
      serviceConfig = {
        Type = "simple";
        ExecStart = pkgs.writeShellScript "battery-charge-limit-loop" ''
          set -eu
          trap 'exit 0' TERM INT
          while true; do
            ${controlScript}/bin/battery-charge-limit apply >/dev/null || true
            sleep ${toString cfg.pollInterval}
          done
        '';
        Restart = "on-failure";
        RemainAfterExit = false;
      };
    };
  };
}