Power Management

This document explains the design and usage of the automated power management system implemented for laptops in this NixOS configuration (enabled via services.power.enable).

Overview

The power management configuration integrates two complementary tools to optimize battery life and system performance on NixOS laptops like ixo:

1. power-profiles-daemon (High-level CPU performance profiles): Coordinates high-level power states (performance, balanced, power-saver) directly with modern CPU drivers (e.g. intel_pstate or AMD equivalents) and BIOS ACPI platform profiles.
2. powertop (Low-level hardware-level optimization): Diagnoses device-level power draw and auto-tunes hardware settings (such as USB autosuspend, SATA link power, PCIe states) to maximize power savings.

Automatic Profile Switcher

While GNOME provides manual sliders to switch power profiles, it does not dynamically adjust them based on battery thresholds. We implement a custom switcher to automate this:

Architecture

The switcher consists of:

• A shell script: Queries the battery capacity and AC status via /sys/class/power_supply and calls powerprofilesctl set.
• A udev rule: Triggers the switcher service whenever the power supply status changes (e.g., plugging or unplugging the charger).
• A systemd timer fallback: Runs the switcher service every 5 minutes in case a udev event is missed or battery capacity changes incrementally without triggering a udev event.

Diagnostics & Verification

Checking Power Profiles

To see the active power profile, run:

powerprofilesctl get

To see all available profiles:

powerprofilesctl list

Checking Low-Level Tuning (powertop)

To run powertop interactively and check active power consumption (in Watts) or inspect the status of hardware tunables, run:

sudo powertop

Checking Switcher Status

You can view the systemd logs for the switcher service to verify that it is triggering correctly:

sudo journalctl -u power-profile-switcher.service

Suspend Power Optimization (s2idle / Modern Standby)

Modern Intel processors (such as the Meteor Lake CPU on ixo) do not support traditional S3 "deep" sleep. Instead, they use s2idle (Suspend-to-Idle / Modern Standby). This can lead to noticeable battery drain (e.g., 10% over several hours or overnight) if peripherals keep waking the CPU or preventing the SoC from entering its deepest package power-saving states (like C10).

To minimize suspend drain, services.power.enable installs a systemd service (disable-wakeup-triggers.service) that automatically disables sleep wakeup triggers for USB controllers (XHCI) and Thunderbolt controllers (TXHC) on system boot.

Diagnostics & Troubleshooting

1. Check active sleep states:

cat /sys/power/mem_sleep
# Output should show `[s2idle]`

1. Check enabled wakeup devices:

cat /proc/acpi/wakeup
# Devices with `*enabled` status can wake the system or keep it from entering deep sleep.

1. Check the status of the wakeup-trigger disabler:

   systemctl status disable-wakeup-triggers.service
   

Recommended BIOS/UEFI Settings

To ensure the hardware is fully entering deep power-saving states during suspend, verify these settings in your ThinkPad's UEFI/BIOS (press F1 during startup):

• Config > Power > Always On USB: Disable this (or disable Charge in Battery Mode). When enabled, USB ports remain powered while suspended, which causes continuous battery drain.
• Config > Network > Wake On LAN / Wake on WLAN: Disable these options so the network cards do not listen for magic packets or scan for networks during sleep.
• Security > Intel AMT (or Intel ME): If you do not require remote enterprise management, disable Intel Active Management Technology (AMT) to allow the processor core to power down completely.