Power Management in Linux – Virtual Devices, USB Wake‑Up & Other Peripherals

Table of Contents:

1. Introduction – Why Peripherals Matter for Suspend/Resume
    1.1 The “Peripheral Problem” in Modern Linux
    1.2 Scope of This Manual
2. Fundamentals of Linux Power-Management
    2.1 ACPI – The Firmware “Contract”
    2.2 Sleep States – S1-S5 in Practice
    2.3 Runtime Power Management (PM)
     2.3.1 Runtime PM Lifecycle
    2.4 systemd Integration
3. Virtual Devices
    3.1 virtio Family
    3.2 vhost-net & vhost-scsi
    3.3 VFIO (PCI Passthrough)
    3.4 USBIP (USB over IP)
    3.5 Other Hypervisor-Presented Devices
4. USB Wake-Up and Suspend
    4.1 Hardware Prerequisites
    4.2 Kernel Configuration
    4.3 sysfs Interface
     4.3.1 Enabling a Device to Wake the System
    4.4 Runtime PM vs. Wake-Up
     4.4.1 Interaction with usbcore
    4.5 Common Troubleshooting Steps
     4.5.1 Check if the Device Is Allowed to Wake
     4.5.2 Verify the Controller’s PME Status
     4.5.3 Inspect the System’s Wake-Up Sources
    4.6 Example: Enabling a USB Keyboard to Wake the System
    4.7 Dealing with “Device Not Allowed to Wake” Errors
    4.8 USB Core Configuration
    4.9 Controlling Autosuspend
    4.10 Example: Enabling Autosuspend for All USB Devices
5. Network Devices
    5.1 Ethernet (Wired)
    5.2 Wireless (Wi-Fi)
     5.2.1 Enabling WoWLAN
    5.3 Virtual Network Interfaces
    5.4 Ethernet
    5.5 Wireless
    5.6 Wi-Fi (iwlwifi)
    5.7 Bluetooth (btusb)
    5.8 Modem (e.g., qmi_wwan)
6. Power Management for Display Devices
    6.1 Intel i915 (Integrated GPU)
    6.2 AMDGPU
    6.3 NVIDIA (Proprietary)
    6.4 Backlight Control
7. Power Management for Input Devices
    7.1 Keyboard & Mouse (HID)
    7.2 Touchpad (Synaptics, Elantech)
    7.3 Keyboard & Mouse (HID) Wake-Up
    7.4 Touchscreen Wake-Up
8. Power Management for Audio Devices
    8.1 HDA Intel (snd_hda_intel)
    8.2 USB Audio (snd_usb_audio)
    8.3 ALSA Power Management
9. Power Management for Storage Devices
    9.1 SATA (ahci, libata)
    9.2 NVMe
    9.3 SATA & AHCI
10. Other Devices
     10.1 Power Management for Sensors & Misc Devices
      10.1.1 Battery & Power Sensors
      10.1.2 Accelerometer (e.g., iio)
     10.2 Power Management for PCI Devices
      10.2.1 General PCI Power Management
      10.2.2 Enabling PME (Power Management Event)
     10.3 PCIe Hot-Plug
     10.4 Power Management for Platform Devices
      10.4.1 ACPI Platform Devices
      10.4.2 Platform Devices Overview
     10.5 Power Management for Embedded Controllers (EC)
     10.6 Power Management for Platform-Specific Subsystems
      10.6.1 Intel-Specific
      10.6.2 AMD-Specific
     10.7 Power Management for Power Supply & Battery
      10.7.1 Power Supply Class
      10.7.2 ACPI Power Supply
     10.8 Power Management for Embedded Devices (e.g., Raspberry Pi)
      10.8.1 Raspberry Pi (BCM2835/BCM2711)
     10.9 Power Management for Embedded Real-Time Clocks (RTC)
      10.9.1 RTC Drivers
      10.9.2 Loop Devices
     10.10 Power Management for Virtual Devices
      10.10.1 Virtual Filesystems (e.g., procfs, sysfs)
     10.11 Power Management for Security Subsystems
      10.11.1 TPM (Trusted Platform Module)
     10.12 Power Management for Video Subsystems
      10.12.1 V4L2 (Video4Linux)

11. Interaction with Systemd & Initramfs
 11.1 Systemd-logind
 11.2 Custom ACPI Wake-Up Overrides

12. Summary of Key Kernel Config Options
     12.1 Core Power-Management Infrastructure
     12.2 USB Sub-System
     12.3 USB-Device Classes
     12.4 Platform / SoC Devices
     12.5 Embedded Controller (EC) – Keyboard / Power-Key Wake-Up
     12.6 Audio Sub-System
     12.7 Video / Display Sub-System
     12.8 Input Sub-System (Keyboards, Mice, Touchpads)
     12.9 SPI / Audio Codecs (e.g., WM8962, WM8742)
     12.10 Network Sub-System (Ethernet, Wi-Fi, Bluetooth)
     12.11 Power Management Core for Devices (PCI, Platform, etc.)
     12.12 Real-Time Clock (RTC)
     12.13 I²C / SPI / MIPI-CSI / DSI Devices
     12.14 Power-Supply / Battery Management
     12.15 GPIO / Pin Control (for Wake-Up from Pins)
     12.16 MIPI-CSI / Camera Sub-System
     12.17 HDMI / DP / DisplayPort Interfaces
     12.18 Audio Subsystem (ALSA, ASoC, HDMI-Audio)
     12.19 Power Management Options (Fine-Tuning)
13. Summary of How to Use This Information

1. Introduction – Why peripherals matter for suspend/resume

1.1 The “peripheral problem” in modern Linux

• Power‑state transitions (S1‑S5, runtime PM, device autosuspend) are global but delegated to individual drivers.
• A single mis‑behaving peripheral can:
  • Prevent the system from entering a low‑power state (e.g., “Failed to suspend device xyz” in dmesg).
  • Cause a resume failure (kernel panic, device “stuck” in D3, I/O errors).
  • Wake the machine unexpectedly (battery drain on laptops, “phantom wake‑ups” on servers).

Bottom line: Peripherals are the weakest link in the power‑management chain. Understanding how each class of device interacts with ACPI, runtime PM, and the user‑space orchestration layer (systemd) is essential for a reliable suspend/resume experience.

1.2 Scope of this manual

• Physical peripherals (PCIe, USB, SATA, Thunderbolt, etc.).
• Virtual devices presented by hypervisors (virtio, vhost, VFIO, USBIP).
• Both system‑wide sleep states (S0‑S5) and runtime power management (autosuspend).
• Focus on Linux kernel 6.6+ (the concepts apply to older kernels but some APIs have changed).

2. Fundamentals of Linux Power‑Management

2.1 ACPI – the firmware “contract”

Tip: acpidump -b -n DSM or acpidump -b -n PRW can be used to inspect the DSDT/SSDT tables for wake‑up capabilities.

2.2 Sleep states – S1‑S5 in practice

The kernel always attempts the deepest state the firmware reports as supported (/sys/power/mem_sleep).

2.3 Runtime Power Management (PM)

• Autosuspend – devices may be put into D3 when idle, independent of system sleep.
• pm_runtime_ API* – used by drivers (pm_runtime_get_sync(), pm_runtime_put_autosuspend()).
• pm_qos – quality‑of‑service constraints (e.g., pm_qos_add_request(PM_QOS_CPU_DMA_LATENCY, 10) prevents deep C‑states).

2.3.1 Runtime PM lifecycle

runtime_suspend   → driver->runtime_suspend()
runtime_resume    → driver->runtime_resume()
runtime_idle      → driver->runtime_idle()

The driver must implement runtime_suspend/runtime_resume. If not, the kernel falls back to “always‑on”.

2.4 systemd integration

Hooks in /usr/lib/systemd/system-sleep/ run before the kernel is asked to sleep (argument pre) and after resume (post).

3. Virtual Devices

Virtual devices are presented by a hypervisor (KVM, Xen, VMware, QEMU) but are driven by Linux kernel drivers. Their power‑management behavior is often different from physical hardware because:

• The underlying physical device may already be powered down.
• The hypervisor may need to freeze the device’s state (e.g., virtio queue) before the guest suspends.
• Some virtual devices expose runtime PM hooks that actually forward to the host.

Below we cover the most common classes.

3.1 virtio family

Key notes

• The virtio_pci driver (or virtio_mmio) provides the generic pm_runtime callbacks. Ensure CONFIG_VIRTIO_PCI and CONFIG_VIRTIO_MMIO are built as modules (m) for flexibility.
• For high‑performance networking, many distros set power/control = on on vhost‑net to avoid latency spikes. If you experience suspend failures, try echo auto > .../power/control.
• Snapshotting (VM suspend to disk) often uses virsh suspend → the guest kernel receives a S3 request; the hypervisor also needs to freeze all virtio devices. Recent QEMU releases have -machine -no-shutdown handling that automatically calls ioctl(VIRTIO_IOTLB_RESET).

3.2 vhost‑net & vhost‑scsi

• vhost is a kernel‑space implementation of virtio backend that bypasses user‑space for better performance.
• Power‑management is slightly different because the kernel owns the device:
  • vhost_net_stop() disables the kick‑fd and frees the ring.
  • vhost_net_open() re‑initialises.
• Potential pitfall: If the host’s NIC driver does not support autosuspend, the vhost device can keep the PCIe link active, preventing the guest from entering S3. Use ethtool -s eth0 wol g on the host to enable wake‑on‑LAN only if needed.

3.3 VFIO (PCI Passthrough)

• Devices are directly assigned to a guest (e.g., GPU, NIC).
• VFIO provides vfio-pci driver with runtime_pm callbacks that simply forward pm_runtime_* to the underlying PCI device.
• Important: When the guest is suspended, the host must also freeze the device:
  • vfio_iommu_type1 implements freeze/unfreeze callbacks.
  • The host’s systemd-suspend.service will call vfio-pci’s runtime_suspend. If the device refuses (e.g., GPU still rendering), the host suspend aborts.

Common work‑arounds

• Set options vfio-pci disable_idle_d3=1 (kernel module parameter) for devices that cannot enter D3 while the guest is alive.
• Use virsh edit <domain> → <features><acpi/></features> to enable ACPI in the guest; the guest’s ACPI tables can then request the host to power down the device.

3.4 USBIP (USB over IP)

• USBIP allows a remote USB device to appear locally as a virtual USB device.
• The client kernel registers a usbip-host driver which creates a usbip bus device under /sys/bus/usb/.
• Power‑management: The underlying physical USB host controller is still managed by the normal USB stack; the virtual usbip device follows the same runtime‑PM rules.

Tip: If a remote USB device is a wake‑up source (e.g., a USB keyboard), you must enable usbip_host wake_up attribute (/sys/bus/usb/devices/<bus>-<port>/power/wakeup). This will forward the remote wake‑up event to the local host.

3.5 Other hypervisor‑presented devices

4. USB Wake‑Up and Suspend

USB devices are the most frequent source of unexpected wake‑ups on laptops and desktops. The Linux kernel supports wake‑up at several layers:

1. Controller level (EHCI, xHCI, UHCI/OHCI).
2. Port level – each physical port can be configured to allow wake‑up.
3. Device level – the device’s driver may expose wake_up attribute.

4.1 Hardware prerequisites

Pro tip: On many laptops, the BIOS disables USB wake‑up while in “fast‑boot” mode. Disable fast‑boot if you need reliable wake‑up.

4.2 Kernel configuration

CONFIG_USB_SUPPORT must be enabled for any USB wake‑up support.

4.3 sysfs interface

• /sys/bus/usb/devices/<bus>-<port>/power/control – auto or on.
• /sys/bus/usb/devices/<bus>-<port>/power/wakeup – enabled or disabled.
• /sys/bus/usb/devices/<bus>-<port>/power/autosuspend_delay – delay before autosuspend (seconds).

4.3.1 Enabling a device to wake the system

# Example: enable wake‑up on USB keyboard (port 1‑2)
echo enabled > /sys/bus/usb/devices/1-2/power/wakeup
# Optional: set autosuspend delay to 5 seconds
echo 5 > /sys/bus/usb/devices/1-2/power/autosuspend_delay

If the device driver does not expose a wake_up file, you can still enable port‑level wake‑up via the controller’s sysfs:

# For xHCI, enable PME on the controller
echo enabled > /sys/bus/pci/devices/0000:00:14.0/power/wakeup

4.4 Runtime PM vs. Wake‑up

When a device is autosuspended, it can still generate a wake‑up event. The kernel keeps a list of “wake‑capable” devices (pm_wakeup_pending). On resume, the kernel checks which device triggered the wake‑up and calls the driver’s resume routine.

4.4.1 Interaction with usbcore

• The USB core maintains a usb_device->do_remote_wakeup flag.
• usb_autoresume can be invoked by the kernel if the device is required for system functionality after wake‑up.

4.5 Common troubleshooting steps

4.5.1 Check if the device is allowed to wake

cat /sys/bus/usb/devices/1-2/power/wakeup
# Output: enabled / disabled

4.5.2 Verify the controller’s PME status

cat /sys/bus/pci/devices/0000:00:14.0/power/wakeup
# Should be “enabled”

4.5.3 Inspect the system’s wake‑up sources

cat /proc/acpi/wakeup
# Lists devices with “enabled/disabled” status

If your keyboard is not listed, it means ACPI does not expose a _PRW entry for that device. You may need to add a custom ACPI override (see Section 9.2).

4.6 Example: Enabling a USB keyboard to wake the system

# 1. Identify the device
lsusb | grep -i keyboard
# e.g., Bus 001 Device 004: ID 046d:c31c Logitech

# 2. Find its sysfs path
readlink -f /sys/bus/usb/devices/1-4
# → /sys/devices/pci0000:00/0000:00:14.0/usb1/1-4

# 3. Enable wake‑up
echo enabled > /sys/bus/usb/devices/1-4/power/wakeup
# 4. Optionally, disable other ports to reduce spurious wakes
for dev in /sys/bus/usb/devices/*; do
    [ "$dev" = "/sys/bus/usb/devices/1-4" ] && continue
    echo disabled > "$dev/power/wakeup"
done

Verification: Suspend the system (systemctl suspend) and press a key on the keyboard. The system should wake instantly.

4.7 Dealing with “device not allowed to wake” errors

When you attempt to enable wake‑up, the kernel may reject it:

[  123.456789] usb 1-4: device not allowed to wake up (err = -22)

Possible causes:

• The controller driver (xhci_hcd) does not support PME# on that port (common on cheap boards).
• The device driver (e.g., usbhid) does not expose wake_up because it does not implement usb_set_device_state().

Solution:

• Patch the driver to add device_set_wakeup_capable(dev, true). For usbhid, the patch is trivial:

static int usbhid_probe(struct usb_interface *intf, const struct usb_device_id *id)
{
    // existing code …
    device_set_wakeup_capable(&intf->dev, true);
    return 0;
}

Re‑compile the driver as a module and load it.

4.8 USB Core Configuration

• CONFIG_USB – core USB support.
• CONFIG_USB_AUTOSUSPEND – enables autosuspend for USB devices.

4.9 Controlling Autosuspend

• Global autosuspend delay: set via kernel parameter usbcore.autosuspend=5.
• Per‑device autosuspend: echo 2 > /sys/bus/usb/devices/1-2/power/autosuspend_delay.

4.10 Example: Enabling autosuspend for all USB devices

# Add to /etc/default/grub
GRUB_CMDLINE_LINUX_DEFAULT="usbcore.autosuspend=5"
# Update grub
update-grub

After reboot, all USB devices will be autosuspended after 5 seconds of inactivity.

5. Network Devices

Network interfaces can also be wake‑up sources (via Wake‑on‑LAN, WoWLAN). They are handled by the net subsystem with additional power‑management hooks.

5.1 Ethernet (wired)

Enabling Wake‑on‑LAN

ethtool -s eth0 wol g
# Verify
ethtool eth0 | grep Wake-on
# Should show “g”

Note: The host NIC must have PME# line routed to the system’s ACPI wake‑up events. On some laptops, this line is not connected; you can still enable “soft‑wake” via systemd-inhibit --what=handle-power-key but true hardware wake‑up will not work.

5.2 Wireless (Wi‑Fi)

• Modern mac80211 drivers (iwlwifi, ath10k) support WoWLAN.
• CONFIG_MAC80211 includes CONFIG_MAC80211_WOWLAN.

5.2.1 Enabling WoWLAN

# Enable WoWLAN for iwlwifi
modprobe iwlwifi wowlan=1
# Or add to /etc/modprobe.d/iwlwifi.conf:
options iwlwifi wowlan=1

After loading, check /sys/class/net/wlan0/device/power/wakeup (should be enabled).

Potential issue: Some firmwares (e.g., Intel) require a special command (iw dev wlan0 set wowlan enable) before the driver will accept wake‑up.

5.3 Virtual network interfaces

• veth (Linux bridge) – does not support wake‑up (no hardware).
• tun/tap – same.

If you need a guest network device to wake the host, you must enable wake_on_lan on the host NIC and configure the guest’s virtual NIC to send a magic packet (e.g., via ethtool -s eth0 wol g inside the guest).

5.4 Ethernet

• CONFIG_NETDEVICES – core network device support.
• ethtool -s eth0 wol g – enable Wake‑on‑LAN (WOL).

5.5 Wireless

• iwconfig – set power management mode (iwconfig wlan0 power on).

5.6 Wi‑Fi (iwlwifi)

• CONFIG_IWLWIFI – includes runtime PM.

Enable power‑save:

# In /etc/modprobe.d/wifi.conf
options iwlwifi power_save=1

5.7 Bluetooth (btusb)

• Follows USB core autosuspend.

Enable autosuspend:

echo 2 > /sys/bus/usb/devices/1-1/power/autosuspend_delay

5.8 Modem (e.g., qmi_wwan)

• CONFIG_USB_NET_QMI_WWAN – runtime PM supported.

Disable wake‑up if causing suspend issues:

echo disabled > /sys/bus/usb/devices/1-2/power/wakeup

6. Power Management for Display Devices

Display devices (GPU, backlight) are another common source of failed suspend. Their power‑management behavior differs based on the driver and hardware generation.

6.1 Intel i915 (integrated GPU)

The i915 driver supports RC6 (deep power‑down) and RC6on (lightweight). Use i915.enable_rc6=1 kernel parameter to enable.

Enable RC6:

# Add to /etc/default/grub
GRUB_CMDLINE_LINUX_DEFAULT="i915.enable_rc6=1 i915.enable_rc6p=1"

Common suspend issue: Some laptops have a “display off while suspended” bug where the GPU fails to enter RC6, causing the system to stay in S0ix (light‑sleep).

Workaround: Add i915.enable_psr=0 to kernel parameters (disables Panel Self‑Refresh).

6.2 AMDGPU

• CONFIG_DRM_AMDGPU – supports PowerPlay and DPM.
• Runtime PM is not fully implemented for all ASICs.

Enabling DPM:

• Use amdgpu.dc=1 to enable Display Core for better power management.

# In /etc/default/grub
GRUB_CMDLINE_LINUX_DEFAULT="amdgpu.dc=1"

6.3 NVIDIA (proprietary)

• nvidia driver does not support runtime PM.
• Use nvidia-suspend scripts from the nvidia-persistenced package.

Alternative: Use the open‑source nouveau driver, which supports runtime PM, but may have poorer performance.

6.4 Backlight control

• acpi_video0 – generic backlight driver.
• /sys/class/backlight/intel_backlight/brightness – set brightness.

Power‑saving: Set acpi_backlight=vendor kernel parameter to use vendor‑specific backlight control (often more power‑efficient).

7. Power Management for Input Devices

7.1 Keyboard & Mouse (HID)

• usbhid driver – supports runtime PM and wake‑up.
• Use usbhid.mousepoll=0 kernel parameter to reduce power usage.

Customization: For keyboards with extra LEDs, you may need to disable LED control to allow deeper suspend (options hid_apple fnmode=2).

7.2 Touchpad (Synaptics, Elantech)

• CONFIG_TOUCHSCREEN_SYNAPTICS_I2C – runtime PM supported.

Wake‑up: Usually not needed; disable wake‑up on the port if causing spurious wakes.

7.3 Keyboard & Mouse (HID) Wake-Up

• usbhid – supports runtime PM and wake‑up.

Enable wake‑up:

modprobe usbhid
echo enabled > /sys/bus/usb/devices/1-3/power/wakeup

For built‑in keyboards (e.g., laptop top‑row), the device may be on the i8042 controller (PS/2). Use i8042.reset and i8042.nomux kernel parameters to ensure proper detection.

7.4 Touchscreen Wake-Up

• hid-multitouch – supports runtime PM.

Disable wake‑up (if causing spurious wakes):

echo disabled > /sys/bus/usb/devices/1-5/power/wakeup

8. Power Management for Audio Devices

Audio devices can prevent suspend if they hold DMA or interrupts.

8.1 HDA Intel (snd_hda_intel)

Common problem: When headphones are plugged in, the codec may stay active. Use hdajackretask to disable detection of jack events that keep the codec awake.

8.2 USB Audio (snd_usb_audio)

• Follows USB core’s autosuspend. Enable wake‑up with echo enabled > /sys/bus/usb/devices/1-5/power/wakeup.

8.3 ALSA Power Management

• Add options snd_hda_intel power_save=1 to /etc/modprobe.d/alsa.conf.
• Verify with cat /sys/module/snd_hda_intel/parameters/power_save.

9. Power Management for Storage Devices

9.1 SATA (ahci, libata)

• CONFIG_ATA – includes runtime PM for SATA.
• autosuspend_delay defaults to 5 seconds.

Enabling SATA link power management (ALPM)

# For a given port
echo min_power > /sys/class/scsi_host/host0/link_power_management_policy

Check /sys/class/scsi_host/host*/link_power_management_policy.

9.2 NVMe

• CONFIG_NVME_CORE – supports runtime PM (nvme_autosuspend).
• autosuspend_delay can be set via /sys/block/nvme0n1/device/power/autosuspend_delay.
• Use nvme_core.default_ps_max_latency_us=0 to enable deepest power‑saving state.

Wake‑up: NVMe devices can generate PCIe wake events; enable with nvme_core.default_ps_max_latency_us=0 kernel parameter.

# In /etc/default/grub
GRUB_CMDLINE_LINUX_DEFAULT="nvme_core.default_ps_max_latency_us=0"

9.3 SATA & AHCI

• Enable Aggressive Link Power Management (ALPM):

echo min_power > /sys/class/scsi_host/host0/link_power_management_policy

Set libata.force=1.5Gbps if link speed issues cause suspend failures.

10. Other Devices

10.1 Power Management for Sensors & Misc Devices

10.1.1 Battery & Power Sensors

• acpi – provides battery status (/sys/class/power_supply/BAT0).
• acpi_fakekey – used to simulate power key events.

11.1.2 Accelerometer (e.g., iio)

• CONFIG_IIO – Industrial I/O subsystem.
• Many sensors support runtime PM and can be configured for low‑power mode via iio_dev_attr_*.

Example: Disable wake‑up for an accelerometer

echo disabled > /sys/bus/iio/devices/iio:device0/power/wakeup

10.2 Power Management for PCI Devices

10.2.1 General PCI Power Management

• CONFIG_PCI_MSI – Message Signaled Interrupts (required for many wake‑up events).
• CONFIG_PCI – core PCI support.

10.2.2 Enabling PME (Power Management Event)

# Identify the PCI device (e.g., network controller)
lspci -nn | grep Ethernet
# e.g., 00:1f.6 Ethernet controller [0200]: Intel Corporation Ethernet Connection

# Enable wake‑up
echo enabled > /sys/bus/pci/devices/0000:00:1f.6/power/wakeup

10.3 PCIe Hot‑plug

• CONFIG_PCIEPORTBUS – required for hot‑plug and runtime PM of PCIe devices.

Note: Some laptops expose the EC (Embedded Controller) as a PCI device that must be kept awake for keyboard wake‑up.

10.4 Power Management for Platform Devices

10.4.1 ACPI Platform Devices

• CONFIG_ACPI – core ACPI support.
• Platform devices are often enumerated under /sys/devices/platform/.

Example: Disable wake‑up for a platform‑specific USB controller

echo disabled > /sys/devices/platform/ehci-omap/power/wakeup

10.4.2 Platform Devices Overview

• Enumerated under /sys/devices/platform/.
• Many SoC‑specific devices (e.g., USB OTG controller, audio codec) are platform devices.

General guideline:

1. Enable runtime PM (pm_runtime_enable) for each device in its driver probe function.
2. Set autosuspend delay appropriately (default 5 s is a good compromise).
3. Control wake‑up via /sys/.../power/wakeup.

10.5 Power Management for Embedded Controllers (EC)

Embedded controllers manage power keys, battery, and many other functions.

• CONFIG_EC – generic EC support.
• ec_sysfs – exposes EC registers under /sys/class/ec/.

Enable wake‑up

# Typically the EC is always awake; to allow suspend, add:
echo enabled > /sys/class/ec/ec0/power/wakeup

10.6 Power Management for Platform-Specific Subsystems

10.6.1 Intel‑specific

• intel_idle.max_cstate=1 – limit deepest C‑state (useful for debugging).
• intel_iommu=on – required for proper DMA remapping.

10.6.2 AMD‑specific

• amd_iommu=on – enable AMD IOMMU.

10.7 Power Management for Power Supply & Battery

10.7.1 Power Supply Class

• Exposed under /sys/class/power_supply/.

Example: Enable wake‑up for AC adapter

echo enabled > /sys/class/power_supply/AC/power/wakeup

10.7.2 ACPI Power Supply

• Exposed under /sys/class/power_supply/.

Set thresholds for battery charge/discharge to reduce wake‑ups.

10.8 Power Management for Embedded Devices (e.g., Raspberry Pi)

10.8.1 Raspberry Pi (BCM2835/BCM2711)

• CONFIG_BCM2835_POWER – power management for GPU and VideoCore.
• CONFIG_ARM_CPUIDLE – ARM idle driver.

Enable deep sleep:

# In /boot/config.txt
dtparam=watchdog=on

10.9 Power Management for Embedded Real‑Time Clocks (RTC)

10.9.1 RTC Drivers

• CONFIG_RTC_CLASS – generic RTC class.
• CONFIG_RTC_HCTOSYS – set system clock from RTC at boot.

Enable wake‑up:

echo enabled > /sys/class/rtc/rtc0/power/wakeup

10.10 Power Management for Virtual Devices

10.10.1 Virtual Filesystems (e.g., procfs, sysfs)

• Not power‑managed; they are always present.

10.9.2 Loop Devices

• CONFIG_BLK_DEV_LOOP – can be autosuspended if underlying storage supports it.

10.11 Power Management for Security Subsystems

10.11.1 TPM (Trusted Platform Module)

• CONFIG_TCG_TPM – TPM driver.
• Can generate wake‑up events; disable if not needed:

echo disabled > /sys/class/tpm/tpm0/power/wakeup

10.12 Power Management for Video Subsystems

10.12.1 V4L2 (Video4Linux)

• CONFIG_VIDEO_DEV – core video device support.
• Many webcams support runtime PM.

Disable wake‑up:

echo disabled > /sys/class/video4linux/video0/power/wakeup

11. Interaction with Systemd & Initramfs

Systemd integrates tightly with the kernel’s power‑management features.

11.1 Systemd‑logind

• logind tracks inhibitors (e.g., systemd-inhibit).
• It also monitors wake‑up sources via udev events.

11.2 Custom ACPI Wake‑up Overrides

If a device does not expose a proper _PRW entry, you can override ACPI tables at boot:

1. Create a custom SSDT (e.g., mywakeup.dsl) with the correct _PRW for the device.

DefinitionBlock ("", "SSDT", 2, "OEM", "MYWAK", 0x00000001)
{
    External (\_SB.PCI0.LNK0, DeviceObj)
    Method (_PRW, 0, NotSerialized) {
        Return (Package(){0x02, \_SB.PCI0.LNK0})
    }
}

2. Compile with iasl:

iasl -tc mywakeup.dsl
# Produces mywakeup.aml

3. Place the AML file in /boot/firmware/acpi/ and add to the boot loader:

# GRUB
GRUB_CMDLINE_LINUX_DEFAULT="acpi_override=mywakeup.aml"

4. Update GRUB and reboot.

Verification: After suspend, check /proc/acpi/wakeup – the device should now be listed as enabled.

12. Summary of Key Kernel Config Options

Below is a check‑list that you can copy‑paste into a notebook or a script.
It groups the kernel configuration symbols that must be built into the kernel (or compiled as modules) and the boot‑time kernel parameters that enable the corresponding runtime‑PM / wake‑up features for the major device classes you listed.

How to use itVerify the symbols are present in your current kernel (zcat /proc/config.gz | grep <SYMBOL>).If a symbol is missing, re‑run make menuconfig (or xconfig/kconfig) and enable it (usually under Device Drivers → …).Add the required kernel command‑line options to your boot‑loader (GRUB, systemd‑boot, etc.) and rebuild the initramfs if the option is consumed by an early‑boot script.After reboot, you can query the run‑time defaults under /sys/module/<driver>/parameters/ or /sys/.../power/.

12.1 Core Power‑Management Infrastructure

Boot parameter (optional):

pm_trace.enable=1          # enable runtime‑PM tracing (debug)

12.2 USB Sub‑system

Boot parameters:

usbcore.autosuspend=5          # default autosuspend delay (seconds)
usbcore.authorized_default=1  # allow autosuspend of newly‑added devices
usbcore.noirq=0                # keep IRQ wake‑up support

12.3 USB‑Device Classes

Typical runtime‑PM tuning (after the driver is loaded):

# Example for any USB device
dev=/sys/bus/usb/devices/1-1   # change to the correct BDF
echo auto   > $dev/power/control      # enable autosuspend
echo 2       > $dev/power/autosuspend_delay   # 2 s (or 5 s default)
echo enabled > $dev/power/wakeup       # allow wake‑up (or “disabled” to block)

12.4 Platform / SoC Devices

Boot‑time parameters (most are optional but useful on laptops/embedded boards):

acpi=force                 # force ACPI on platforms that support it
acpi_osi=Linux             # tell BIOS we are a Linux system (helps WOL)
acpi_no_static_ssdt=y      # ignore static SSDTs that may keep devices awake
intel_idle.max_cstate=1   # (debug) limit deepest C‑state
intel_iommu=on             # required for many Intel platforms
amd_iommu=on               # same for AMD

12.5 Embedded Controller (EC) – Keyboard / Power‑Key Wake‑up

Typical steps:

# Enable EC as a wake source (most laptops need this)
echo enabled > /sys/class/ec/ec0/power/wakeup

12.6 Audio Sub‑System

Boot parameters (optional):

snd_hda_intel.power_save=1        # Intel HDA power‑save
snd_hda_intel.enable_rc6=1       # Enable RC6 (deep idle)
snd_hda_intel.enable_rc6p=1

12.7 Video / Display Sub‑System

Kernel parameters:

i915.enable_rc6=1                # Intel RC6 deep idle
i915.enable_rc6p=1               # RC6p (more aggressive)
amdgpu.dc=1                      # Enable AMD Display Core (better power‑save)
drm.debug=0x1e                   # (optional) see suspend/resume messages in dmesg

12.8 Input Sub‑system (Keyboards, Mice, Touchpads)

Typical runtime‑PM tuning:

# Enable autosuspend for the PS/2 controller (usually platform device)
dev=/sys/devices/platform/i8042/serio0
echo auto   > $dev/power/control
echo enabled > $dev/power/wakeup   # allow wake‑up from this device

12.9 SPI / Audio Codecs (e.g., WM8962, WM8742)

Example:

# For any SPI device
dev=/sys/bus/spi/devices/spi0.0
echo auto   > $dev/power/control
echo enabled > $dev/power/wakeup

12.10 Network Sub‑system (Ethernet, Wi‑Fi, Bluetooth)

Network‑related parameters:

net.ifnames=0                 # keep classic ethX names (some BIOSes keep them awake)

12.11 Power Management Core for Devices (PCI, Platform, etc.)

Boot parameters:

no_console_suspend            # keep console active during suspend (debug)
pcie_aspm=force               # force ASPM on (may improve power)
pcie_aspm_policy=performance  # or “powersave”

12.12 Real‑Time Clock (RTC)

Typical use:

# Enable wake‑up from RTC alarm
dev=/sys/class/rtc/rtc0
echo enabled > $dev/power/wakeup

12.13 I²C / SPI / MIPI‑CSI / DSI Devices

Boot parameters (often used on ARM platforms):

mipi_dsi.enable=1            # enable DSI runtime‑PM (if supported)
dwc_otg.lpm_enable=0        # disable Link Power Management for USB‑OTG (if causing wake‑ups)

12.14 Power‑Supply / Battery Management

Parameters:

acpi_battery=poll_interval=60   # poll battery every 60 s (optional)

12.15 GPIO / Pin Control (for wake‑up from pins)

Example: enable a GPIO as a wake source:

# Suppose GPIO 23 is exported as /sys/class/gpio/gpio23
echo enabled > /sys/class/gpio/gpio23/power/wakeup

12.16 MIPI‑CSI / Camera Sub‑system

Tuning:

# Autosuspend the CSI host controller
dev=/sys/bus/platform/devices/mipi-csi0
echo auto > $dev/power/control
echo enabled > $dev/power/wakeup

12.17 HDMI / DP / DisplayPort Interfaces

Example:

# Enable wake‑up from the DP/HDMI controller
dev=/sys/bus/platform/devices/rcar-du
echo enabled > $dev/power/wakeup

12.18 Audio Subsystem (ALSA, ASoC, HDMI‑audio)

Typical:

# Autosuspend the HDA controller
dev=/sys/bus/pci/devices/0000:00:1f.3
echo auto > $dev/power/control
echo enabled > $dev/power/wakeup

12.19 Power Management Options (Fine‑tuning)

System‑wide kernel boot parameters for power management:

no_console_suspend
intel_idle.max_cstate=1          # limit deepest C‑state (useful on some hardware)
acpi=noirq                      # disable ACPI IRQ routing (debug)
acpi_osi=Linux                  # set OSI string (helps some laptops)
acpi_backlight=vendor           # for backlight control (may affect wake‑up)
acpi=force                      # force ACPI even if BIOS reports otherwise

13. Summary of How to Use This Information

1. Identify the device you want to be a wake source (e.g., a USB keyboard, a GPIO pin, a camera, an HDMI hot‑plug event, etc.).
2. Check that the driver is compiled with the appropriate CONFIG_… option (most modern kernels have them enabled by default; if you are building a custom kernel, make sure they are not disabled).
3. For GPIO‑based wake‑up, export the GPIO and set the wake‑up flag.
4. For RTC alarms, set the alarm time via /dev/rtc0 or the sysfs interface and enable wake‑up.

Test the configuration with:

systemctl suspend
# or
echo mem > /sys/power/state

Then trigger the device (press a key, move the mouse, toggle the GPIO, etc.) and verify that the system wakes.

Enable the wake‑up if it is not already:

echo enabled > /sys/<bus>/<device>/power/wakeup

Verify the device’s runtime‑PM and wake‑up attributes in /sys:

cat /sys/<bus>/<device>/power/control   # should be "auto" or "on"
cat /sys/<bus>/<device>/power/wakeup    # should be "enabled"

By ensuring that the relevant kernel configuration options are enabled (as listed above) and that the appropriate power/wakeup sysfs attribute is set to enabled, you can make almost any supported device—USB, I²C, SPI, HDMI, DisplayPort, camera, network, audio codec, battery, or GPIO—act as a wake‑source from suspend.