Part 16: Power Management

Power management allows devices to save energy when idle and properly handle system sleep states.

Two Types of PM

Type	When	Use Case
Runtime PM	Device idle while system runs	Save power during gaps in activity
System Sleep	System suspending (S3/S4)	Laptop lid close, `systemctl suspend`

flowchart TB
    subgraph RuntimePM["Runtime PM"]
        ACTIVE["Active<br/>(device in use)"]
        SUSPENDED["Suspended<br/>(device idle)"]
        ACTIVE -->|"pm_runtime_put()"| SUSPENDED
        SUSPENDED -->|"pm_runtime_get()"| ACTIVE
    end

    subgraph SystemPM["System Sleep"]
        RUNNING["System Running"]
        SLEEP["System Suspended<br/>(S3/S4)"]
        RUNNING -->|"suspend()"| SLEEP
        SLEEP -->|"resume()"| RUNNING
    end

    style RuntimePM fill:#7a8f73,stroke:#2e7d32
    style SystemPM fill:#738f99,stroke:#0277bd

When to Use What

Runtime PM:

Network card with no active connections
USB device with no pending transfers
Display controller with screen off

System Sleep:

User closes laptop
System enters hibernate
Scheduled sleep

Key Concept: Reference Counting

Runtime PM uses reference counting:

/* Before using device */
pm_runtime_get_sync(dev);   /* Ensures device is active */

/* ... use device ... */

/* When done */
pm_runtime_put(dev);        /* May suspend if count reaches 0 */

dev_pm_ops

Both types use the same structure:

static const struct dev_pm_ops my_pm_ops = {
    /* System sleep */
    .suspend = my_suspend,
    .resume = my_resume,

    /* Runtime PM */
    .runtime_suspend = my_runtime_suspend,
    .runtime_resume = my_runtime_resume,
};

Chapters

Chapter	What You’ll Learn
Concepts	Runtime PM vs system sleep, reference counting
Implementation	dev_pm_ops, pm_runtime_* API
Wakeup Sources	Wakeup capability, enable_irq_wake
System Sleep States	Suspend phases, freeze/thaw, debugging

Example

PM Driver - Platform driver with both runtime and system PM

Prerequisites

Platform drivers (Part 6)
Device model basics

Part 16: Power Management

Two Types of PM

When to Use What

Key Concept: Reference Counting

dev_pm_ops

Chapters

Example

Prerequisites

Further Reading

Table of contents