Power Management Concepts

Understand the difference between runtime PM and system sleep.

Runtime PM: Idle Power Saving

Runtime PM suspends devices when they’re not in use while the system is running.

Example scenario: Network driver

No packets for 5 seconds
Driver calls pm_runtime_put()
If reference count → 0, runtime_suspend() is called
Device enters low-power mode
Packet arrives → pm_runtime_get() → runtime_resume()

flowchart LR
    A["Device Active<br/>refcount > 0"] -->|"pm_runtime_put()<br/>refcount--"| B{"refcount == 0?"}
    B -->|Yes| C["runtime_suspend()"]
    C --> D["Device Suspended"]
    D -->|"pm_runtime_get()<br/>refcount++"| E["runtime_resume()"]
    E --> A
    B -->|No| A

    style C fill:#738f99,stroke:#0277bd
    style E fill:#7a8f73,stroke:#2e7d32

System Sleep: Suspend/Resume

System sleep happens when the entire system enters a low-power state (S3 = suspend to RAM, S4 = hibernate).

Example scenario: Laptop lid close

User closes lid
System calls suspend() on all devices
Devices save state, disable hardware
System enters S3
User opens lid
System calls resume() on all devices
Devices restore state

Reference Counting

Runtime PM uses reference counting to track active users:

/* Increment count - ensures device is active */
pm_runtime_get(dev);       /* Async, returns immediately */
pm_runtime_get_sync(dev);  /* Blocks until device is active */

/* Decrement count - may trigger suspend */
pm_runtime_put(dev);       /* Async, may schedule suspend */
pm_runtime_put_sync(dev);  /* Blocks if suspend happens */

/* Check status */
bool active = pm_runtime_active(dev);

Rule: Every get needs a matching put.

Always use pm_runtime_get_sync() before accessing hardware to ensure the device is powered.

Autosuspend

To avoid rapid suspend/resume cycles, use autosuspend:

/* In probe: enable autosuspend with delay */
pm_runtime_set_autosuspend_delay(dev, 1000);  /* 1 second */
pm_runtime_use_autosuspend(dev);

/* In code: mark idle, will suspend after delay */
pm_runtime_mark_last_busy(dev);
pm_runtime_put_autosuspend(dev);

This is useful for devices with bursty activity.

PM States

State	Meaning	Power
Active	Device fully powered, processing	High
Runtime Suspended	Device idle, may be powered down	Low
System Suspended	System in S3/S4, device state saved	Very Low

Callbacks Summary

Callback	When Called
`runtime_suspend`	Device idle, refcount → 0
`runtime_resume`	Device needed, `pm_runtime_get()`
`suspend`	System entering sleep
`resume`	System waking up

Summary

Concept	Description
Runtime PM	Saves power when device idle, system running
System Sleep	Saves power when system suspended
Reference count	Tracks active users of device
Autosuspend	Delays suspend to avoid rapid cycles