UM0851 datasheet(223/245 Pages) STMICROELECTRONICS

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UM0851

Power management section

Doc ID 16604 Rev 2

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10.2.1

Linux power management PM framework

The Power Management framework provides the basic framework for system wide power

management. The framework in Linux is a BIOS-less implementation of the existing

Standard APM (Advanced Power Management) model.

The framework supports the low power state transitions in Suspend-To-RAM or Standby

modes. The details of the modes are as follows:

●

Standby mode

: This state offers minimal, though real, power savings, while providing

a very low-latency transition back to a working system. Devices are suspended and

their clocks are gated, system and device state are saved in memory but no operating

state is lost (the CPU, Devices retains power), so the system easily starts up again

where it left off. The memory is placed in self refresh mode.

●

Suspend-to-RAM (STR)

: This state offers significant power savings as everything in

the system is put into a low-power state, except for memory, which is placed in self-

refresh mode to retain its contents. System and device state is saved and kept in

memory. All devices are suspended. In many cases, all peripheral buses lose power

when entering STR, so devices must be able to handle the transition back to the On

state. STR requires some minimal boot-strapping code to resume the system from

STR.

Note:

In the current software release LSPv2.3, both Suspend to RAM and Standby modes are

treated in an identical way. The system goes into sleep mode, everything except the wake-

up sources is suspended.

Features of the framework

●

Support for System Sleep model: The drivers can enter low power states as part of

entering system-wide low-power states like "Suspend-To-RAM".

●

Role of device drivers: The Device, Bus, and Class drivers collaborate by implementing

various role-specific suspend and resume methods to cleanly power down hardware

and software subsystems, then reactivate them without loss of data.

●

Static implementation: that is, the System Suspend-To-RAM is triggered from user

space through sysfs.

10.2.2

Linux clock framework

The clock framework has been devised to model the hardware clock tree, track

dependencies, maintain usage counts, disable clocks when unused by devices, propagate

changes to the dependents.

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