Power Management Specification

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1 Introduction
2 Rationale
3 Work in Progress

Introduction

The goal of power management technology is to allow Consumer Electronic
(CE) device manufacturers to optimize the power use of their products,
including a variety of mobile devices, home appliances, and other
end-user devices.

Rationale

In CE products, battery-driven mobile devices have particularly
stringent requirements for power management, and this affects the
product’s competitiveness significantly in specific product categories.
Mobile phones and other handheld devices are driven by secondary
batteries, not wired power supply. Efficient power usage is required to
increase the utilization time of such products.

Power Dissipation Elements in CMOS Circuits

CMOS(Complementary Metal Oxide Semiconductor) circuit has three elements
of power dissipation; leakage current, dynamic current, short-circuit
current power dissipation. The leakage current power dissipation is
caused by the leakage current. The leakage current power dissipation is
quadratically dependent on operating voltage but not dependent on clock
frequency. The dynamic current power dissipation is caused by the
dynamic current which is required for signal transitions. The dynamic
power dissipation is quadratically dependent on the voltage and linearly
dependent on the frequency of signal transitions. The short-circuit
current dissipation is generated by short circuit current when NMOS and
PMOS transistors are activated simultaneously. Short-circuit current
power dissipation is either linearly or quadratically dependent on the
supply voltage, depending on the size of the length of channel between
PMOS and NMOS transistors. Of those three elements, the dynamic current
power dissipation is a dominant factor because the frequency of signal
transitions is very high.

Power Reduction Methods

There are various methods for reducing power consumption: voltage/clock
scaling, clock gating, power supply gating, input signal selection and
so on.

- The voltage/clock scaling is the method that reduces both the dynamic power dissipation and the static power dissipation by decreasing voltage and clock dependently or independently.
- The clock gating reduces the dynamic power dissipation by disconnecting the clock from an unused circuit block.
- The power supply gating disconnects the supply power from unused circuit blocks, which are components of whole circuit. In clock/power supply gating methods, it is important to decide whether the block is used or not and to restore the blocks when they are used again.
- The input signal selection affects the reduction of the leakage power dissipation. The input signals of CPU pins should be adjusted to minimize the usage of power when the system is suspended.

Voltage scaling is the key to achieve power reduction and energy
reduction. As clear from the analysis of the three power dissipation
elements, operating voltage plays a major role in power dissipation.
Let’s take an example to see how voltage/clock scaling can reduce power
and energy. If operating voltage is down-scaled from 2.0V to 1.0V,
operating frequency is linearly dependent on the operating voltage in
practice. So let us assume clock frequency at 1.0V is half of the
frequency at 2.0V. In this case, nearly 87.5% of power can be reduced
theoretically. In terms of energy, the total energy for finishing a job
need to be compared. Please note that energy is the integration of power
dissipation over a period of time. The time taken to finish a job at
1.0V is twice as that of 2.0V. So the total energy required to finish a
job will be reduced by 75%.

http://tree.celinuxforum.org/docs/PM_formula.jpg

http://tree.celinuxforum.org/docs/PM_DVS.jpg

Clock gating and power supply gating can be used to reduce power usage
by unused blocks. It is important to predict correctly the idle period
of a hardware block because incorrect prediction may lead to performance
degradation due to the latency of reconnecting clock or power to the
hardware blocks.

Power Management Architecture

In designing power management architecture for Linux, the followings are
kept in mind.

- To design a system-level optimization of power usage involving interactions among application, OS, and hardware.
- To provide a generic power management framework for various hardware platforms

System-level power management is important for coordinating the
performance and power dissipation of hardware components in a system.
Recently, CPU chip vendors deploy dynamic clock/voltage scaling support
in their processors to enable OS-level power management capability. The
power reduction techniques offered by hardware components should be
synthesized, managed, and adjusted to minimize the power dissipation of
the whole system while the requirements of system should be satisfied as
well. Existing applications need to be adapted to use the power
management capability of operating system and such applications are said
to be “Energy-Aware”.

The power management architecture is designed to provide a generic
framework for various hardware architectures. Hardware specific power
control interfaces are abstracted by a power control layer. Power
management can be implemented for various hardware platforms by adapting
the machine-specific power control layer as necessary.

http://tree.celinuxforum.org/docs/PM_Arch.jpg

In this specification, power management technologies are classified into
three categories; platform suspend/resume, device power management, and
platform dynamic power management. Platform suspend/resume is aimed at
major power state changes at infrequent intervals, while platform
dynamic power management is aimed at subtle changes across a wider range
of power states at frequent intervals. Platform suspend/resume
technology is used for reducing most of power dissipation of a product
while the product is idle for a long time. In contrast, dynamic power
management is used for reducing power dissipation while a product is
being used. Dynamic power management covers also reducing power for very
short idle periods of time while a product is busy. Device power
management is to suspend/resume devices in a platform, which is used by
platform suspend/resume and dynamic power management technologies.

The rest of this specification is organized as follows. In Terminology
section, general power management related terminologies are defined.
Platform suspend/resume, device power management, and platform dynamic
power management are described in separate sections. Work in Progress
section describes important on-going projects which are not included in
CELF 1.0 spec but need to be considered for next CELF specification.

Work in Progress

Platform suspend/resume

Protocol between Energy-Aware Application and Policy Manager

1. Energy-aware(EA) application SHOULD register itself to policy
  manager.
2. Policy manager SHOULD send system suspend request to all
  applications registered as EA application.
3. Energy-aware application SHOULD respond to the system suspend
  request from policy manager appropriately.
  1. If EA application agree to system suspend, it invokes its
    suspend handler.
    1. The suspend handler may store important data including
      user data to a non-volatile storage.
    2. The suspend handler SHOULD send acknowledgement to
      policy manager and SHOULD wait until it receives resume
      request from policy manager.
  2. If EA application disagree to system suspend, it SHOULD send
    negative acknowledgement to policy manager.
1. If system inactivity which is longer than system timeout
  threshold is detected, policy manager MUST send system suspend
  request to EA applications and wait for responses from EA
  applications.
  1. The policy manager SHOULD provide user interface for
    initializing the system timeout threshold.
  2. The policy manager SHOULD initialize system timeout
    threshold to a default value.
  3. The policy manager MAY check device inactivity by
    periodically reading last access time of device.
2. If a user has requested system suspend, policy manager SHOULD
  enforce every EA application to suspend itself and wait for
  responses from EA applications. An EA application which is
  enforced to suspend by policy manager SHOULD invoke its suspend
  handler unconditionally.

Device Power Management

Automatic Device Power Saving

1. The policy manager set the length of inactivity period for each
  device.
2. The period of device inactivity can be detected by
  PM-engine/device drivers and notified to policy manager via
  signal.

Dynamic Power Management

Energy-Aware(EA) Application Specification

This specification shall define requirements of energy-aware
applications so that application can interact tightly with in-kernel
PM-engine to control system power states dynamically adaptive to various
application specific environments.

EA application may choose one appropriate operating state among system
PM policy. The system policy may be given to the EA application by
policy manager or hard-coded, which is dependent on design decisions by
system administrator. If the system policy information should be given
by policy manager, EA application need to set up a IPC mechanism with
the policy manager while it starts up.

For real-time EA applications, the information regarding the real-time
task scheduling such as the period of execution and their priorities
should be given to in-kernel PM engine via some whatever existing API
which we have to choose after extensive discussion on pros and cons of
existing APIs. (Linux kernel need to support scheduling
periodic real-time tasks.)

There are some cases that an EA application need to change system
operating state according to the task’s environment. This situation is
highly dependent on system constraints which system administrator should
know correctly. If a system administrator firmly believe that the
operating state of an EA task need to be changed, the decision by system
administrator should be respected and the in-kernel PM engine will
proceed with the decision. It should be noted that system performance
and power consumption efficiency might be greatly hampered if the
decision of the system administrator is incorrect. So the system
administrator should be very much considerate on changing application
operating state.

Performance vs. Battery Lifetime Policy

A kernel SHOULD provide userspace interface for specifying
requirements on the lower limit of battery lifetime. If the battery
lifetime requirement exists, a kernel SHOULD set the operating state
of the task to allocate the remaining energy resource amongst all
tasks, based on the task priorities, to satisfy this requirement.

Category:

Specification