Power Management in Linux-Based Systems

Implementing power management in any system is a complex task. Here's how to manage your system's transitions from normal run state to power-saving modes.

Power management (PM) software is a crucial component in battery-powered systems, such as PDAs and laptops, because it helps conserve power when the system is inactive. As a simple example, power may be conserved by switching off the display when a system is inactive for some time. Conserving power in this manner extends battery life, so one can work more hours before having to recharge the battery.

Hardware support is vital for power management to work, and software intelligently exercises that support. The degree of power management support available in hardware varies from device to device. Some devices, such as a display, simply provide two power states, on and off. Other devices, like the SA1110 CPU, may support more complex power-saving features, including frequency scaling.

Implementing power management in any system is a complex task, considering that several non-interacting subsystems need to be brought together under a single set of guidelines. This article explains how power management works in Linux (2.4.x) and how it can be implemented in battery-powered systems based on an APM standard, at both the device driver and application levels.

Two Power Management Standards

Power management for computer systems has matured over the years and several standards exist. The two popular ones are advanced power management (APM) and advanced configuration and power interface (ACPI). APM is a standard proposed by Microsoft and Intel for system power management, and it consists of one or more layers of software to support power management. It standardizes the information flow across those layers. In the APM model, BIOS plays a key role. ACPI is the newer of the two technologies, and it is a specification by Toshiba, Intel and Microsoft for defining power management standards. ACPI allows for more intelligent power management, as it is managed more by the OS than by the BIOS. Although both standards are more popular in x86-based systems, it is possible to implement them in other architectures.

Power Management Implementation

Before implementing power management, it is important to understand what hardware support is available for saving power. One of the important goals of power management software is to keep all devices in their low power states as much as possible.

A possible approach for implementing power management is first to define a power state transition diagram. This defines several power states for the system and also defines the rules and events governing state transitions.

As an example, consider a PDA that has the following devices: Intel SA1110 CPU, real-time clock, DRAM, Flash, LCD, front light, UART, audio codec, touchscreen, keys and power button. The Intel SA1110 CPU supports several power-saving features, including frequency scaling, where the core clock frequency can be configured by software. Lowering clock frequency reduces the CPU's power consumption, but at the cost of reduced CPU speed. This CPU also supports several modes of operation:

  • Run mode: the normal state of operation for the SA1110 when it is executing code. All power supplies are enabled, all clocks are running and every on-chip resource is functional.

  • Idle mode: allows software to stop the CPU when not in use. In this mode, the CPU clock is stopped, representing some savings in power. All other on-chip resources are active. When an interrupt occurs, the CPU is reactivated.

  • Sleep mode: offers the greatest power savings and consequently the lowest level of available functionality. In this mode, power is switched off to the majority of the processor. Some preprogrammed event, such as a power button press, wakes up the CPU from this mode.

As you can see, software is responsible for transitioning the CPU either to idle mode or sleep mode.

In such a PDA, DRAM cells normally are refreshed periodically by the memory controller logic present inside the CPU. In sleep state, however, the majority of the CPU is shut off, which results in DRAM cells not being refreshed, which in turn leads to loss of data in DRAM. To avoid this loss, most DRAMs support a mode called self-refresh wherein the DRAM itself takes care of refreshing its cells. In such cases, software can put DRAM in its self-refresh mode by writing to a few control registers before transitioning the CPU to its sleep mode, thereby preserving the DRAM contents.

The top power-hungry devices in this PDA can be the CPU, DRAM and display back light. Hence, they should be kept in their low power states as much as possible.

Figure 1. Power State Transition Diagram

Figure 1 shows a possible power state transition diagram for this PDA. Here is a brief description of the power states:

  • Run state: system falls into this default state when it reboots. Power consumption is maximum in this state, as all devices are turned on or active.

  • Standby state: system falls into this state due to inactivity. LCD and display back light are turned off, and CPU clock speed is reduced to save some power.

  • Sleep state: system falls into this state due to continued inactivity. Power is conserved aggressively by putting the CPU in sleep mode, which in turn powers off most devices. DRAM, however, is put in its self-refresh mode to preserve the machine state (system and application text/data loaded in memory) while the system is sleeping. The system awakens from sleep state when a preprogrammed event occurs. When it wakes up, it transitions to the run state and machine state is restored.

  • Shutdown state: system falls into this state when the shutdown command is issued. The system reboots when it exits from this state. This means it is not necessary to preserve the machine state in DRAM, and hence DRAM can be powered off. The shutdown state then represents the lowest power consumption state of all.