ACPI Implementation on FreeBSD

Prior to the introduction of the Advanced Configuration and Power Management Interface (ACPI), PCs did not have a unified standard mechanism that allowed the operating system to enumerate, configure, and manage the power usage and thermal properties of built-in hardware devices. Instead, these devices were either left unmanaged, or they were managed by special BIOS-level code such as Plug-and-Play BIOS (PnP BIOS), Advanced Power Management BIOS (APM), or other vendor-specific BIOS code. These firmware-driven methods increase firmware costs, and the resulting BIOS code is difficult to alter or debug. Device management issues are becoming more important, especially in moble computing environments where fine-grain power management is often necessary. ACPI replaces PnP BIOS, APM, and a number of ad hoc methods while providing a management framework that allows increased flexibility in hardware design. Unfortunately, the increased power and flexibility of ACPI comes with a cost: it requires substantial software support from the operating system kernel. In this paper we describe ACPI, how it is implemented in FreeBSD, and the lessons we learned from working with ACPI.