Sunday, June 23, 2013

ACPI AC Adapter (1) - Introduction

In ACPI Control Method Battery (2), we see how ACPI's definition for control method battery and read the data in Ubuntu Linux. However, battery does not usually go alone - ACPI AC Adapter always goes with it.

AC Adapter Interface

Unlike the complexity of a control method battery, AC adapter is much simpler and three objects are mandatory:
  • _HID (Hardware ID) - needs to be ACPI0003
  • _PSR (Power Source) - indicates whether AC is online
  • _PCL(1) (Power Consumer List) - refers what is powered by AC

Below is a sample for AC adapter that returns whether AC is online from an EC register in _PSR:

Plugging and Unplugging AC Adapter

When AC adapter is plugged or unplugged, BIOS needs to issue a notification event as the example:

It is worth noting that only 0x80 is needed. Some BIOS will also issue Notify(_SB.AC, 0x81) that is not only redundant but also incorrect. Some BIOS also issues Notify(_SB.BAT, 0x80) and Notify(_SB.BAT, 0x81). While OS is smart enough to know that battery status is also changed, one may argument that issuing notification to battery is correct and at least won't break anything.

In my opinions, it is not incorrect to issue Notify(_SB.BAT, 0x80) as Battery State in _BST is definitely changed to discharging, assuming only one AC source and one battery are present in a system; however, issuing Notify(_SB.BAT, 0x81) and asking OS to re-enumerate _BIF/_BIX is only a waste of CPU cycles.

What is ACPI AC Adapter Affecting?

It is definitely not surprising to know the little AC icon in OS's taskbar uses the return value of AC._PSR - that's  the only way that an OS knows whether a system is powered by an AC adapter. But... what else?

It is true that hardware state is different from powered-by-AC to powered-by-battery - at least from the viewpoint of the power circuit; however, this is only small part of the system board and we often see much more difference and affecting the system. When a system is powered by AC source, it is, in most cases, running at performance mode - CPU can spend more time in maximum frequency, power-saving features are less aggressive, fan runs at higher speed, and the panel may be brighter. All of these can be changed when OS finds out AC is offline from AC._PSR.

The implication? If one sees stability issues without AC only, playing around _PSR may help identify whether it is the power circuit or is an OS beahviours, i.e. one device and its driver are too aggressive with its power-saving features. One way to distinguish whether it is always reports 1 in _PSR - if the problem persists, it is the power circuit (hardware) that causes the problem. I have seem a number of times that bad-quality power components that hang the system and many other times that a device or its driver failed the system. This technique always helps a quick and good direction to real problems.


How does Linux kernel handle ACPI AC adapter?


  1. It seems _PCL is defined but is not used in Linux kernel.


  • ACPI Specification 5.0 - Sec. 10.3
  • ACPI Specification 5.0 - Table 5-121
  • Linux kernel source code (3.10)

1 comment:

  1. Hi! Some guy comment to me, that the _PCL do not return a (\SB) according to ACPI, _PCL has to return variable-length packages containing a list of references to devices or buses