2004.03_Apci-Implementation in Linux 2.6.pdf

KNOW HOW

ACPI

The Small Sleeper

The new kernel has seen the continued advance of ACPI into the Linux mainstream. The standardized hardware configu-

ration facilities should be familiar to most people from the kernel 2.4, but power management has now been enhanced

with the addition of several sleep modes. BY TIMO HÖNIG

any laptop, pre-configured

PC, components or operating

systems you look at claim to be ACPI-

aware. Since the introduction of the first

version of the Advanced Configuration

and Power Interface way back in 1996,

the specification has aimed to finally

send off the APM (Advanced Power

Management) standard and the in-

flexible plug & play BIOS into retirement.

ACPI is often seen merely as a replace-

ment for APM. But that is not strictly

true: the C for Configuration is an inte-

gral part of ACPI, which provides a

uniform and (as far as this is feasible)

operating system independent hardware

setup interface. Linux 2.6 uses ACPI to

route PCI interrupts. If you want to use

the Advanced Programmable Interrupt

Controller (APIC) on a single-CPU sys-

tem, instead of the venerable PIC – to

avoid resource conflicts or interrupt

sharing – there is no alternative to ACPI.

Also, the ACPI subsystem tells the kernel

which PCI devices are hotplug capable,

and how to configure these devices. The

routines are to be found in drivers/pci/

hotplug/acpiphp_glue.c .

ACPI uses the ACPI Source Language

(ASL) and its compiled offshoot, the

ACPI Machine Language to provide an

abstraction layer for hardware-depen-

dent functions. AML describes the

hardware and the steps needed to access

it. Every ACPI-compatible operating sys-

tem has an AML interpreter for AML

bytecode.

The second, or high, level is part of the

operating system and uses the ACPI core

and ACPI drivers to provide an API that

the low level ACPI components can

access. The AML interpreter is part of

this level.

Two-Layered Specification

The ACPI specification [1] divides the

ACPI architecture into two layers. The

first of them, low level, comprises the

following ACPI architecture:

•ACPI tables

•ACPI BIOS

•ACPI registers

The ACPI tables describe the ACPI

hardware and its configuration in AML.

The central data structure of each ACPI

system uses definition blocks to stipulate

how to access the hardware. When a

system boots, the ACPI BIOS stores the

ACPI tables in memory. The ACPI BIOS

is also involved in sleep and resume

operations (Suspend-to-RAM, Suspend-

to-Disk).

ACPI on Linux 2.6

The kernel configuration groups the ACPI

power management functions [2] below

Power management options (ACPI, APM)

(see Figure 1). If you want to use Sus-

pend-to-RAM , you will first need to

select Prompt for development and/or

incomplete code/drivers below Code

maturity level options to display the Sleep

States option with the other ACPI options.

On booting, the kernel calls acpi_

boot_init() to access the ACPI tables and

parses them via the ACPI BIOS. The Dif-

ferentiated System Description Table

Timo Hönig studies Computer Sci-

ence in Augsburg, Germany. He has

lived in the world of networks, desk-

tops, Open Source and Unix for over

ten years now. His current passion is

an Open Source Project called FnFX, a

Linux ACPI daemon for Toshiba lap-

tops (http://fnfx.sourceforge.net).

March 2004

www.linux-magazine.com

ACPI Implementation in Linux 2.6

W hat a success story. Almost

ACPI

KNOW HOW

(DSDT) is of particular inter-

est. It contains the speci-

fications for ACPI-compatible

hardware components and

their configurations.

It is well-known that some

manufacturers use faulty

DSDTs on their systems. Even

if the manufacturer does not

provide a BIOS upgrade with

an error-free DSDT, and the

Relaxed AML kernel option

( CONFIG_ACPI_RELAXED_

AML ) is no help, don’t panic.

Debugged DSDTs for many

systems with faulty DSDTs

are available from [3]. The

steps are described at [4].

If you are out of luck, and cannot

locate an error-free DSDT for your sys-

tem, a manual repair may be possible.

Intel provides an ASL compiler and the

AML disassembler iasl [5]. This allows

you to copy the faulty DSDT from the

Proc filesystem ( cat /proc/acpi/dsdt >

dsdt.aml ), and disassemble it using the

AML compiler. The iasl -d dsdt.aml com-

mand will disassemble the DSDT and

output the ASL source to dtds.dsl (see

Listing 1).

The next step is to locate the errors in

the ASL source, remove them, and re-

compile ( iasl -tc dtds.dsl ). Try to eliminate

any warnings and errors, by modifying

the ASL source ( dtds.dsl ), before using

the re-compiled DSDT. Tips on patching

faulty DSDTs are available at [6].

After booting Linux 2.6 with ACPI sup-

port, it makes sense to inspect the ACPI

entries on the proc and sys filesystems.

memory, the chipset and the

peripheral devices (hard

disks, USB and so on). To

prevent loss of memory con-

tent, power is supplied to the

RAM. Suspend-to-RAM theo-

retically allows the system to

go to sleep, and wake up

again, within a few seconds.

My experience is that Sus-

pend-to-RAM rarely works. A

good thing that the kernel

still has it tagged as experi-

mental .

Suspend-to-Disk (S4)

saves more power than any

other sleep state, as it stores

the current memory content, the regis-

ters, and the states of peripheral devices

on the hard disk (Linux uses a large swap

partition to do this) before completely

halting the system. On re-booting, Sus-

pend-to-Disk reads the content of main

memory, and the previous system state,

from the hard disk, and restores the

previous state. Linux 2.6 supports Sus-

pend-to-Disk independently of ACPI and

APM. For comparison’s sake: Linux 2.4

does not support Suspend-to-RAM at all,

although Suspend-to-Disk is possible,

using the kernel patches provided by the

Swsusp [7] project.

You can use the entries in /proc/acpi and

/sys to access the ACPI interface. Root

privileges are required to write to these

locations. If you have not mounted sysfs ,

enter the following to do so:

mkdir /sys

mount -t sysfs sysfs /sys

The sysfs /sys sysfs defaults 0 0 entry in

the /etc/fstab file ensures that sysfs will

mount automatically when you boot.

Sleep States

The individual sleep states (see Table 1)

can be manipulated via /sys/power/state .

cat /sys/power/state will tell you the

states your system supports. echo -n

“Sleep_State” /sys/ power/state sets the

system to the specified state.

Suspend-to-RAM (S3) switches off the

whole system with the exception of the

main memory: that is, the CPU, cache

Software Suspend and

Suspend-to-Disk

Confusingly, Linux 2.6 has two alterna-

tive Suspend-to-Disk (S4) methods:

Software Suspend ( CONFIG_SOFTWARE_

SUSPEND ) and Suspend-to-Disk ( CON-

FIG_PM_ DISK ). The former is tagged as

Listing 1: Disassembling a DSDT with iasl

01 [root@sunshine:~]$ cp

/proc/acpi/dsdt ~/dsdt

02 [root@sunshine:~]$ iasl -d

./dsdt

03 Intel ACPI Component

Architecture

04 ASL Optimizing Compiler / AML

Disassembler version 20030918

[Sep 18 2003]

Intel Corporation

06 Supports ACPI Specification

Revision 2.0b

09 Acpi table [DSDT] successfully

installed and loaded

10 Pass 1 parse of [DSDT]

11 Pass 2 parse of [DSDT]

12 Parsing Deferred Opcodes

(Methods/Buffers/Packages/Regi

ons)

18 * Intel ACPI Component

Architecture

19 * AML Disassembler version

20030918

20 *

21 * Disassembly of dsdt, Thu

Dec 11 17:28:16 2003

22 */

23 DefinitionBlock ("DSDT.aml",

"DSDT", 1, "TOSHIB", "2000 ",

537003284)

24 {

25 Name (\_S0, Package (0x04)

26 {

27 [root@sunshine:~]$

..............................

..................

14 Parsing completed

15 Disassembly completed, written

to "dsdt.dsl"

16 [root@sunshine:~]$ head

dsdt.dsl

17 /*

08 Loading Acpi table from file

dsdt

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March 2004

Figure 1: Kernel 2.6 options for ACPI-based power management.

KNOW HOW

ACPI

experimental . In contrast, Suspend-to-

Disk support is a stable branch of

Suspend-to-Disk.

Software Suspend assumes a resume

boot prompt parameter – it is not possi-

ble to compile this into the kernel. The

option tells the kernel the swap partition

it should use to store the memory content

and status information. It makes sense to

create a permanent boot prompt parame-

ter in the boot manager configuration file,

for example resume=/dev/hda2 .

The alternative Suspend-to-Disk sup-

port provided by Software Suspend

allows you to specify the swap partition

in the kernel parameter pmdisk . In con-

trast to resume , you can bind pmdisk to

the kernel.

Table 1: ACPI Sleep States in Linux 2.6

ACPI Sleep State Sleep_State off Description

Standby (S1) standby System empowered, no processing by CPU.

Suspend-to-RAM (S3) mem System switched off except for main memory.

Suspend-to-Disk (S4) disk System switched off, main memory and status are saved to disk.

Note: States S0 (Running) and S5 (Soft Off) are not ACPI sleep states.

/sys/power/state , first remove the driver

module, and re-instate it after resuming.

The whole suspend process is easily

scripted. Listing 2 shows an example

that helped perform Suspend-to-RAM

gracefully, although the USB host con-

troller ( ohci-hcd.o driver module) had

previously caused kernel panic on

resuming.

If you are experiencing difficulty both

with Suspend-to-Disk (Software Suspend

or Suspend-to-Disk), and on resuming,

you can edit the kernel parameters man-

ually. resume=noresume or pmdisk=off

will prevent the kernel from attempting

to restore the previous state on booting.

The Software Suspend variant addition-

ally requires you re-initialize the swap

partition, mkswap /dev/hda2 in our

example, and then re-assign it to the sys-

tem: swapon /dev/hda2 .

cpu0/cpufreq in sysfs: cpuinfo_max_ freq

and cpu_ min_freq contain the maximum

and minimum processor speeds for the

current CPU. However, scaling_max_freq

and scaling_min _freq define the clock

speeds between which the CPU will actu-

ally scale. echo speed > scaling_max_freq

and echo speed > scaling_min_freq allow

root to change the values. speed must be a

value between cpuinfo_max_ freq and

cpuinfo_min_freq .

The scaling_available_governors entry

tells you what kind of clock speed manip-

ulation strategies the system provides:

powersave , userspace , or perform- ance .

The currently enabled strategy is stored

in scaling_governor . Root can enter echo

Governor > scaling_governor to enable a

different strategy, where Governor must

be one of the three known strings defined

in scaling_available_governors .

To r emove the need for command line

echoes, the Cpudyn [8] project has devel-

oped a daemon that uses the cpufreq

interface. Cpudyn helps you modify the

clock speed strategies of modern CPUs to

reflect your own requirements.

Insomnia

Suspend-to-RAM and Suspend-to-Disk

both assume that the current drivers are

suspend and resume-aware. The driver

model was modified to accomplish this

on Linux 2.6. Device drivers are required

to provide suspend and resume functions

that the kernel will call at the beginning

or end of a sleep state.

If the system still hangs, or fails to

resume, the following workaround might

help. Working as root, first inspect the

kernel logfiles, then re-compile the dri-

ver that is causing the issue as a module

– if this has not been done previously.

Before calling echo Sleep_State >

Dynamic Clock Speed for

Mobile CPUs

More recent mobile CPUs can reduce or

increase their clock speeds depending on

the current CPU load (Intel Speedstep

and AMD Power Now!). To leverage this

feature with Linux 2.6, enable cpufreq as

shown in Figure 2 and make sure you

select the correct cpufreq driver. The

cpufreq interface changed during devel-

opment of Linux 2.5, and is to be found

in sysfs in the final release. If you intend

to use a kernel compiled with these fea-

tures, you should use dmesg | grep

cpufreq to check if the kernel was able to

initialize the feature (see Listing 3).

If cpufreq quits with an error message,

you should first re-check that you have

the correct cpufreq driver. As strange as

this may sound, we had to enable Plug &

Play BIOS kernel support ( CONFIG_PNP

and CONFIG_PNPBIOS ) in our lab to get

the cpufreq ACPI driver to run.

Listing 2: Suspend script

ACPI Throttling

Many CPUs that do not support dynamic

clock speed manipulation can use ACPI-

based throttling instead. This typically

has a positive effect on heat and noise

levels, and might be a good idea for

servers with nothing to do outside of

office hours.

The CPU will need to support throt-

tling. To find out whether your CPU has

this support, check /proc/acpi/processor/

CPUx/ throttling for a list of available

throttling states. The T0 state tells the

CPU to run at full speed. In any other

throttling state the CPU is slowed down

by the percentage defined in /proc/

acpi/processor/CPUx/throttling .

01 #!/bin/sh

02 rmmod usblp usb-storage hid;

# Remove USB Module

03 rmmod ohci-hcd; # Remove Host

Controller Module

04 rmmod usbcore; # Remove USB

Core Module

06 echo -n "mem" >

/sys/power/state; # Suspend

to RAM

08 modprobe usbcore; # Load USB

Core Module

09 modprobe ohci-hcd; # Load

Host Controller Module

10 modprobe usblp usb-storage

hid; # Load USB Module

Entries

Assuming that cpufreq has initialized cor-

rectly, the following entries should be

available below /sys/devices/system/cpu/

ACPI for Laptops

The ACPI features described so far are

especially interesting for laptop users.

/proc/acpi/battery/BATx/info provides

12 exit 0;

March 2004

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ACPI

KNOW HOW

details on your machine’s

battery status. The charge

state, capacity, and voltage are

available in /proc/acpi/battery/

BATx/state . If the value drops

below a specific threshold,

which can be defined by typ-

ing echo threshold /proc/acpi/

battery/BATx/ alarm , ACPI can

use /proc/acpi/events to alert

the user.

The /proc/acpi/ac_adapter/

ADPx/state entry tells you if

the mains adapter is attached.

Plugging in and unplugging

the adapter is logged in /proc/

acpi/events . The button driver

( CONFIG_ACPI_BUTTON ker-

nel option) and the ( CONFIG_

ACPI_THERMAL ) thermal driver use the

same approach. The button driver alerts

when the power, sleep, or lid buttons are

pressed. The thermal driver monitors the

CPU temperature (the current value is

available in /proc/acpi/thermal_zone/

THRM/temperature ) and triggers when

the threshold defined in /proc/acpi/ther-

mal_zone/THRM/trip_points is reached.

Laptop owners can use the ACPI dae-

mon (acpid [9]) to respond to the

messages in /proc/acpi/events on their

machines. When the lid is closed, acpid

automatically tells the machine to sus-

pend to disk.

Despite the variety of ACPI functions,

most owners of modern laptops will

soon become disillusioned when they

notice that they cannot use keyboard

shortcuts to change the display bright-

ness on Linux, and that the function [Fn]

keys do not provide the expected func-

tionality. Good news for Asus and

Medion notebook users: the ASUS/

Medion Laptop Extras ( CONFIG_ACPI_

ASUS ) driver option creates entries

below /proc/acpi/asus that allow you to

modify the brightness of your display,

problems that accompanied

MS Windows 2000. The

goals of the ACPI specifi-

cation with systematic hard-

ware configuration and uni-

fied power management, are

urgent and important.

On the upside, the recent

implementation of PCI inter-

rupt IRQ routing via ACPI on

Linux is approaching sta-

bility. However, Linux 2.6

rightly tags the new power

management Suspend-to-

RAM feature as experimental

– it provides too little sup-

port to too few machines.

What’s worse, but this is

not Linux’ fault, is that

many systems on the market have faulty

DSDTs – even though some of them are

by manufacturers who are members of

the ACPI consortium. Mere mortal users

will probably be out of their depth if

expected to provide patched DSDTs to set

things straight. A lot of work still needs

to be done before ACPI can really

assume the role of a central inter-

face that will configure any known

hardware.

and control your laptop’s LEDs. The dri-

ver converts [Fn] keyboard combinations

to ACPI events. A userspace daemon [10]

is available for all other functions.

Toshiba notebooks can also leverage a

special ACPI driver known as the

Toshiba Laptop Extras ( CONFIG_ACPI

_TOSHIBA ). Unfortunately, the module

simply uses /proc/acpi/toshiba/keys in

the proc filesystem to pass [Fn] key-

presses, instead of using standard ACPI

events. The other /proc/acpi/toshiba

entries manipulate the brightness and

the fan. There are userspace daemons

available for the Toshiba extensions [11].

■

INFO

[1] ACPI: http://www.acpi.info

[2] ACPI4Linux: http://acpi.sf.net

[3] ACPI4Linux DSDT overview:

http://acpi.sf.net/dsdt/index.php

[4] DSDT initrd Patch:

http://gaugusch.at/kernel.shtml

[5] Intel IASL: http://www.intel.com/

technology/iapc/acpi/downloads.htm

[6] Repairing faulty DSDTs:

http://www.cpqlinux.com/acpi-howto.

html#fix_broken_dsdt

[7] Software Suspend for Linux:

http://swsusp.sf.net

[8] CPU Dynamic Frequency Control:

http://mnm.uib.es/~gallir/cpudyn/

[9] ACPI Event Daemon:

http://acpid.sf.net , http://sf.net/project/

showfiles.php?group_id=33140

[10] ASUS/Medion Laptop Extras:

http://julien.lerouge.free.fr

[11] Toshiba ACPI tools:

http://fnfx.sf.net , http://sourceforge.net/

projects/tclkeymon/

[12] ACPI4Linux mailing list:

http://acpi.sf.net/mailinglists.html

Where’s the Docs?

Users experiencing issues with Linux 2.6

and ACPI can try following their noses,

and searching the Web for documenta-

tion. Unfortunately, there isn’t any.

Current special publications collectively

steer clear of the topic “ACPI as a

Replacement for the Plug&Play BIOS”.

The documentation on the kernel itself is

the only exception, but it is fairly insub-

stantial apart from a few notes on

ACPI-based power management. The

best results that a spot of googling

returned were the Acpi4linux mailing list

at [12] and a few newsgroups, where at

least you can join forces with other peo-

ple facing similar problems.

Listing 3: The cpufreq test

01 [root@sunshine:~]$ dmesg |

grep cpufreq

02 cpufreq: CPU0 - ACPI

performance management activated.

03 cpufreq: *P0: 750 MHz, 22000

mW, 250 uS

04 cpufreq: P1: 350 MHz, 9800 mW,

250 uS

Conclusion

The current spate of ACPI hardware will

put the operating system developers

under pressure to do something about it.

Some of you may remember the ACPI

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March 2004

Figure 2: cpufreq ACPI options for current mobile CPUs.

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