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><H1
CLASS="SECT1"
><A
NAME="POWERMGM"
></A
>1. Power Management</H1
><P
>If you have a relatively recent x86 laptop, odds are it supports either
Advanced Power Management (APM) or Advanced Configuration and Power Interface
(ACPI). ACPI is the newer of the two technologies and puts power management in
the hands of the operating system, allowing for more intelligent power
management than is possible with BIOS controlled APM. This is most useful for
battery-powered laptops.
You can only have one
power management interface in control of your machine at a time, so it's
important you decide which method best suits your situation.</P
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="APM"
></A
>1.1. Advanced Power Management (APM)</H2
><P
><EM
>Advanced Power Management (APM)</EM
>
allows your computer's BIOS to control your system's power management
without the knowledge of the operating system.
The advantages to APM under Linux are that it's stable, well supported by
Linux vendors and has a solid history behind it.
However, not much development has been done with it over the past few
years.</P
><P
>To use it, you'll need to enable APM in the kernel:</P
><TABLE
BORDER="1"
BGCOLOR="#E0E0E0"
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><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="SCREEN"
> [*] Power Management support
<*> Advanced Power Management BIOS support
[ ] Ignore USER SUSPEND (NEW)
[ ] Enable PM at boot time (NEW)
[ ] Make CPU Idle calls when idle (NEW)
[ ] Enable console blanking using APM (NEW)
[ ] RTC stores time in GMT (NEW)
[ ] Allow interrupts during APM BIOS calls (NEW)
[ ] Use real mode APM BIOS call to power off (NEW)</PRE
></FONT
></TD
></TR
></TABLE
><P
>Most of the other APM
options exist as work-arounds for known problems with specific hardware
devices,
so you'll probably only want to enable the first one (CONFIG_APM).</P
><P
></P
><UL
><LI
><P
>Advanced Power Management BIOS support (CONFIG_APM):
You'll need to enable this in order to do anything
useful
with APM. User-space programs will receive
notification of
APM events (e.g., battery status change) and a
/proc/apm
device will provide you with battery status
information.</P
></LI
><LI
><P
>Ignore USER SUSPEND (CONFIG_APM_IGNORE_USER_SUSPEND):
This is a workaround for NEC Versa M
notebooks.</P
></LI
><LI
><P
>Enable PM at boot time (CONFIG_APM_DO_ENABLE):
Although it sounds nifty, most machines do
not require this feature to be enabled and in fact can
hang
some systems at boot time.</P
></LI
><LI
><P
>Make CPU Idle calls when idle (CONFIG_APM_CPU_IDLE):
On some machines,
this option provides increased power savings. On
others, it will
hang the system at boot time. Use with
caution.</P
></LI
><LI
><P
>Enable console blanking using APM
(CONFIG_APM_DISPLAY_BLANK):
Instead of blanking the virtual console actually turn
off the
screen. This won't work with X-Windows and actually
can cause more
problems that it solves.</P
></LI
><LI
><P
>RTC stores time in GMT (CONFIG_APM_RTC_IS_GMT): If you
want to
store GMT (Greenwich Mean Time) in your RTC (Real Time
Clock),
say yes here.</P
></LI
><LI
><P
>Allow interrupts during APM BIOS calls
(CONFIG_APM_ALLOW_INTS):
This is a workaround for some IBM Thinkpads that hang
while
suspending.</P
></LI
><LI
><P
>Use real mode APM BIOS call to power off
(CONFIG_APM_REAL_MODE_POWER_OFF):
This is a workaround for a number of broken BIOSes.
If your computer
crashes instead of powering off properly, turn this
on.</P
></LI
></UL
><P
>You'll want to install the APM daemon from
<A
HREF="http://www.worldvisions.ca/~apenwarr/apmd/"
TARGET="_top"
>http://www.worldvisions.ca/~apenwarr/apmd/</A
>
and configure your system startup scripts to activate it on boot:</P
><TABLE
BORDER="1"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="SCREEN"
># Start the APM daemon if it exists and if APM is enabled in the kernel
if [ -x /usr/sbin/apmd -a -d /proc/apm ]; then
if cat /proc/apm 1> /dev/null 2> /dev/null ; then
echo "Starting APM daemon: /usr/sbin/apmd"
/usr/sbin/apmd
fi
fi</PRE
></FONT
></TD
></TR
></TABLE
><P
>The APM daemon is actually made up of three primary programs:</P
><P
></P
><UL
><LI
><P
><B
CLASS="COMMAND"
>apmd</B
> - handles power management
tasks</P
></LI
><LI
><P
><B
CLASS="COMMAND"
>apm</B
> - a command-line tool to print
the current battery status or suspend the computer</P
></LI
><LI
><P
><B
CLASS="COMMAND"
>xapm</B
> - a simple battery meter for
X</P
></LI
></UL
><P
>If you're looking for a simple, "works out of the box" approach to power
management for your Laptop, APM is definitely the way to go.</P
><P
>A simple script to notify you how much battery time is remaining can be
added to your <TT
CLASS="FILENAME"
>~/.profile</TT
> file:</P
><TABLE
BORDER="1"
BGCOLOR="#E0E0E0"
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><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="SCREEN"
>if [ -f /proc/apm ]; then
DUMMY=`cat /proc/apm | cut -d" " -f 7`
# Don't display when fully charged
if [ "$DUMMY" != "99%" ]; then
LEVEL=`cat /proc/apm | sed -e "s/^.*% //"`
echo "Battery at $DUMMY ($LEVEL)"
fi
fi</PRE
></FONT
></TD
></TR
></TABLE
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="ACPI"
></A
>1.2. Advanced Configuration and Power Interface (ACPI)</H2
><P
><EM
>Advanced Configuration and Power Interface (ACPI)</EM
>
is the successor to APM, which places the responsibility of power management
away from the BIOS and into the hands of the operating system.
ACPI Linux is newer than APM Linux, more flexible in responding to power
management events, has seen
much more development as of late, and as a result of all this is prone to its
own share of bugs from
time to time.</P
><P
>If you're into cutting-edge development and are not intimidated with
kernel
builds and applying patches against source code, ACPI is worth
consideration.</P
><P
>There are two parts to ACPI under Linux: The ACPI driver built into the
kernel itself, and the ACPI daemon (ACPID). ACPID in its current incarnation
is
pretty simple: monitor /proc/acpi/event and do things in response. Even if
you don't load the daemon, you'll still get the benefit of ACPI features built
into the kernel such as processor thermal support.</P
><P
>You can determine which version
of the ACPI driver you are using, along with supported suspend states, by
using:</P
><TABLE
BORDER="1"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="SCREEN"
>bash $ cat /proc/acpi/info
version: 20030619
states: S0 S1 S3 S4 S4 S5</PRE
></FONT
></TD
></TR
></TABLE
><P
>ACPI development is progressing at a steady rate, so you might want to
consider
<A
HREF="http://sourceforge.net/project/showfiles.php?group_id=36832"
TARGET="_top"
>patching</A
>
your kernel against any recent updates to the kernel-level ACPI code. Once
you have downloaded the
patch for your specific kernel, you can patch it with something like:</P
><TABLE
BORDER="1"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="SCREEN"
>bash$ gunzip acpi-[version-kernel].diff.gz
bash# cd /usr/src/linux-[version]
bash# patch -Np1 -i ../acpi-[version-kernel].diff</PRE
></FONT
></TD
></TR
></TABLE
><P
>You'll want to recompile your kernel after this, of course:</P
><TABLE
BORDER="1"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="SCREEN"
> [*] ACPI Support
[ ] CPU Enumeration Only
<*> AC Adapter
<*> Battery
<*> Button
<*> Fan
<*> Processor
<*> Thermal Zone
< > ASUS Laptop Extras
< > Toshiba Laptop Extras
[*] Debug Statements</PRE
></FONT
></TD
></TR
></TABLE
><P
>You'll also want to install the ACPID daemon from
<A
HREF="http://sourceforge.net/project/showfiles.php?group_id=33140"
TARGET="_top"
>http://sourceforge.net/project/showfiles.php?group_id=33140</A
>
and configure your system startup scripts to activate it on boot:</P
><TABLE
BORDER="1"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="SCREEN"
>if [ -x /usr/sbin/acpid -a -d /proc/acpi ]; then
echo "Starting ACPID Daemon: /usr/sbin/acpid"
/usr/sbin/acpid
fi</PRE
></FONT
></TD
></TR
></TABLE
><P
>A bit of history... Microsoft was the first vendor to implement ACPI.
This is both good and
bad. It is good because when you buy a system, you can pretty much guarantee
that it has passed Microsoft's hardware compliance tests, including the test
of its ACPI implementation. However, these tests come up short in that they
do not indicate compliance with the ACPI specification, but rather with
Microsoft's implementation of ACPI. When that same machine is used with
Linux, some classes of errors that did not manifest themselves under Windows
may become apparent. To protect against this problem, the Linux ACPI driver
maintains a "bad BIOS" blacklist of known BIOS's that are known to not be ACPI
compliant, and as a result will refuse to enable ACPI if your system is
listed.</P
><P
>Many manufacturers are now validating that their systems run on Linux.
However, they use major Linux distributions with the default kernel. This
means that it is somewhat difficult to get OEMs to ensure that their systems
work with ACPI-enabled Linux until a major Linux distribution ships an ACPI
kernel. This presents a slight dilemma in that Linux distributions want to
ship kernels that run on as many systems as possible, but there have been some
positive moves in this area lately.</P
><P
>To conserve energy while remaining quickly available, ACPI-compatible
PCs may enter system sleep states. The ACPI specification defines five of
these states, known as S-states. Unlike processor sleep states, no work is
done by the system under S-states. Each state introduces greater power savings
but requires commensurately more time to awaken and begin performing work.
These are patterned on system states from the APM standard, a predecessor of
ACPI.</P
><P
>Full details on ACPI sleep states are available at
<A
HREF="http://acpi.sourceforge.net/documentation/sleep.html"
TARGET="_top"
>http://acpi.sourceforge.net/documentation/sleep.html</A
>.
Processor states are described at
<A
HREF="http://acpi.sourceforge.net/documentation/processor.html"
TARGET="_top"
>http://acpi.sourceforge.net/documentation/processor.html</A
>.</P
><P
>For more specific background information on ACPI itself, you can visit
the ACPI website at
<A
HREF="http://www.acpi.info"
TARGET="_top"
>http://www.acpi.info</A
></P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="WHICHONE"
></A
>1.3. APM vs. ACPI: Which one?</H2
><P
>There are currently two competing standards for providing power
management: APM and ACPI. Both cannot be used at the same time, so which one
is best for your situation? If you have a relatively recent (>2.4.20) kernel
and are not intimidated by kernel builds and patching source code, you'll find
many benefits with the flexibility of ACPI. If you just want to enable
generic power management, or are using an older machine, choose APM. Neither
method spins down idle hard drives; use hdparm for that instead. Either way,
your system's BIOS must correctly support the power management scheme you'd
like to use as well; if your system does not fully support either standard,
some of the power management options might crash your system and/or cause data
loss. You have been warned!</P
><P
>Even if you don't enable power management on your x86-laptop, Linux will
always issue the HLT instruction to your
processor whenever nothing needs to be done
<A
NAME="AEN101"
HREF="#FTN.AEN101"
><SPAN
CLASS="footnote"
>[1]</SPAN
></A
>.
Many Microsoft Windows CPU cooling program use this technique. This results
in lowering the power consumption of your CPU. Note that the system doesn't
power down when it receives the HLT instruction; it just stops executing
instructions until there is an interrupt.</P
><P
>There is generally no advantage to
enabling either type of power management on servers or workstations that do
not fall into
these categories.</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="HYPERTHREAD"
></A
>1.4. SMP, Hyper-Threading, IA64 & NUMA</H2
><P
>Some SMP system manufacturers may have omitted the pre-ACPI tables used
for SMP configurations. In this case, ACPI is required.</P
><P
>If you have a newer system that supports
<A
HREF="http://www.intel.com/technology/hyperthread/"
TARGET="_top"
>Hyper-Threading</A
>,
you will need to enable ACPI (and, of course, SMP). Without it, your Linux system
may be unable to discover and initialize all of the virtual processors.</P
><P
>IA64 machines require ACPI as well. Additionally, NUMA servers are
starting to require it for proper initialization.</P
></DIV
></DIV
><H3
CLASS="FOOTNOTES"
>Notes</H3
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><A
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HREF="powermgm.html#AEN101"
><SPAN
CLASS="footnote"
>[1]</SPAN
></A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>source/arch/i386/kernel/process.c</P
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