Pentium M undervolting and underclocking

2006-03-21T06:40:50Z

Blackholesun: /* pentiums do not "melt down", they shut off when the get too hot */

== Background ==

Intel Pentium M processors from the [[Intel Pentium M (Dothan)|Dothan]] and [[Intel Pentium M (Banias)|Banias]] families can be instructed to operate at voltage and clock frequencies lower than the nominal ones recommended by Intel and used by ThinkPads by default. Experience shows that the processor may continue working correctly at lower-than-nominal voltages and frequencies, thereby reducing power consumption, heat and fan noise.

In the Pentium M, speed and voltage are controlled by software (through the MSR registers). It is up to the operating system to choose the right voltage for each frequency. Normally this is done according to tables published by Intel or according to ACPI tables. However, this can be overriden - in the case of Linux, by a kernel patch.

{{WARN|Following this instructions will operate your CPU under conditions it was not designed for. Even if your system seems stable, it may still suffer transient faults leading to arbitrary data corruption. In addition, errors in following these instructions (or changes between processor models) may operate the CPU ''above'' its nominal parameters, which, if taken too far, can cause kernel panics or even possibly hardware damage}}

For an example of what may be achieved, on one ThinkPad {{T43}} all voltages could be reduced by 20-30% and moreover, the minimal frequency reduced from 800MHz to 533MHz; this reduced stable CPU temperature by 7-10deg under both idle and burn-in conditions. Combined with [[how to control fan speed|fan speed control]], this greatly reduced the [[Problem with fan noise|problem with fan noise]].

{{NOTE|While under''volting'' has a clear measurable effect, it's not clear if under''clocking'' really works. <tt>/proc/cpuinfo</tt> reflects the underclocked frequency, but enabling debug output on <tt>cpufreq</tt> causes it to say things like "<tt>CPU frequency out of sync: cpufreq and timing core thinks of 533000, is 800000 kHz.</tt>" where the former is the chosen underclocked frequency and the latter is the documented minimum frequency. This discrepancy also causes [[Software Suspend 2]] to oops during suspend.}}

There are several methods to control CPU voltage in Linux, and currently all of them require a patched kernel.

== Alternative 1: patch with hard-coded voltages ==
=== Instructions ===

# Determine the stable voltages for each frequency, using some overclocking/undervolting utility that has specific support for Pentium M ("Centrino") processors. It seems that no such utility exists for Linux. For windows, one good choice is [http://www.pbus-167.com/chc.htm Notebook Hardware Control (NHC)], which conveniently also includes a table of nominal frequencies and voltags (in its help file).<br> Ascertain that at the undervolted settings the CPU actually performs correct computation ([http://www.mersenne.org/freesoft.htm Prime95] in "Tortute Test" mode seems to be a good partial test emphasizing FPU and memory access).<br> Note that beside reducing voltages (undervolting), you can also try to add lower frequencies (underclocking).
# Determine the model name string reported by the CPU, e.g., via the "<tt>Model name</tt>" line in {{path|/proc/cpuinfo}}, and likewise the <tt>cpu_family</tt>, <tt>model</tt> and <tt>stepping</tt>.
# Grab a copy of the example patch below and update it to reflect the parameters, frequencies and voltages you found.
# Apply the patch to your kernel (tested with 2.6.13.1 and 2.6.14-rc2).
# To prevent the ACPI table from overriding your table, disable the <tt>CONFIG_X86_SPEEDSTEP_CENTRINO_ACPI</tt> kernel option.
# Compile and install the new kernel.

The changes should be active now. If you want to see debug information attesting to thus, do the following:

# Reboot in <tt>single</tt> mode.
# {{cmdroot|echo 2 > /sys/module/cpufreq/parameters/debug}}
# {{cmdroot|modprobe speedstep-centrino}}
# {{cmdroot|dmesg}} and check for the message "<tt>speedstep-centrino: found "Intel(R) Pentium(R) M processor </tt>..."

If you don't see that message, you got the <tt>cpu_id</tt> or <tt>model_name</tt> parmaeters wrong. If your system crashes, re-test the voltages and adjust accordingly.

Please report your results (including voltages)!

=== Linux kernel patch (example) ===

The following example shows how to undervolt a Pentium M 750 (1.86GHz) on a ThinkPad {{T43}}. As discussed above, the parameters are specific to this one CPU. You will need to experimentally find the correct settings for your own CPU and adjust the patch accordingly.

Note that the Pentium M 750 has a 533MHz FSB (quad-pumped 133MHz), hence the use of <tt>OP133</tt>. If you have a Pentium M with 400MHz FSB (i.e., quad-pumped 100MHz, found in [[Intel Pentium M (Banias)|Banias]] and older [[Intel Pentium M (Dothan)|Dothan]]) then in the voltage table change <tt>OP133</tt> to <tt>OP</tt>.

<pre>--- linux-2.6.13.1-vanilla/arch/i386/kernel/cpu/cpufreq/speedstep-centrino.c 2005-09-10 05:42:58.000000000 +0300
+++ linux-2.6.13.1/arch/i386/kernel/cpu/cpufreq/speedstep-centrino.c 2005-10-18 04:46:01.000000000 +0200
@@ -54,6 +54,7 @@ enum {
CPU_DOTHAN_A1,
CPU_DOTHAN_A2,
CPU_DOTHAN_B0,
+ CPU_DOTHAN_C0,
CPU_MP4HT_D0,
CPU_MP4HT_E0,
};
@@ -63,6 +64,8 @@ static const struct cpu_id cpu_ids[] = {
[CPU_DOTHAN_A1] = { 6, 13, 1 },
[CPU_DOTHAN_A2] = { 6, 13, 2 },
[CPU_DOTHAN_B0] = { 6, 13, 6 },
+ [CPU_DOTHAN_C0] = { 6, 13, 8 },
[CPU_MP4HT_D0] = {15, 3, 4 },
[CPU_MP4HT_E0] = {15, 4, 1 },
};
@@ -94,6 +97,12 @@ static struct cpufreq_driver centrino_dr
.frequency = (mhz) * 1000, \
.index = (((mhz)/100) << 8) | ((mv - 700) / 16) \
}
+/* Likewise, for processors with 133MHz FSB. */
+#define OP133(mhz, mv) \
+ { \
+ .frequency = (mhz) * 1000, \
+ .index = (((mhz)/133) << 8) | ((mv - 700) / 16) \
+ }

/*
* These voltage tables were derived from the Intel Pentium M
@@ -202,7 +211,26 @@ static struct cpufreq_frequency_table ba
OP(1700, 1484),
{ .frequency = CPUFREQ_TABLE_END }
};
+
+/* Intel Pentium M 750, drastically undervoltaged */
+static struct cpufreq_frequency_table pentium_m_750[] =
+{
+ /* MHz mV */
+ OP133( 800, 700),
+ OP133(1066, 780),
+ OP133(1200, 828),
+ OP133(1333, 876),
+ OP133(1466, 924),
+ OP133(1600, 972),
+ OP133(1733, 1020),
+ OP133(1866, 1068),
+ { .frequency = CPUFREQ_TABLE_END }
+};
+
#undef OP
+#undef OP133

#define _BANIAS(cpuid, max, name) \
{ .cpu_id = cpuid, \
@@ -225,6 +253,12 @@ static struct cpu_model models[] =
BANIAS(1500),
BANIAS(1600),
BANIAS(1700),
+ {
+ .cpu_id = &cpu_ids[CPU_DOTHAN_C0],
+ .model_name = "Intel(R) Pentium(R) M processor 1.86GHz",
+ .max_freq = 1866000,
+ .op_points = pentium_m_750
+ },

/* NULL model_name is a wildcard */
{ &cpu_ids[CPU_DOTHAN_A1], NULL, 0, NULL },

</pre>

Don't forget to disable <tt>CONFIG_X86_SPEEDSTEP_CENTRINO_ACPI</tt>.

==Alternative 2: patch with user-space control==
The [http://fabrice.bellamy.club.fr/bdz.undervolt.2005.10.22.a.patch bdz.undervolt.2005.10.22.a.patch] patch, written by Gentoo-Wiki user "Bdz", allows voltages to be changed without reboot.

It presents a userspace interface, as in the following example:
:{{cmdroot|cat /sys/devices/system/cpu/cpu0/cpufreq/voltage_table}}
:{{cmdresult|1356,1356,1356,1356,1356,1356,1356,1244,1116,988}}
:{{cmdroot|echo "1084,1084,1084,1084,1084,1084,1084,988,908,860" >/sys/devices/system/cpu/cpu0/cpufreq/voltage_table}}

The relevant [http://gentoo-wiki.com/HOWTO_Undervolt_a_Pentium_M_CPU page] on Gentoo-Wiki providues further information and some helpful hints and scripts for voltage adjustment.

When applying this patch be sure to keep the "ACPI tables for decoding frequency pairs" option -- this patch only allows adjustment of the voltages for the normal clock speeds as reported by the ACPI table in the BIOS. On some models (e.g., ThinkPad {{T43}}) this does not include all clock speeds supported by the processor.

===Initscript===
This is a Gentoo initscript for the patch from Gentoo-Wiki user "Bdz".

Configuration file
<pre>
# /etc/conf.d/undervoltage

# voltage table sysfs interface
VTABLE_SYSFS="/sys/devices/system/cpu/cpu0/cpufreq/voltage_table"

# switch back to DEFAULT_VTABLE if undervoltage is stopped? [yes/no]
SWITCH_BACK="no"

# pentium-m banias 1,7GHz default voltages
DEFAULT_VTABLE="1484,1308,1228,1116,1004,956"
# pentium-m banias 1,7GHz lowered voltages [ -208mV ]
MOD_VTABLE="1276,1100,1020,908,796,748"
</pre>

Initscript
<pre>
#!/sbin/runscript
# Copyright 1999-2005 Gentoo Foundation
# Distributed under the terms of the GNU General Public License v2
# $Header: $

sysfs_check() {

if ! [ -e ${VTABLE_SYSFS} ]; then
logger "No sysfs voltage_table present. Modifying vcore voltage failed."
eerror "It seems that the undervolting patch has not been applied to the kernel."
eerror "see http://gentoo-wiki.com/HOWTO_Undervolt_a_Pentium_M_CPU for further information"
return 1
fi
}

start() {
sysfs_check || return 1
ebegin "Switching to modified voltage table [${MOD_VTABLE}]"
echo ${MOD_VTABLE} > ${VTABLE_SYSFS}
eend $?
}

# I think it is not necessary to switch to the default voltage table on shutdown
stop() {
if [ $SWITCH_BACK = "yes" ]; then
ebegin "Switching back to default voltage table [${DEFAULT_VTABLE}]"
echo ${DEFAULT_VTABLE} > ${VTABLE_SYSFS}
eend $?
fi
}
</pre>

===Tested frequencies===

If you have done excessive testing with user defined frequencies (stable system for over a month).
and no lookups occur when computing primes with mprime http://mersenne.org/ (gentoo: 'emerge gimps')
you can post your frequencies here. But note that this is just a rough indication, since the stable values will differ between individual processors, even in the same model.

{| border="1" cellpadding="2" cellspacing="0"
! CPU Type !! GHz !! Frequency Steps
|-
| Pentium-M Banias || 1.7 || 1276,1100,1020,908,796,748
|-
| Pentium-M Dothan || 1.86 || 1068,972,876,780,700
|-
| Pentium-M SL6F7 || 1.6 || 1196,1052,956,860,780,732
|}

==Alternative 3: another patch with user-space control==

The [http://avkrok.net/nw8240/centrino-voltages.diff centrino-voltages.diff] patch, written by Rickard Holmberg, also provides user-space control. See [http://avkrok.net/nw8240/ here] and [http://mailman.linux-thinkpad.org/pipermail/linux-thinkpad/2005-December/030772.html here] for usage.

==External Sources==

* A [http://forums.anandtech.com/messageview.aspx?catid=51&threadid=1677980 forum discussion] of Pentium M undervolting

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Pentium M undervolting and underclocking