Pentium M undervolting and underclocking

Background

Intel Pentium M processors from the Dothan and 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.

ATTENTION!

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, with effects up to and including laptop meltdown.

On one ThinkPad T43, undervolting reduced stable CPU temperature by 7-10deg under both idle and burn-in conditions. Combined with an ACPI fan control script, this greatly reduced the problem with fan noise.

HOWTO

To be added.

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 OP133. If you have a Pentium M with 400MHz FSB (i.e., quad-pumped 100MHz, found in Dothan and older Banias) then in the voltage table change OP133 to OP.

Don't forget to disable CONFIG_X86_SPEEDSTEP_CENTRINO_ACPI.

--- 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_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( 533,  700),
+	OP133( 666,  700),
+	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 },