lenovo/x200,t400: enable C4 cpu low power state

This enables the C4 low power state on the lenovo x200 and t400.
It's inspired by the thread on the mailinglist:
"[coreboot] Lenovo X200 running Coreboot drains 3-4W more power
than with Vendor BIOS".

What this does, is to enable a C3 state using MWAIT(C3) request
and set the southbridge config c4onc3_enable to automatically
upgrade C3 to the lower power C4 state.
The latency (0x37) is the same value used by the vendor bios.

With C4 enabled the idle power consumption is about ~2-3W lower.

TEST= build and install on target. Use powertop top to measure power
usage. To manually disable c-state to compare them,
do (tested on linux 4.4):
echo 1 > /sys/devices/system/cpu/cpu*/cpuidle/stateX/disable

Change-Id: I1a1663a7662ebc7157a965667680688ad6a33545
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/15251
Tested-by: build bot (Jenkins)
Reviewed-by: Swift Geek
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Nico Huber <nico.h@gmx.de>

1 files changed, 1 insertions, 1 deletions

diff --git a/src/mainboard/lenovo/t400/devicetree.cb b/src/mainboard/lenovo/t400/devicetree.cb
index 43a610ab5f..6bf25fab38 100644
--- a/src/mainboard/lenovo/t400/devicetree.cb
+++ b/src/mainboard/lenovo/t400/devicetree.cb
@@ -63,7 +63,7 @@ chip northbridge/intel/gm45
 			register "sata_traffic_monitor"		= "0"
 
 			# Set c-state support
-			register "c4onc3_enable"		= "0"
+			register "c4onc3_enable"		= "1"
 			register "c5_enable"			= "1"
 			register "c6_enable"			= "1"