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Subject[PATCH v4 5/5] x86/umwait: Document umwait control sysfs interfaces
Since two new sysfs interface files are created for umwait control, add
an ABI document entry for the files:

Signed-off-by: Fenghua Yu <>
Reviewed-by: Ashok Raj <>
.../ABI/testing/sysfs-devices-system-cpu | 21 +++++++++++++++++++
1 file changed, 21 insertions(+)

diff --git a/Documentation/ABI/testing/sysfs-devices-system-cpu b/Documentation/ABI/testing/sysfs-devices-system-cpu
index 1528239f69b2..d22cdadd3161 100644
--- a/Documentation/ABI/testing/sysfs-devices-system-cpu
+++ b/Documentation/ABI/testing/sysfs-devices-system-cpu
@@ -538,3 +538,24 @@ Description: Intel Energy and Performance Bias Hint (EPB)

This attribute is present for all online CPUs supporting the
Intel EPB feature.
+What: /sys/devices/system/cpu/umwait_control
+ /sys/devices/system/cpu/umwait_control/enable_c02
+ /sys/devices/system/cpu/umwait_control/max_time
+Date: May 2019
+Contact: Linux kernel mailing list <>
+Description: Umwait control
+ enable_c02: Read/write interface to control umwait C0.2 state
+ Read returns C0.2 state status:
+ 0: C0.2 is disabled
+ 1: C0.2 is enabled
+ Write 'Yy1' or [oO][nN] for on to enable C0.2 state.
+ Write 'Nn0' or [oO][fF] for off to disable C0.2 state.
+ max_time: Read/write interface to control umwait maximum time
+ in TSC-quanta that the CPU can reside in either C0.1
+ or C0.2 state. The time is an unsigned 32-bit number.
+ Note that a value of zero means there is no limit.
+ Low order two bits must be zero.
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