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diff --git a/Documentation/ABI/testing/sysfs-devices-sun b/Documentation/ABI/testing/sysfs-devices-sun
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+Whatt:          /sys/devices/.../sun
+Date:           October 2012
+Contact:        Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>
+Description:
+                The file contains a Slot-unique ID which provided by the _SUN
+                method in the ACPI namespace. The value is written in Advanced
+                Configuration and Power Interface Specification as follows:
+
+                "The _SUN value is required to be unique among the slots of
+                the same type. It is also recommended that this number match
+                the slot number printed on the physical slot whenever possible."
+
+                So reading the sysfs file, we can identify a physical position
+                of the slot in the system.