diff --git a/Documentation/power/interface.txt b/Documentation/power/interface.txt
index a66bec2..74311d7 100644
--- a/Documentation/power/interface.txt
+++ b/Documentation/power/interface.txt
@@ -30,6 +30,17 @@
 that is known a priori. But, the user may choose 'shutdown' or
 'reboot' as alternatives. 
 
+Additionally, /sys/power/disk can be used to turn on one of the two testing
+modes of the suspend-to-disk mechanism: 'testproc' or 'test'.  If the
+suspend-to-disk mechanism is in the 'testproc' mode, writing 'disk' to
+/sys/power/state will cause the kernel to disable nonboot CPUs and freeze
+tasks, wait for 5 seconds, unfreeze tasks and enable nonboot CPUs.  If it is
+in the 'test' mode, writing 'disk' to /sys/power/state will cause the kernel
+to disable nonboot CPUs and freeze tasks, shrink memory, suspend devices, wait
+for 5 seconds, resume devices, unfreeze tasks and enable nonboot CPUs.  Then,
+we are able to look in the log messages and work out, for example, which code
+is being slow and which device drivers are misbehaving.
+
 Reading from this file will display what the mode is currently set
 to. Writing to this file will accept one of
 
@@ -37,6 +48,8 @@
        'platform'
        'shutdown'
        'reboot'
+       'testproc'
+       'test'
 
 It will only change to 'firmware' or 'platform' if the system supports
 it.