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diff --git a/Documentation/power/interface.txt b/Documentation/power/interface.txt
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+++ b/Documentation/power/interface.txt
@@ -52,3 +52,18 @@ suspend image will be as small as possible.
52 52
53Reading from this file will display the current image size limit, which 53Reading from this file will display the current image size limit, which
54is set to 500 MB by default. 54is set to 500 MB by default.
55
56/sys/power/pm_trace controls the code which saves the last PM event point in
57the RTC across reboots, so that you can debug a machine that just hangs
58during suspend (or more commonly, during resume). Namely, the RTC is only
59used to save the last PM event point if this file contains '1'. Initially it
60contains '0' which may be changed to '1' by writing a string representing a
61nonzero integer into it.
62
63To use this debugging feature you should attempt to suspend the machine, then
64reboot it and run
65
66 dmesg -s 1000000 | grep 'hash matches'
67
68CAUTION: Using it will cause your machine's real-time (CMOS) clock to be
69set to a random invalid time after a resume.