path: root/Documentation/power
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authorFrans Pop <elendil@planet.nl>2008-10-15 22:01:21 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2008-10-16 11:21:29 -0700
commitb25f29b0da23f4f784f9bcae954b157e1f45cc69 (patch)
treef8319a9635282d066832bd2dd8b80e460a6a5804 /Documentation/power
parent1bfcf1304ea79c46efc3724e548b13b4b442b418 (diff)
pm: document use of RTC in pm_trace
As pm_trace uses the system's hardware clock to save its magic value, users of that option should be warned that using this debug option will result in an incorrect system time after resume. Signed-off-by: Frans Pop <elendil@planet.nl> Acked-by: Pavel Machek <pavel@suse.cz> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'Documentation/power')
1 files changed, 18 insertions, 0 deletions
diff --git a/Documentation/power/s2ram.txt b/Documentation/power/s2ram.txt
index b05f512130ea..2ebdc6091ce1 100644
--- a/Documentation/power/s2ram.txt
+++ b/Documentation/power/s2ram.txt
@@ -54,3 +54,21 @@ used to run with "radeonfb" (it's an ATI Radeon mobility). It turns out
that "radeonfb" simply cannot resume that device - it tries to set the
PLL's, and it just _hangs_. Using the regular VGA console and letting X
resume it instead works fine.
+pm_trace uses the system's Real Time Clock (RTC) to save the magic number.
+Reason for this is that the RTC is the only reliably available piece of
+hardware during resume operations where a value can be set that will
+survive a reboot.
+Consequence is that after a resume (even if it is successful) your system
+clock will have a value corresponding to the magic mumber instead of the
+correct date/time! It is therefore advisable to use a program like ntp-date
+or rdate to reset the correct date/time from an external time source when
+using this trace option.
+As the clock keeps ticking it is also essential that the reboot is done
+quickly after the resume failure. The trace option does not use the seconds
+or the low order bits of the minutes of the RTC, but a too long delay will
+corrupt the magic value.