Commit b25f29b0 authored by Frans Pop's avatar Frans Pop Committed by Linus Torvalds

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: default avatarFrans Pop <elendil@planet.nl>
Acked-by: default avatarPavel Machek <pavel@suse.cz>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 1bfcf130
...@@ -54,3 +54,21 @@ used to run with "radeonfb" (it's an ATI Radeon mobility). It turns out ...@@ -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 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 PLL's, and it just _hangs_. Using the regular VGA console and letting X
resume it instead works fine. resume it instead works fine.
NOTE
====
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.
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