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pm: document use of RTC in pm_trace

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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>
This commit is contained in:
Frans Pop 2008-10-15 22:01:21 -07:00 committed by Linus Torvalds
parent 1bfcf1304e
commit b25f29b0da
1 changed files with 18 additions and 0 deletions
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18
Documentation/power/s2ram.txt
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@ -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.