Re: pm: System date and time set incorrectly after suspend/resume todisk

From: Shuah Khan
Date: Tue Jun 18 2013 - 14:14:13 EST

On 06/17/2013 08:14 PM, John Stultz wrote:
> On Mon, Jun 17, 2013 at 6:30 PM, Shuah Khan <> wrote:
>> I am seeing a problem on my system after a suspend to disk in reboot or
>> shutdown mode. When pm_trace is 0 (which is the default), I can't
>> reproduce the problem easily. I have to run a few more suspend tests
>> before I see the problem. When pm_trace is disabled, the problem showed
>> up when I ran suspend in platform mode in a row. However when pm_ptrace
>> is enabled, problem happens on the very first spend to disk in reboot mode.
>> Steps to run into the issue:
>> echo 1 > pm_print_times
>> echo 1 > pm_trace
>> echo reboot > disk
>> echo disk > state
>> or
>> echo 1 > pm_print_times
>> echo 1 > pm_trace
>> echo shutdown > disk
>> echo disk > state
>> When system comes back up, system time set to incorrect time.
> So I this is by design. Please see Documentation/power/s2ram.txt for details:
> "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 number 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."
> thanks
> -john

Thanks. It definitely is working as designed and documented. :) Is there
a reason why pm_trace_store() couldn't/shouldn't print a warning
message (pr_warn) along the lines of:

"CAUTION: this option will cause your machine's real-time clock to be
set to an invalid time after a resume. Remember to set correct time
after resume!!"

It will serve as a reminder to correct the time after resume. I could
send a patch in to add a warning message.

-- Shuah

Shuah Khan, Linux Kernel Developer - Open Source Group Samsung Research
America (Silicon Valley) | (970) 672-0658
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