Suspend: RTC wakeup, sleep

Paul Fox pgf at
Wed Jul 28 11:44:24 EDT 2010

hal wrote:
 > Can somebody give me a pointer to some sample code that will wake up a 
 > suspended system in 5 minutes?  I'm assuming there is some way to do this 
 > using the alarm interrupt from the RTC.

    rtcwake -s 600 -m mem

to wake the system in 600 seconds, after suspending it to S3. 
see the man page -- but other -m options work, including "no", i
believe.  (although, if someone tries it and it does work, i'll
stand corrected.)

 > Can somebody confirm that sleep does what I expect on suspended systems?
 > My expectation is that the sleep timer logically ticks when suspended, but 
 > that the system won't get woken up when the sleep timer expires.
 > For example, suppose my program does a sleep(100), and shortly after that the 
 > system suspends.
 > If the next wakeup is 200 seconds after the start of the sleep, my program 
 > should run then (along with whatever caused the wakeup).
 > Or if the system wakes up after 50 seconds and doesn't suspend again, my 
 > program should run 100 seconds after it started to sleep.

i'm afraid not.  your sleep will be stretched by the duration of
the suspend.  see the following.  a 30 second sleep starts at
15:42:41.  the system suspends for 15 seconds, and wakes (that's
where the "+r" gets printed) at 15:43:01 (2 seconds later than it
expected to, btw).  the sleep terminates, and prints "sleep
ended" and the date at 15:43:25.  that's roughly 45 seconds after
the 30 second sleep started.

    [root at xo-a7-2a-c8 dev]# touch /var/run/powerd-inhibit-suspend/1
    [root at xo-a7-2a-c8 dev]# (date; sleep 30; echo sleep ended; date) &
    [2] 3122
    Wed Jul 28 15:42:41 GMT 2010
    [root at xo-a7-2a-c8 dev]# rtcwake -s 15 -m mem; date
    rtcwake: assuming RTC uses UTC ...
    rtcwake: wakeup from "mem" using /dev/rtc0 at Wed Jul 28 15:42:59 2010

    +rWed Jul 28 15:43:01 GMT 2010
    [root at xo-a7-2a-c8 dev]# sleep ended
    Wed Jul 28 15:43:25 GMT 2010

 paul fox, pgf at

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