This adds a new "wakealarm" sysfs attribute to RTC class devices which support
alarm operations and are wakeup-capable:

 - It reads as either empty, or the scheduled alarm time as seconds
   since the POSIX epoch.  (That time may already have passed, since
   nothing currently enforces one-shot alarm semantics.)

 - It can be written with an alarm time in the future, again seconds
   since the POSIX epoch, which enables the alarm.

 - It can be written with an alarm time not in the future (such as 0,
   the start of the POSIX epoch) to disable the alarm.

Usage examples (some need GNU date) after "cd /sys/class/rtc/rtcN":

    alarm after 10 minutes:
	# echo $(( $(cat since_epoch) + 10 * 60 )) > wakealarm
    alarm tuesday evening 10pm:
	# date -d '10pm tuesday' "+%s" > wakealarm
    disable alarm:
    	# echo 0 > wakealarm

This resembles the /proc/acpi/alarm file in that nothing happens when the
alarm triggers ...  except possibly waking the system from sleep.  It's also
like that in a nasty way: not much can be done to prevent one task from
clobbering another task's alarm settings.

It differs from that file in that there's no in-kernel date parser.

Note that a few RTCs ignore rtc_wkalrm.enabled when setting alarms, or aren't
set up correctly, so they won't yet behave with this attribute.
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Acked-by: Pavel Machek <pavel@ucw.cz>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: Greg KH <greg@kroah.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>