Re: Document sysfs interface to RTC system wakeup

[Pavel Machek <pavel@suse.cz>]

Add wakealarm description to rtc.txt, now including comments from
Michael and others.

Signed-off-by: Pavel Machek <pavel@suse.cz>

--- clean-cg/Documentation/rtc.txt	2008-02-07 10:28:47.000000000 +0100
+++ linux/Documentation/rtc.txt	2008-12-09 13:51:06.000000000 +0100
@@ -187,6 +187,64 @@
 
 If all else fails, check out the rtc-test.c driver!
 
+	    Even newer /sys interface
+	    ~~~~~~~~~~~~~~~~~~~~~~~~~
+	    (thanks to tino.keitel@gmx.de)
+
+How to use /sys/class/rtc/rtcX/wakealarm
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+This file can be used to view / set the time of an alarm (wakeup event).  The
+contents of the file are time measured in seconds since the Epoch
+(00:00:00h, 1 Jan 1970, UTC).
+
+     - 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.
+
+If the alarm was already enabled, a new alarm can only be set after the
+old alarm is disabled.
+
+    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.
+
+
+
+Migration from /proc/acpi/alarm
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+Users of /proc/acpi/alarm have to change their code to supply the
+seconds since the Epoch instead of a date string. (The date string in
+/proc is quite and ugly hack, and it will be removed one day, with
+rest of /proc/acpi).  
+
+For shell scripts, this can be done using the date command, e.g. like
+this:
+
+date -d tomorrow "+%s"
+
+This returns the seconds since the Epoch of the current time on the
+following day.
+
+Please note that you have to disable the old alarm first, if you want
+to set a new alarm. Otherwise, you get an error. Example:
+
+# Toggle example wakeup devices. Be sure to do 'cat wakeup' to check
+# the status when you are done.
+cd /proc/acpi
+echo EXP0 > wakeup
+echo PCI1 > wakeup
+echo HDEF > wakeup
+cd /sys/class/rtc/rtc0
+echo 0 > wakealarm
+echo $(( $(cat since_epoch) + 20 )) > wakealarm
+
 
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-- 
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