[PATCH v2 2/7] rtc: alarmtimer: Use maximum alarm time offset

[Guenter Roeck <linux@roeck-us.net>]

Some userspace applications use timerfd_create() to request wakeups after
a long period of time. For example, a backup application may request a
wakeup once per week. This is perfectly fine as long as the system does
not try to suspend. However, if the system tries to suspend and the
system's RTC does not support the required alarm timeout, the suspend
operation will fail with an error such as

rtc_cmos 00:01: Alarms can be up to one day in the future
PM: dpm_run_callback(): platform_pm_suspend+0x0/0x4a returns -22
alarmtimer alarmtimer.4.auto: platform_pm_suspend+0x0/0x4a returned -22 after 117 usecs
PM: Device alarmtimer.4.auto failed to suspend: error -22

This results in a refusal to suspend the system, causing substantial
battery drain on affected systems.

To fix the problem, use the maximum alarm time offset as reported by rtc
drivers to set the maximum alarm time. While this will result in brief
spurious wakeups from suspend, it is still much better than not suspending
at all.

Cc: Brian Norris <briannorris@chromium.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
---
v2: Rename range_max_offset -> alarm_offset_max

 kernel/time/alarmtimer.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/kernel/time/alarmtimer.c b/kernel/time/alarmtimer.c
index 8d9f13d847f0..895e3a6d6444 100644
--- a/kernel/time/alarmtimer.c
+++ b/kernel/time/alarmtimer.c
@@ -290,6 +290,19 @@ static int alarmtimer_suspend(struct device *dev)
 	rtc_timer_cancel(rtc, &rtctimer);
 	rtc_read_time(rtc, &tm);
 	now = rtc_tm_to_ktime(tm);
+
+	/*
+	 * If the RTC alarm timer only supports a limited time offset, set
+	 * the alarm time to the maximum supported value.
+	 * The system will wake up earlier than necessary and is expected
+	 * to go back to sleep if it has nothing to do.
+	 * It would be desirable to handle such early wakeups without fully
+	 * waking up the system, but it is unknown if this is even possible.
+	 */
+	if (rtc->alarm_offset_max &&
+	    rtc->alarm_offset_max * MSEC_PER_SEC < ktime_to_ms(min))
+		min = ms_to_ktime(rtc->alarm_offset_max * MSEC_PER_SEC);
+
 	now = ktime_add(now, min);
 
 	/* Set alarm, if in the past reject suspend briefly to handle */
-- 
2.39.2