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

[Guenter Roeck]

On Thu, Aug 24, 2023 at 08:52:44PM -0700, John Stultz wrote:
> On Thu, Aug 17, 2023 at 3:55 PM Guenter Roeck <linux@xxxxxxxxxxxx> wrote:
> >
> > 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@xxxxxxxxxxxx>
> > Signed-off-by: Guenter Roeck <linux@xxxxxxxxxxxx>
> > ---
> > 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);
> 
> I don't really have an objection here, but I wonder if this would be
> better abstracted by a rtc_ function?
> 
> ktime_t rtc_bound_ktime_interval(ktime interval)

Probably more like like rtc_bound_alarm_interval(),
but fine with me.

> {
>   if (!rtc->alarm_offset_max)
>       return interval;
>   return ms_to_ktime(min(rtc->alarm_offset_max, ktime_to_ms(interval)));

alarm_offset_max is in seconds, so that would need some tweaking.

Guenter

> }
> 
> (simple enough to throw into rtc.h maybe as an inline function?)
> 
> Then the above would be tweaked to:
> min = rtc_bound_interval(min);
> 
> thanks
> -john