linux-kernel - Re: [PATCH 1/7] rtc: Add support for limited alarm timer offsets

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Date:   Wed, 16 Aug 2023 12:12:56 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Alexandre Belloni <alexandre.belloni@...tlin.com>
Cc:     Alessandro Zummo <a.zummo@...ertech.it>,
        Benson Leung <bleung@...omium.org>,
        Miquel Raynal <miquel.raynal@...tlin.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        John Stultz <jstultz@...gle.com>,
        Stephen Boyd <sboyd@...nel.org>, linux-rtc@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Brian Norris <briannorris@...omium.org>
Subject: Re: [PATCH 1/7] rtc: Add support for limited alarm timer offsets

On Wed, Aug 16, 2023 at 06:19:05PM +0200, Alexandre Belloni wrote:
> On 16/08/2023 08:24:39-0700, Guenter Roeck wrote:
> > On Wed, Aug 16, 2023 at 04:57:30PM +0200, Alexandre Belloni wrote:
> > > On 16/08/2023 06:39:30-0700, Guenter Roeck wrote:
> > > > Some alarm timers are based on time offsets, not on absolute times.
> > > > In some situations, the amount of time that can be scheduled in the
> > > > future is limited. This may result in a refusal to suspend the system,
> > > > causing substantial battery drain.
> > > > 
> > > > Some RTC alarm drivers remedy the situation by setting the alarm time
> > > > to the maximum supported time if a request for an out-of-range timeout
> > > > is made. This is not really desirable since it may result in unexpected
> > > > early wakeups.
> > > > 
> > > > To reduce the impact of this problem, let RTC drivers report the maximum
> > > > supported alarm timer offset. The code setting alarm timers can then
> > > > decide if it wants to reject setting alarm timers to a larger value, if it
> > > > wants to implement recurring alarms until the actually requested alarm
> > > > time is met, or if it wants to accept the limited alarm time.
> > > > 
> > > > Only introduce the necessary variable into struct rtc_device.
> > > > Code to set and use the variable will follow with subsequent patches.
> > > > 
> > > > Cc: Brian Norris <briannorris@...omium.org>
> > > > Signed-off-by: Guenter Roeck <linux@...ck-us.net>
> > > > ---
> > > >  include/linux/rtc.h | 1 +
> > > >  1 file changed, 1 insertion(+)
> > > > 
> > > > diff --git a/include/linux/rtc.h b/include/linux/rtc.h
> > > > index 1fd9c6a21ebe..b6d000ab1e5e 100644
> > > > --- a/include/linux/rtc.h
> > > > +++ b/include/linux/rtc.h
> > > > @@ -146,6 +146,7 @@ struct rtc_device {
> > > >  
> > > >  	time64_t range_min;
> > > >  	timeu64_t range_max;
> > > > +	timeu64_t range_max_offset;
> > > 
> > > While range_min and range_max are for the wall clock time, I would
> > > prefer using a name that would clearly mark this as an alarm related
> > > variable.
> > 
> > Sure, no problem. Do you have a suggestion ? alarm_range_max or
> > alarm_range_max_offset, maybe ? I'd also be happy to use some other
> > term for 'offset' if you have a suggestion.
> 
> I don't really know, what about alarm_offset_max? This is the maximum
> value of the offset for the alarm to the current time.
> 
Sounds good to me.

Thanks,
Guenter

> 
> -- 
> Alexandre Belloni, co-owner and COO, Bootlin
> Embedded Linux and Kernel engineering
> https://bootlin.com