linux-kernel - Re: [PATCH v2 1/7] rtc: Add support for limited alarm timer offsets

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Message-ID: <20230823225129d2bc7dcd@mail.local>
Date:   Thu, 24 Aug 2023 00:51:29 +0200
From:   Alexandre Belloni <alexandre.belloni@...tlin.com>
To:     Guenter Roeck <linux@...ck-us.net>
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 v2 1/7] rtc: Add support for limited alarm timer offsets

Hello,

On 23/08/2023 09:50:47-0700, Guenter Roeck wrote:
> Hi Alexandre,
> 
> On Thu, Aug 17, 2023 at 03:55:31PM -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>
> 
> I guess it is a bit late to get the series into v6.6, but would it be
> possible to apply it to a -next branch to get some more test coverage ?
> 

I'm probably going to take 1 and 3-7 for 6.6 once I get a reliable
internet access. I can't take 2/7 without a review or ack from the time
maintainers.

> Either case, do you have any additional comments / feedback ?
> 

The main issue that remains is that after 2/7, the rtc_device structure
is not opaque anymore to its user as alarmtimer_suspend now directly
accesses one of the members. But I'd have to find which RTCs have an
absolute limit so we can design a proper API. I may also decide that it
is good enough to require that the alarm range must cover the registered
RTC range.

> Thanks,
> Guenter
> 
> > ---
> > v2: Rename range_max_offset -> alarm_offset_max
> > 
> >  include/linux/rtc.h | 1 +
> >  1 file changed, 1 insertion(+)
> > 
> > diff --git a/include/linux/rtc.h b/include/linux/rtc.h
> > index 1fd9c6a21ebe..4c0bcbeb1f00 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 alarm_offset_max;
> >  	time64_t start_secs;
> >  	time64_t offset_secs;
> >  	bool set_start_time;

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