linux-kernel - Re: [PATCH] rtc: cros-ec: Limit RTC alarm range if needed

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Date:   Wed, 2 Nov 2022 11:48:04 -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>, linux-rtc@...r.kernel.org,
        chrome-platform@...ts.linux.dev, linux-kernel@...r.kernel.org,
        Brian Norris <briannorris@...omium.org>
Subject: Re: [PATCH] rtc: cros-ec: Limit RTC alarm range if needed

Alexandre,

On Mon, Oct 31, 2022 at 04:07:51PM -0700, Guenter Roeck wrote:
[ ... ]
> > > 
> > > On a side note, I tried an alternate implementation by adding a retry into
> > > alarmtimer_suspend(), where it would request a smaller timeout if the
> > > requested timeout failed. I did not pursue/submit this since it seemed
> > > hacky. To solve that problem, I'd rather discuss extending the RTC API
> > > to provide a maximum offset to its users. Such a solution would probably
> > > be desirable, but that it more longer term and would not solve the
> > > immediate problem.
> > 
> > Yes, this is what I was aiming for. This is something that is indeed
> > missing in the RTC API and that I already thought about. But indeed, it
> > would be great to have a way to set the alarm range separately from the
> > time keeping range. This would indeed have to be a range relative to the
> > current time.
> > 
> > alarmtimer_suspend() can then get the allowed alarm range for the RTC,
> > and set the alarm to max(alarm range, timer value) and loop until the
> > timer has expired. Once we have this API, userspace can do the same.
> > 
> > I guess that ultimately, this doesn't help your driver unless you are
> > wanting to wakeup all the chromebooks at least once a day regardless of
> > their EC.
> 
> That is a no-go. It would reduce battery lifetime on all Chromebooks,
> including those not affected by the problem (that is, almost all of them).
> 
> To implement reporting the maximum supported offset, I'd probably either
> try to identify affected Chromebooks using devicetree information,
> or by sending am alarm request > 24h in the future in the probe function
> and setting the maximum offset just below 24h if that request fails.
> We'd have to discuss the best approach internally.
> 
> Either case, that doesn't help with the short term problem that we
> have to solve now and that can be backported to older kernels. It also
> won't help userspace - userspace alarm requests, as Brian has pointed out,
> are separate from limits supported by the RTC hardware. We can not change
> the API for CLOCK_xxx_ALARM to userspace, and doing so would not make
> sense anyway since it works just fine as long as the system isn't
> suspended. Besides, changing alarmtimer_suspend() as you suggest above
> would solve the problem for userspace, so I don't see a need for a
> userspace API/ABI change unless I am missing something.
>

Would you be open to accepting this patch, with me starting to work
on the necessary infastructure changes as suggested above for a more
comprehensive solution ?

Thanks,
Guenter