linux-kernel - Re: [PATCH] rtc: Add support for century bits to m41t62 (rv4162) RTC devices

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Message-ID: <20191003134329.GB575@piout.net>
Date:   Thu, 3 Oct 2019 15:43:29 +0200
From:   Alexandre Belloni <alexandre.belloni@...tlin.com>
To:     Lukasz Majewski <lukma@...x.de>
Cc:     Alessandro Zummo <a.zummo@...ertech.it>, linux-rtc@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rtc: Add support for century bits to m41t62 (rv4162) RTC
 devices

On 03/10/2019 15:14:34+0200, Lukasz Majewski wrote:
> Hi Alexandre,
> 
> > On 03/10/2019 14:21:50+0200, Lukasz Majewski wrote:
> > > > 
> > > > See the datasheet:
> > > > 
> > > > "During any year which is a multiple of 4, the RV-4162 RTC will
> > > > automatically insert leap day, February 29.  Therefore, the
> > > > application software must correct for this during the exception
> > > > years (2100, 2200, etc.) as noted above."  
> > > 
> > > I'm wondering what the phrase "application software" means here?
> > > 
> > > If it is the userland SW, then we shall at least be able to set
> > > 2099 in this device and then count on software correction.
> > > 
> > > If the "application software" is the kernel driver - the date
> > > correction shall be done there (maybe some lookup table?).
> > > 
> > > Personally, I do prefer the first option - this means that with this
> > > patch we can set the time to e.g. 2234 year and then rely on
> > > userland software (or libc) to do the correction.
> > >   
> > 
> > It is not possible to ensure this correction is properly done in
> > software, there is no point in letting the user set those bits.
> > 
> > 
> 
> I see your point.
> 
> However, could you share your idea on testing setting RTC time to year
> 2100 on this particular IC (by using hctosys and friends)?
> 

You can use rtc from
https://git.kernel.org/pub/scm/linux/kernel/git/abelloni/rtc-tools.git/

You can also use rtc-range with your patch to observe that it fails in
2100.

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