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

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Date:   Thu, 3 Oct 2019 16:10:53 +0200
From:   Lukasz Majewski <lukma@...x.de>
To:     Alexandre Belloni <alexandre.belloni@...tlin.com>
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

Hi Alexandre,

> 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.

Sorry, but I do see some inconsistency here.

The application note [1] says that the correction shall be done in
application SW.

The rtc-range.c program [2] sets and reads the time via ioctl (e.g.
RTC_SET_TIME, RTC_RD_TIME).

To pass your tests one needs to do the correction in linux kernel
driver for drivers/rtc/rtc-m41t80.c. 

Please correct me if I'm wrong, but IMHO it shall be enough to adjust
2100, 2200, 2300, years in this driver (the submitted patch shall be
adjusted to support it - I can prepare proper v2).

> > > 
> > >   
> > 
> > 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.

Thanks for sharing useful links.

> 


Note:

[1] -
https://www.microcrystal.com/fileadmin/Media/Products/RTC/App.Manual/RV-4162-C7_App-Manual.pdf
(point 4.5 and 4.6).

[2] -
https://git.kernel.org/pub/scm/linux/kernel/git/abelloni/rtc-tools.git/tree/rtc-range.c

Best regards,

Lukasz Majewski

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