From: Alexandre Belloni <alexandre.belloni@bootlin.com>
To: Lukasz Majewski <lukma@denx.de>
Cc: Alessandro Zummo <a.zummo@towertech.it>,
linux-rtc@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] rtc: Add support for century bits to m41t62 (rv4162) RTC devices
Date: Thu, 3 Oct 2019 23:34:52 +0200 [thread overview]
Message-ID: <20191003213452.GT4106@piout.net> (raw)
In-Reply-To: <20191003164906.2f4a1676@jawa>
On 03/10/2019 16:49:06+0200, Lukasz Majewski wrote:
> Hi Alexandre,
> I'm rather thinking about following use cases:
>
> I. Adjusting time:
>
> 1. I start with time < 01.01.2099
>
> 2. I issue ioctl to set the time to e.g. 2100
>
> - When driver receives such request I setup century bits
>
> - and also perform in kernel driver time correction (and store
> corrected time in RTC)
>
> 3. Subsequent reads from rtc use century bits to provide the time
> (after year 2100). Century bits are set, so the correction may be
> performed if needed.
>
>
> II. The system is started at year 2098 and is supposed to run for e.g. 3
> years:
>
> 1. The time is read from the rtc - the "passing" of centuries need to
> be detected.
>
> From the documentation [1] (point 4.5):
>
> "The two century bits, CB1 and CB0, are bits 7 and 6, respectively, in
> the Month / Century register at address 06h. Together, they comprise a
> 2 - bit counter which increments at the turn of each century. CB1 is
> the most significant bit."
>
> If those bits increment when we pass century boundaries, we can detect
> this fact and correct time when ioctl is issued.
>
No, you can't because you simply don't know if you still need to
correct the time or if you already did it the last time the system was
started.
Example:
Date is set to 2100-02-28, some time pass, the rtc now thinks it is
2100-02-29. You correct it to 2100-03-01, fine.
Now, date is set to 2100-02-28, the system is shutdown, some time pass,
it starts on 2100-03-02, the rtc thinks 2100-03-01 you can't correct it
because you can't know whether a day has been missed.
> > The only useful range for an RTC is its fully contiguous range.
>
> Does the automatic increment of century bits count to "contiguous
> range" ?
>
No, because of the leap day issue.
--
Alexandre Belloni, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com
prev parent reply other threads:[~2019-10-03 21:34 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-09-11 15:48 [PATCH] rtc: Add support for century bits to m41t62 (rv4162) RTC devices Lukasz Majewski
2019-09-30 7:56 ` Lukasz Majewski
2019-10-03 11:48 ` Alexandre Belloni
2019-10-03 12:21 ` Lukasz Majewski
2019-10-03 12:35 ` Alexandre Belloni
2019-10-03 13:14 ` Lukasz Majewski
2019-10-03 13:43 ` Alexandre Belloni
2019-10-03 14:10 ` Lukasz Majewski
2019-10-03 14:23 ` Alexandre Belloni
2019-10-03 14:49 ` Lukasz Majewski
2019-10-03 21:34 ` Alexandre Belloni [this message]
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