linux-kernel - Re: [PATCH 00/51] rtc: stop using rtc deprecated functions

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Message-ID: <20170621083907.y3gadsmsoufa5niv@piout.net>
Date:   Wed, 21 Jun 2017 10:39:07 +0200
From:   Alexandre Belloni <alexandre.belloni@...e-electrons.com>
To:     Pavel Machek <pavel@....cz>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Russell King - ARM Linux <linux@...linux.org.uk>,
        Benjamin Gaignard <benjamin.gaignard@...aro.org>,
        Baruch Siach <baruch@...s.co.il>,
        "patches@...nsource.wolfsonmicro.com" 
        <patches@...nsource.wolfsonmicro.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        "linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
        Thierry Reding <thierry.reding@...il.com>,
        "x86@...nel.org" <x86@...nel.org>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Chen-Yu Tsai <wens@...e.org>, Ingo Molnar <mingo@...hat.com>,
        Sylvain Lemieux <slemieux.tyco@...il.com>,
        Sebastian Hesselbarth <sebastian.hesselbarth@...il.com>,
        Len Brown <len.brown@...el.com>,
        "linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
        Jason Cooper <jason@...edaemon.net>,
        "rtc-linux@...glegroups.com" <rtc-linux@...glegroups.com>,
        "linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
        Hans Ulli Kroll <ulli.kroll@...glemail.com>,
        "adi-buildroot-devel@...ts.sourceforge.net" 
        <adi-buildroot-devel@...ts.sourceforge.net>,
        Vladimir Zapolskiy <vz@...ia.com>,
        John Stultz <john.stultz@...aro.org>,
        Gregory Clement <gregory.clement@...e-electrons.com>,
        Michael Chan <michael.chan@...adcom.com>,
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        <linux-arm-kernel@...ts.infradead.org>,
        Alessandro Zummo <a.zummo@...ertech.it>,
        Barry Song <baohua@...nel.org>,
        Support Opensource <Support.Opensource@...semi.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Steve Twiss <stwiss.opensource@...semi.com>,
        Maxime Ripard <maxime.ripard@...e-electrons.com>
Subject: Re: [PATCH 00/51] rtc: stop using rtc deprecated functions

On 21/06/2017 at 09:51:52 +0200, Pavel Machek wrote:
> Hi!
> 
> > > I agree with that but not the android guys. They seem to mandate an RTC
> > > that can store time from 01/01/1970. I don't know much more than that
> > > because they never cared to explain why that was actually necessary
> > > (apart from a laconic "this will result in a bad user experience")
> > > 
> > > I think tglx had a plan for offsetting the time at some point so 32-bit
> > > platform can pass 2038 properly.
> > 
> > Yes, but there are still quite some issues to solve there:
> > 
> >      1) How do you tell the system that it should apply the offset in the
> >      	first place, i.e at boot time before NTP or any other mechanism can
> >      	correct it?
> 
> I'd not do offset. Instead, I'd select a threshold (perhaps year of
> release of given kernel?) and
> 
> 	if (rtc_time < year_of_release_of_kernel)
> 	   rtc_time += 0x100000000;
> 
> Ok, we'll have to move away from "rtc_time == 0 indicates zero", as
> seen in some drivers.
> 
> >      2) Deal with creative vendors who have their own idea about the 'start
> >      	of the epoch'
> 
> If someone uses different threshold, well, there will be
> confusion. But only for users that have their rtc set to the past,
> which is quite unusual.
> 

Or not, having an RTC set in the past is actually quite common. I'd find
it weird to have a new device boot and be set to a date in the future.

Also note that the threshold or offset thing may seem like a good idea
but fails with many RTCs because of how they handle leap years.


-- 
Alexandre Belloni, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com