From mboxrd@z Thu Jan  1 00:00:00 1970
From: alexandre.belloni@free-electrons.com (Alexandre Belloni)
Date: Wed, 21 Jun 2017 10:39:07 +0200
Subject: [PATCH 00/51] rtc: stop using rtc deprecated functions
In-Reply-To: <20170621075152.GA15996@amd>
References: <20170620213507.urobmtg34vzubrdq@piout.net>
 <alpine.DEB.2.20.1706202343120.2157@nanos>
 <20170621075152.GA15996@amd>
Message-ID: <20170621083907.y3gadsmsoufa5niv@piout.net>
To: linux-arm-kernel@lists.infradead.org
List-Id: linux-arm-kernel.lists.infradead.org

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