linux-kernel - Re: [PATCH] rtc: adapt allowed RTC update error

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Date:   Thu, 3 Dec 2020 16:10:30 -0400
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Alexandre Belloni <alexandre.belloni@...tlin.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Miroslav Lichvar <mlichvar@...hat.com>,
        linux-kernel@...r.kernel.org, John Stultz <john.stultz@...aro.org>,
        Prarit Bhargava <prarit@...hat.com>,
        Alessandro Zummo <a.zummo@...ertech.it>,
        linux-rtc@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] rtc: adapt allowed RTC update error

On Thu, Dec 03, 2020 at 05:07:53PM +0100, Alexandre Belloni wrote:
> On 03/12/2020 11:52:49-0400, Jason Gunthorpe wrote:
> > On Thu, Dec 03, 2020 at 03:10:47AM +0100, Alexandre Belloni wrote:
> > 
> > > IIRC, used in conjunction with rtc_hctosys which also adds
> > > inconditionnaly 500ms this can ends up with the system time
> > > being one second away from the wall clock time which NTP will take quite
> > > some time to remove.
> > 
> > I can't remember the details, but this was not the intention. 
> > 
> > As long as systohc and hctosys exist then the design goal of rtclib
> > should be to provide sub-second accuracy on the round trip of time
> > through the RTC.
> > 
> > Otherwise what is the point?
> 
> hctosys never had a sub second accuracy because there is no way to
> accurately read the rtc time without being ready to wait up to a
> second.

Yes, I know. I was talking about a goal..

If that is not the goal then just delete it all and update the docs
that userspace needs to deal with all of this and the kernel stuff
isn't supposed to be useful.

Jason