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

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Message-ID: <20201203161622.GA1317829@ziepe.ca>
Date:   Thu, 3 Dec 2020 12:16:22 -0400
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Alexandre Belloni <alexandre.belloni@...tlin.com>,
        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 04:39:21PM +0100, Thomas Gleixner wrote:

> The logic in sync_cmos_clock() and rtc_set_ntp_time() is different as I
> pointed out: sync_cmos_clock() hands -500ms to rtc_tv_nsec_ok() and
> rtc_set_ntp_time() uses +500ms, IOW exactly ONE second difference in
> behaviour.

I understood this is because the two APIs work differently, rmk
explained this as:

> 1. kernel/time/ntp.c assumes that all RTCs want to be told to set the
>    time at around 500ms into the second.
>
> 2. drivers/rtc/systohc.c assumes that if the time being set is >= 500ms,
>    then we want to set the _next_ second.

ie one path is supposed to round down and one path is supposed to
round up, so you get to that 1s difference..

IIRC this is also connected to why the offset is signed..

Jason