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Date:   Wed, 2 Dec 2020 22:04:27 -0400
From:   Jason Gunthorpe <jgg@ziepe.ca>
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     Miroslav Lichvar <mlichvar@redhat.com>,
        linux-kernel@vger.kernel.org, John Stultz <john.stultz@linaro.org>,
        Prarit Bhargava <prarit@redhat.com>,
        Alessandro Zummo <a.zummo@towertech.it>,
        Alexandre Belloni <alexandre.belloni@bootlin.com>,
        linux-rtc@vger.kernel.org, Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH] rtc: adapt allowed RTC update error
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On Thu, Dec 03, 2020 at 02:14:12AM +0100, Thomas Gleixner wrote:

> If anyone involved seriously believes that any of this solves a real
> world problem, then please come forth an make your case.

The original commit 0f295b0650c9 ("rtc: Allow rtc drivers to specify
the tv_nsec value for ntp") was tested by myself and RMK on various
ARM systems and did work as advertised. Here is the giant thread,
RMK's post explains the problem and gives his measurements of several
different RTCs:

https://lore.kernel.org/linux-arm-kernel/20170920112152.GL20805@n2100.armlinux.org.uk/

And the patch that resulted:

https://lore.kernel.org/linux-arm-kernel/20171013175433.GA22062@obsidianresearch.com/

There is a lot of detail in there.. Keep in mind none of this was for
the mc146818 style RTCs.

I can't recall any more why no drivers use the set_offset_nsec. I'm
surprised, maybe I forgot to send the patch for the RTCs I tested or
maybe it got dropped someplace.. It certainly was needed for some
maxim I2C chips.

The thread shows rmk had even written a hrtimer patch to go with this,
but it also got lost for some reason. Maybe all the arguing killed
further effort?

Jason