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

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Message-ID: <87wnxybmqx.fsf@nanos.tec.linutronix.de>
Date:   Thu, 03 Dec 2020 22:31:02 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Jason Gunthorpe <jgg@...pe.ca>
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 22:05, Thomas Gleixner wrote:
> On Thu, Dec 03 2020 at 12:16, Jason Gunthorpe wrote:
> So now we have two options to fix this:
>
>    1) Use a negative sync_offset for devices which need #1 above
>       (rtc_cmos & similar)
>
>       That requires setting tsched to t2 - abs(sync_offset)
>
>    2) Use always a positive sync_offset and a flag which tells
>       rtc_tv_nsec_ok() whether it needs to add or subtract.
>
> #1 is good enough. All it takes is a comment at the timer start code why
> abs() is required.
>
> Let me hack that up along with the hrtimer muck.

That comment in rtc.h makes me cry:

	/* Number of nsec it takes to set the RTC clock. This influences when
	 * the set ops are called. An offset:
	 *   - of 0.5 s will call RTC set for wall clock time 10.0 s at 9.5 s
	 *   - of 1.5 s will call RTC set for wall clock time 10.0 s at 8.5 s
	 *   - of -0.5 s will call RTC set for wall clock time 10.0 s at 10.5 s
	 */

Setting the wall clock time 10.0 at 10.5 is only possible for time
traveling RTCs. It magically works, but come on ...

Thanks,

        tglx