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Message-ID: <20201207205532.GJ5487@ziepe.ca>
Date: Mon, 7 Dec 2020 16:55:32 -0400
From: Jason Gunthorpe <jgg@...pe.ca>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: LKML <linux-kernel@...r.kernel.org>,
Alexandre Belloni <alexandre.belloni@...tlin.com>,
Miroslav Lichvar <mlichvar@...hat.com>,
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 4/8] rtc: core: Make the sync offset default more
realistic
On Sun, Dec 06, 2020 at 10:46:17PM +0100, Thomas Gleixner wrote:
> The offset which is used to steer the start of an RTC synchronization
> update via rtc_set_ntp_time() is huge. The math behind this is:
>
> tsched twrite(t2.tv_sec - 1) t2 (seconds increment)
>
> twrite - tsched is the transport time for the write to hit the device.
>
> t2 - twrite depends on the chip and is for most chips one second.
>
> The rtc_set_ntp_time() calculation of tsched is:
>
> tsched = t2 - 1sec - (t2 - twrite)
>
> The default for the sync offset is 500ms which means that twrite - tsched
> is 500ms assumed that t2 - twrite is one second.
>
> This is 0.5 seconds off for RTCs which are directly accessible by IO writes
> and probably for the majority of i2C/SPI based RTC off by an order of
> magnitude. Set it to 10ms which should bring it closer to reality.
>
> The default can be adjusted by drivers (rtc_cmos does so) and could be
> adjusted further by a calibration method which is an orthogonal problem.
>
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> drivers/rtc/class.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> +++ b/drivers/rtc/class.c
> @@ -201,7 +201,7 @@ static struct rtc_device *rtc_allocate_d
> device_initialize(&rtc->dev);
>
> /* Drivers can revise this default after allocating the device. */
> - rtc->set_offset_nsec = NSEC_PER_SEC / 2;
> + rtc->set_offset_nsec = 10 * NSEC_PER_MSEC;
So the old value is clearly wrong for CMOS, and I have a strong
feeling this was an error and it should have been -NSEC_PER_SEC/2
I have no idea if CMOS behavior or 0s behavior is more common in the
rtclib drivers, but it seems since nobody noticed the huge offset
mistake in 3 years it doesn't actually really matter.
Reviewed-by: Jason Gunthorpe <jgg@...dia.com>
Jason
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