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SubjectRe: [patch 4/8] rtc: core: Make the sync offset default more realistic
Hi,

On 06/12/2020 22:46:17+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@linutronix.de>
> ---
> drivers/rtc/class.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> --- a/drivers/rtc/class.c
> +++ 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;

I did retest, on a slow 100kHz i2c bus, with a fairly inconvenient RTC,
The maximum offset to set the RTC was 4845533ns so I'd say 10ms is too
large. Should we make that 5ms ?

Apart from that, on the series, you can add my
Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

--
Alexandre Belloni, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com

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