rtc: core: Make the sync offset default more realistic

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 5ms 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>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20201206220541.960333166@linutronix.de

drivers/rtc/class.c

@@ -201,7 +201,7 @@ static struct rtc_device *rtc_allocate_device(void)
 	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 =  5 * NSEC_PER_MSEC;
 
 	rtc->irq_freq = 1;
 	rtc->max_user_freq = 64;