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@xxxxxxxxxxxxx> > 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@xxxxxxxxxx> Jason
