On Sun, Dec 06, 2020 at 10:46:16PM +0100, Thomas Gleixner wrote: > The offset for rtc_cmos must be -500ms to work correctly with the current > implementation of rtc_set_ntp_time() due to the following: > > tsched twrite(t2.tv_sec - 1) t2 (seconds increment) > > twrite - tsched is the transport time for the write to hit the device, > which is negligible for this chip because it's accessed directly. > > t2 - twrite = 500ms according to the datasheet. > > But rtc_set_ntp_time() calculation of tsched is: > > tsched = t2 - 1sec - (t2 - twrite) > > The default for the sync offset is 500ms which means that the write happens > at t2 - 1.5 seconds which is obviously off by a second for this device. > > Make the offset -500ms so it works correct. > > Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx> > --- > drivers/rtc/rtc-cmos.c | 3 +++ > 1 file changed, 3 insertions(+) Reviewed-by: Jason Gunthorpe <jgg@xxxxxxxxxx> Matches what sync_cmos_clock() does at least. My recollection is this change was not supposed to change anything, so either I got things mixed up and this is wrong: drivers/rtc/class.c: rtc->set_offset_nsec = NSEC_PER_SEC / 2; Or it faithfully preserved some nonsense from before.. Jason
