ntp: Make the RTC sync offset less obscure

The current RTC set_offset_nsec value is not really intuitive to
understand. 

  tsched       twrite(t2.tv_sec - 1) 	 t2 (seconds increment)

The offset is calculated from twrite based on the assumption that t2 -
twrite == 1s. That means for the MC146818 RTC the offset needs to be
negative so that the write happens 500ms before t2.

It's easier to understand when the whole calculation is based on t2. That
avoids negative offsets and the meaning is obvious:

 t2 - twrite:     The time defined by the chip when seconds increment
      		  after the write.

 twrite - tsched: The time for the transport to the point where the chip
 	  	  is updated. 

==> set_offset_nsec =  t2 - tsched
    ttransport      =  twrite - tsched
    tRTCinc         =  t2 - twrite
==> set_offset_nsec =  ttransport + tRTCinc

tRTCinc is a chip property and can be obtained from the data sheet.

ttransport depends on how the RTC is connected. It is close to 0 for
directly accessible RTCs. For RTCs behind a slow bus, e.g. i2c, it's the
time required to send the update over the bus. This can be estimated or
even calibrated, but that's a different problem.

Adjust the implementation and update comments accordingly.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20201206220542.263204937@linutronix.de

1 files changed, 1 insertions, 1 deletions

diff --git a/drivers/rtc/rtc-cmos.c b/drivers/rtc/rtc-cmos.c
index 7728faca01b47..c5bcd2adc9fe7 100644
--- a/drivers/rtc/rtc-cmos.c
+++ b/drivers/rtc/rtc-cmos.c
@@ -869,7 +869,7 @@ cmos_do_probe(struct device *dev, struct resource *ports, int rtc_irq)
 		goto cleanup2;
 
 	/* Set the sync offset for the periodic 11min update correct */
-	cmos_rtc.rtc->set_offset_nsec = -(NSEC_PER_SEC / 2);
+	cmos_rtc.rtc->set_offset_nsec = NSEC_PER_SEC / 2;
 
 	/* export at least the first block of NVRAM */
 	nvmem_cfg.size = address_space - NVRAM_OFFSET;