[tip: timers/core] rtc: cmos: Make rtc_cmos sync offset correct

["tip-bot2 for Thomas Gleixner" <tip-bot2@linutronix.de>]

The following commit has been merged into the timers/core branch of tip:

Commit-ID:     b0ecd8e8c5ef376777277c4c2db7de92ac59f23f
Gitweb:        https://git.kernel.org/tip/b0ecd8e8c5ef376777277c4c2db7de92ac59f23f
Author:        Thomas Gleixner <tglx@linutronix.de>
AuthorDate:    Sun, 06 Dec 2020 22:46:16 +01:00
Committer:     Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Fri, 11 Dec 2020 10:40:52 +01:00

rtc: cmos: Make rtc_cmos sync offset correct

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@linutronix.de>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20201206220541.830517160@linutronix.de

---
 drivers/rtc/rtc-cmos.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/rtc/rtc-cmos.c b/drivers/rtc/rtc-cmos.c
index c633319..7728fac 100644
--- a/drivers/rtc/rtc-cmos.c
+++ b/drivers/rtc/rtc-cmos.c
@@ -868,6 +868,9 @@ cmos_do_probe(struct device *dev, struct resource *ports, int rtc_irq)
 	if (retval)
 		goto cleanup2;
 
+	/* Set the sync offset for the periodic 11min update correct */
+	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;
 	if (rtc_nvmem_register(cmos_rtc.rtc, &nvmem_cfg))