[lkml]   [2020]   [Dec]   [11]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
Patch in this message
Subject[tip: timers/core] rtc: core: Make the sync offset default more realistic
The following commit has been merged into the timers/core branch of tip:

Commit-ID: 354c796b9270eb4780e59e3bdb83a3ae4930a832
Author: Thomas Gleixner <>
AuthorDate: Sun, 06 Dec 2020 22:46:17 +01:00
Committer: Thomas Gleixner <>
CommitterDate: Fri, 11 Dec 2020 10:40:52 +01:00

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 <>
Reviewed-by: Jason Gunthorpe <>
Acked-by: Alexandre Belloni <>

drivers/rtc/class.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/rtc/class.c b/drivers/rtc/class.c
index 7c88d19..d795737 100644
--- a/drivers/rtc/class.c
+++ b/drivers/rtc/class.c
@@ -201,7 +201,7 @@ static struct rtc_device *rtc_allocate_device(void)

/* 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;
 \ /
  Last update: 2020-12-11 11:11    [W:0.194 / U:1.680 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site