Messages in this thread Patch in this message | | | Date | Sun, 06 Dec 2020 22:46:17 +0100 | From | Thomas Gleixner <> | Subject | [patch 4/8] 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 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@linutronix.de> --- drivers/rtc/class.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
--- a/drivers/rtc/class.c +++ 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; rtc->irq_freq = 1; rtc->max_user_freq = 64;
| |