Messages in this thread Patch in this message | | | Date | Fri, 11 Dec 2020 10:07:48 -0000 | From | "tip-bot2 for Thomas Gleixner" <> | Subject | [tip: timers/core] rtc: core: Make the sync offset default more realistic |
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The following commit has been merged into the timers/core branch of tip:
Commit-ID: 354c796b9270eb4780e59e3bdb83a3ae4930a832 Gitweb: https://git.kernel.org/tip/354c796b9270eb4780e59e3bdb83a3ae4930a832 Author: Thomas Gleixner <tglx@linutronix.de> AuthorDate: Sun, 06 Dec 2020 22:46:17 +01:00 Committer: Thomas Gleixner <tglx@linutronix.de> 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 <tglx@linutronix.de> Reviewed-by: Jason Gunthorpe <jgg@nvidia.com> Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com> Link: https://lore.kernel.org/r/20201206220541.960333166@linutronix.de
--- 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) 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 = 5 * NSEC_PER_MSEC; rtc->irq_freq = 1; rtc->max_user_freq = 64;
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