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