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    SubjectRe: [PATCH] rtc: Fix set RTC time delay 500ms on some Zhaoxin SOCs
    On 17/08/2021 19:09:28+0800, tonywwang-oc@zhaoxin.com wrote:
    >
    >
    > On August 16, 2021 8:36:48 PM GMT+08:00, Alexandre Belloni <alexandre.belloni@bootlin.com> wrote:
    > >On 16/08/2021 18:03:13+0800, Tony W Wang-oc wrote:
    > >>
    > >> On 16/08/2021 16:24, Alexandre Belloni wrote:
    > >> > Hello,
    > >> >
    > >> > On 16/08/2021 21:47:18+0800, Tony W Wang-oc wrote:
    > >> >> When the RTC divider is changed from reset to an operating time
    > >base,
    > >> >> the first update cycle should be 500ms later. But on some Zhaoxin
    > >SOCs,
    > >> >> this first update cycle is one second later.
    > >> >>
    > >> >> So set RTC time on these Zhaoxin SOCs will causing 500ms delay.
    > >> >>
    > >> >
    > >> > Can you explain what is the relationship between writing the
    > >divider and
    > >> > the 500ms delay?
    > >> >> Isn't the issue that you are using systohc and set_offset_nsec is
    > >set to
    > >> > NSEC_PER_SEC / 2 ?
    > >> >
    > >> No.
    > >> When using #hwclock -s to set RTC time and set_offset_nsec is
    > >> NSEC_PER_SEC / 2, the function mc146818_set_time() requires the first
    > >> update cycle after RTC divider be changed from reset to an operating
    > >> mode is 500ms as the MC146818A spec specified. But on some Zhaoxin
    > >SOCs,
    > >> the first update cycle of RTC is one second later after RTC divider
    > >be
    > >> changed from reset to an operating mode. So the first update cycle
    > >after
    > >> RTC divider be changed from reset to an operation mode on These SOCs
    > >> will causing 500ms delay with current mc146818_set_time()
    > >implementation.
    > >>
    > >
    > >What happens with hwclock --delay=0 -s ?
    >
    > With "hwclock --delay=0 -s" still have this problem. Actually, this 500ms delay caused by writing the RTC time on these Zhaoxin SOCs.
    > As I've tested, with hwclock --delay=0 -w can fix it too.
    >

    Both -s and -w end up calling set_hardware_clock_exact() so both should
    end up with the correct time. If this is not the case, then hwclock
    needs to be fixed.


    --
    Alexandre Belloni, co-owner and COO, Bootlin
    Embedded Linux and Kernel engineering
    https://bootlin.com

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