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Date:   Tue, 26 Oct 2021 19:42:12 +0800
From:   <tonywwang-oc@zhaoxin.com>
To:     Alexandre Belloni <alexandre.belloni@bootlin.com>
CC:     <a.zummo@towertech.it>, <linux-rtc@vger.kernel.org>,
        <linux-kernel@vger.kernel.org>, <TimGuo-oc@zhaoxin.com>,
        <CooperYan@zhaoxin.com>, <QiyuanWang@zhaoxin.com>,
        <HerryYang@zhaoxin.com>, <CobeChen@zhaoxin.com>,
        <YanchenSun@zhaoxin.com>
Subject: Re: [PATCH] rtc: Fix set RTC time delay 500ms on some Zhaoxin SOCs
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On August 18, 2021 11:54:20 AM GMT+08:00, tonywwang-oc@zhaoxin.com wrote:
>
>
>On August 17, 2021 9:21:03 PM GMT+08:00, Alexandre Belloni
><alexandre.belloni@bootlin.com> wrote:
>>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.
>
>I checked Util-linux-2.37.2, hwclock -w will call
>set_hardware_clock_exact() and hwclock -s will not.
>Please correct me if I'm wrong.
>
>Sincerely
>TonyWWang-oc

As explained before, the root cause of this problem is: these Zhaoxin SOCs

which belong to X86 architecture do not meet the requirement of

MC146818A compatible RTC about “When the divider is changed from reset

to an operating time base, the first update cycle is one-half second later”.

Actually the first update cycle on these Zhaoxin SOCs is one second later in

this case.

 
This problem is not only happened when running “hwclock -w”. On X86 platform,

the 0.5s delay is default for both “hwclock –w” and NTP driver’s invoke of

sync_cmos_clock().  So set RTC time caused by NTP driver also has this problem.

 
As have been test pass, skip operate the RTC_REG_A (which divider-control bits in)

with these Zhaoxin SOCs in function mc146818_set_time() can  fix this problem.

I think this patch seems appropriate.

Sincerely
TonyWWang-oc