linux-kernel - Re: [PATCH] rtc: Fix set RTC time delay 500ms on some Zhaoxin SOCs

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Date:   Wed, 18 Aug 2021 11:54:20 +0800
From:   <tonywwang-oc@...oxin.com>
To:     Alexandre Belloni <alexandre.belloni@...tlin.com>
CC:     <a.zummo@...ertech.it>, <linux-rtc@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <TimGuo-oc@...oxin.com>,
        <CooperYan@...oxin.com>, <QiyuanWang@...oxin.com>,
        <HerryYang@...oxin.com>, <CobeChen@...oxin.com>,
        <YanchenSun@...oxin.com>
Subject: Re: [PATCH] rtc: Fix set RTC time delay 500ms on some Zhaoxin SOCs



On August 17, 2021 9:21:03 PM GMT+08:00, Alexandre Belloni <alexandre.belloni@...tlin.com> wrote:
>On 17/08/2021 19:09:28+0800, tonywwang-oc@...oxin.com wrote:
>> 
>> 
>> On August 16, 2021 8:36:48 PM GMT+08:00, Alexandre Belloni
><alexandre.belloni@...tlin.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