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

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Message-ID: <7EA395FF-EB66-4274-9EDE-EC28450A0259@zhaoxin.com>
Date:   Tue, 17 Aug 2021 19:09:28 +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 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. 

Sincerely
TonyWWang-oc