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

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Date:   Tue, 17 Aug 2021 15:21:03 +0200
From:   Alexandre Belloni <alexandre.belloni@...tlin.com>
To:     tonywwang-oc@...oxin.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 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.


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