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Date:   Tue, 11 Jul 2023 09:27:14 +0200
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Subject: Re: [PATCH] rtc: pcf-8563: Report previously detected low-voltage via
 RTC_VL_BACKUP_LOW
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From:   Jan Kiszka <jan.kiszka@siemens.com>
To:     Alexandre Belloni <alexandre.belloni@bootlin.com>
Cc:     Alessandro Zummo <a.zummo@towertech.it>, linux-rtc@vger.kernel.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On 12.06.23 08:49, Jan Kiszka wrote:
> On 12.06.23 00:16, Alexandre Belloni wrote:
>> On 11/06/2023 18:28:22+0200, Jan Kiszka wrote:
>>> On 11.06.23 17:11, Alexandre Belloni wrote:
>>>> On 11/06/2023 15:38:04+0200, Jan Kiszka wrote:
>>>>> On 10.06.23 10:31, Alexandre Belloni wrote:
>>>>>> Hello Jan,
>>>>>>
>>>>>> On 09/06/2023 23:04:12+0200, Jan Kiszka wrote:
>>>>>>> From: Jan Kiszka <jan.kiszka@siemens.com>
>>>>>>>
>>>>>>> The VL bit in the seconds register remains set only until seconds are
>>>>>>> written under main power. As this often happens during boot-up after
>>>>>>> picking up a network time, make sure to preserve the low battery state
>>>>>>> across this, caching it and returning it via the RTC_VL_BACKUP_LOW bit.
>>>>>>>
>>>>>>> To permit userspace clearing this state during runtime, also implement
>>>>>>> RTC_VL_CLR that works against the cached state.
>>>>>>>
>>>>>>> This is emulating RTCs which have a battery voltage check that works
>>>>>>> under main power as well.
>>>>>>>
>>>>>>
>>>>>> Emulating doesn't work well and I deliberately chose to not implement
>>>>>> it. For example, in your scenario, if you boot twice without using
>>>>>> VL_READ, you anyway have lost the information. This makes emulating
>>>>>> unreliabl. The fix you need is in userspace where you have to ensure you
>>>>>> read the status before setting the time.
>>>>>
>>>>> Then let's make sure the bit is also set in the hardware register. Then
>>>>> also the reboot issue (which is practically a minor one) is solved. The
>>>>> current situation is far from optimal.
>>>>
>>>> This doesn't work because then the time will be considered invalid. I'm
>>>> not sure why you don't want to fix your userspace.
>>>>
>>>
>>> Nope, that could be easily avoided in software. The actual problem is
>>> that the VL bit is not settable (clear-on-write). And that means we
>>> can't do anything about losing the low battery information across
>>> reboots - but that's no difference to the situation with the existing
>>> driver.
>>>
>>> There is no "fix" for userspace as there is no standard framework to
>>> read-out the status early and retrieve it from there when the user asks
>>> for it. That's best done in the kernel.
>>
>> That's not true, nothing prevents userspace from reading the battery
>> status before setting the time and destroying the information which is
>> exactly what you should be doing.
> 
> What is your "userspace"? Mine is stock Debian with systemd and
> timesyncd enabled. But there is no framework to read the status early
> enough and propagate that after timesyncd did its job. Any concrete
> suggestion to "fix" userspace?
> 

Ping - I still have seen no suggestion to improve this situation otherwise.

>>
>>>
>>> In that light, I still believe my patch is an improvement over the
>>> current situation without making anything worse.
>>
>> The information goes from behaving deterministically to being unreliable
>> which makes the situation worse.
> 
> Nope, not at all. You already lose the VL bit today during reboot when
> you have written a new value (which is standard). So this here is not
> making things worse. It's rather improving the situation for the first
> boot at least. Deterministically.
> 
> Jan
> 

Thanks,
Jan

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