linux-kernel - Re: [PATCH] rtc: pcf-8563: Report previously detected low-voltage via RTC_VL_BACKUP

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Date:   Sun, 11 Jun 2023 15:38:04 +0200
From:   Jan Kiszka <jan.kiszka@...mens.com>
To:     Alexandre Belloni <alexandre.belloni@...tlin.com>
Cc:     Alessandro Zummo <a.zummo@...ertech.it>, linux-rtc@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] rtc: pcf-8563: Report previously detected low-voltage via
 RTC_VL_BACKUP_LOW

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@...mens.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.

Jan

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