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

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Message-ID: <9ac4b2a5-7cc8-4fce-7ea0-61b26d6ef223@siemens.com>
Date:   Sun, 11 Jun 2023 18:28:22 +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 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@...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.
> 
> 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.

In that light, I still believe my patch is an improvement over the
current situation without making anything worse.

Jan

-- 
Siemens AG, Technology
Competence Center Embedded Linux