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Date:   Tue, 5 May 2020 22:07:44 +0200
From:   Alexandre Belloni <alexandre.belloni@bootlin.com>
To:     Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Cc:     Bruno Thomsen <bruno.thomsen@gmail.com>,
        Per =?iso-8859-1?Q?N=F8rgaard?= Christensen 
        <per.christensen@prevas.dk>, LKML <linux-kernel@vger.kernel.org>
Subject: Re: battery switch-over detection on pcf2127
Message-ID: <20200505200744.GV34497@piout.net>
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On 05/05/2020 21:54:47+0200, Rasmus Villemoes wrote:
> Hi Bruno
> 
> I just noticed your "rtc: pcf2127: add tamper detection support"
> (03623b4b04) from 5.4. Unfortunately, clearing the BTSE bit breaks a use
> case of ours:
> 
> We rely on the battery switch-over detection to distinguish a powerfail
> during boot from a PORESET by the external watchdog (in the latter case,
> the RTC is still powered throughout, meaning there is no battery
> switch-over event). OTOH, we do not use the tamper detection - in fact,
> the TS signal is unconnected on our board.
> 
> We're currently still on 4.19, but we will eventually upgrade to a
> kernel containing the above commit. So I was wondering if we could
> figure out a way that would work for both of us - either some CONFIG
> knob, or perhaps something in the device-tree. Any ideas?
> 

Yes, I was working on a patch series last week allowing to read BF. I'm
not sure clearing BTSE is your issue but clearing BF is.

I'm going to send it tonight, I'll copy you, let me now if that works
for you. You can then read BF using the RTC_VL_READ ioctl. The
RTC_VL_BACKUP_SWITCH flag will be set if a switchover happened.
The RTC_VL_CLR ioctl can be used to clear the flag.

I think clearing BTSE is still the right thing to do.

Regards,

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