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Date:   Wed, 6 May 2020 00:14:46 +0200
From:   Alexandre Belloni <>
To:     Rasmus Villemoes <>
Cc:     Bruno Thomsen <>,
        Per Nørgaard Christensen 
        <>, LKML <>
Subject: Re: battery switch-over detection on pcf2127

On 05/05/2020 23:01:19+0200, Rasmus Villemoes wrote:
> Thanks for the quick replies, both. Unfortunately, being able to read BF
> from linux is not relevant to us - all the handling happens early in the
> bootloader (including clearing BF, so that we can detect that the
> previous boot failed only because of power fail - hence whether the
> linux driver clears BF or not is not relevant). We really just want
> linux to not touch the bits in CTRL3 at all.

Well, in that case, Linux doesn't touch the BF bit anymore unless
userspace uses the ioctls so you should be ok using it from your

> Hm, wait. Re-reading the above suggests that BF can get set even if BTSE
> is not, and a quick experiment shows that is true - I must have misread
> the data sheet. While I think that's fine for now (currently I only
> print the time of last switch-over as a diagnostic), I did have some use
> case in mind for comparing that timestamp to the current time and make
> decisions based on that. But until I figure out exactly what I want to
> use it for, and until we actually upgrade to 5.4+, there's no rush.

Configuring the timestamps is something else I want to do but I still
didn't finish to design the proper interface.

It is definitively on the todo list.

Alexandre Belloni, Bootlin
Embedded Linux and Kernel engineering

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