Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Wed, 6 May 2020 00:14:46 +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: <20200505221446.GX34497@piout.net>
References: <a0ed6b56-33b1-b5ab-00d1-268fcd61b754@prevas.dk>
 <20200505200744.GV34497@piout.net>
 <CAH+2xPB7LJSxsr1vYhc=u+Qweu6_-S5oEQETUVZSVu1ATW=BYw@mail.gmail.com>
 <62944a2d-f593-5a98-34be-f1f86ac6111c@prevas.dk>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <62944a2d-f593-5a98-34be-f1f86ac6111c@prevas.dk>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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
bootloader.

> 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
https://bootlin.com