linux-kernel - Re: [PATCH 0/1] power: supply: bq27xxx: expose battery data when CI=1

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Message-ID: <Yl84bed9pZaF7H4x@tp440p.steeds.sam>
Date:   Wed, 20 Apr 2022 00:32:13 +0200
From:   Sicelo <absicsz@...il.com>
To:     Pali Rohár <pali@...nel.org>
Cc:     sre@...nel.org, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/1] power: supply: bq27xxx: expose battery data when CI=1

On Wed, Apr 20, 2022 at 12:09:59AM +0200, Pali Rohár wrote:
> NAC (Nominal Available Capacity) is exported via CHARGE_NOW property.
> 
> So what about implementing CHARGE_NOW property also writable and write
> operation would do this WRTNAC seeding? So userspace would be able to do
> 'echo new_value > charge_now' and for Nokia N900 case just directly
> 'cat .../rx51_battery/charge_full_design > .../bq27200/charge_now'
Thank you for this suggestion. I'm afraid implementing it would be a bit
too involved for my current level of experience.  If someone beats me to
implementing WRTNAC ability, all the better.  Otherwise I will
definitely want to look into it in the future.

The challenge with seeding via rx51_battery is that it provides
CHARGE_FULL_DESIGN, so one must still calculate a NAC value from the
voltage. In my userspace script [1], I am currently using a linear
calculation, but obviously this is not really correct.

I might mention, in passing, that I also wish to later add the 
VOLTAGE_MIN_DESIGN property (which will be taken from EDVF). It may help
userspace (e.g. upower) know when to shutdown the device if CI is set,
as capacity cannot be trusted.

Regards
Sicelo

[1] https://gist.github.com/sicelo/0a0e895d81a6b73b26555d215dee296d#file-rx51_seed_bq27xxx-sh-L34-L42