linux-kernel - Re: [RFC PATCH v3 3/9] power: supply: Support DT originated temperature-capacity tables

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Date:   Thu, 18 Nov 2021 03:10:35 +0100
From:   Linus Walleij <linus.walleij@...aro.org>
To:     Matti Vaittinen <matti.vaittinen@...rohmeurope.com>
Cc:     Matti Vaittinen <mazziesaccount@...il.com>,
        Sebastian Reichel <sre@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Lee Jones <lee.jones@...aro.org>, rostokus@...il.com,
        fan.chen@...iatek.com, linux-pm@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-power@...rohmeurope.com
Subject: Re: [RFC PATCH v3 3/9] power: supply: Support DT originated
 temperature-capacity tables

On Tue, Nov 16, 2021 at 1:26 PM Matti Vaittinen
<matti.vaittinen@...rohmeurope.com> wrote:

> Support obtaining the "capacity degradation by temperature" - tables
> from device-tree to batinfo.
>
> Signed-off-by: Matti Vaittinen <matti.vaittinen@...rohmeurope.com>

Same questions as on the binding patch.

If we already support different degradation by temperature tables,
why do we need a second mechanism for the same thing?

I'd just calculate a few tables per temperature and be done with
it.

At least documentation needs to be updated to reflect that the two methods
are exclusive and you can only use one of them.

+ * Usually temperature impacts on battery capacity. For systems where it is
+ * sufficient to describe capacity change as a series of temperature ranges
+ * where the change is linear (Eg delta cap = temperature_change * constant +
+ * offset) can be described by this structure.

But what chemistry has this property? This seems to not be coming from
the real physical world. I would perhaps accept differential equations
but *linear* battery characteristics?

If the intent is only for emulation of something that doesn't exist in
reality I doubt how useful it is, all battery technologies I have seen
have been nonlinear and hence we have the tables.

If you want to simulate a linear discharge, then just write a few tables
with linear dissipation progression, it's easier I think.

Yours,
Linus Walleij