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

From: Linus Walleij <linus.walleij@linaro.org>
To: "Vaittinen, Matti" <matti.vaittinen@fi.rohmeurope.com>
Cc: Matti Vaittinen <mazziesaccount@gmail.com>,
	Sebastian Reichel <sre@kernel.org>,
	Rob Herring <robh+dt@kernel.org>,
	Lee Jones <lee.jones@linaro.org>,
	"rostokus@gmail.com" <rostokus@gmail.com>,
	"fan.chen@mediatek.com" <fan.chen@mediatek.com>,
	"linux-pm@vger.kernel.org" <linux-pm@vger.kernel.org>,
	"devicetree@vger.kernel.org" <devicetree@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	linux-power <linux-power@fi.rohmeurope.com>
Subject: Re: [RFC PATCH v3 3/9] power: supply: Support DT originated temperature-capacity tables
Date: Tue, 30 Nov 2021 02:34:27 +0100	[thread overview]
Message-ID: <CACRpkdZ9EEmWSb4mq7WJywma4dFCms+Z3BAKfntcsb9y_1THNQ@mail.gmail.com> (raw)
In-Reply-To: <57f97c5f-aedb-7f7e-d269-90bd97bbba09@fi.rohmeurope.com>

Hi Matti,

not so much time so trying to answer the central question here!

On Sun, Nov 28, 2021 at 9:51 AM Vaittinen, Matti
<Matti.Vaittinen@fi.rohmeurope.com> wrote:

> I am pretty
> sure the current power-supply framework allows us to expose the current
> full_cap to userlans - so building your Tesla example with the star
> should be doable - if the drivers can somehow get the information about
> the absolute capacity drop.

To do this we would need to export a capacity at current temperature
and capacity at nominal temperature (which is usually 25 deg C)
so you can scale to something. This isn't in sysfs today but we could
probably add it (and then the world of UI:s battery icons need to change
in response).

> > Then the question is: is the method used by Rohm universal and
> > well-known and something many chargers will do exactly this
> > way, so it should be in the core, or is it a particularity that should
> > be in your driver?
>
> I am not sure this is the correct question. I'd rephrase it to: Would it
> be beneficial for drivers to come if the core did support this
> functionality - or is the feature unnecessary, or are there better
> alternatives. If core does not provide the support - then it is a high
> mountain for one to climb if he wants to use such improvement.

I think we need this.

> I think that the agreement we can do is that the OCV+temperature => SOC
> tables do not provide quite same information as the suggested
> temperature => capacity-drop tables would. Whether there are better
> alternatives - or if this is generally useful remains to be discussed -
> and this is something where I _do_ appreciate your (and everyone else's)
> input!

temperature + OCV => SOC isn't enough I think.

We probably need something to tell us what the total usable
capacity will be under different temperatures. I suspect an
interpolated table is best though, this is going to be quite
nonlinear in practice.

Yours,
Linus Walleij