Re: [PATCH v2 2/3] ARM: dts: imx: Add devicetree for Kobo Clara 2E

[Matti Vaittinen <mazziesaccount@gmail.com>]

On 24/10/2024 17:22, Andreas Kemnade wrote:
> Adds a devicetree for the Kobo Clara 2E Ebook reader. It is based
> on boards marked with "37NB-E60K2M+4A2" or "37NB-E60K2M+4B0". It is
> equipped with an i.MX6SLL SoC.
> 
> Expected to work:
>    - Buttons
>    - Wifi
>    - Bluetooth
>      (if Wifi is initialized first, driver does not handle regulators
>       yet)
>    - LED
>    - uSD
>    - USB
>    - RTC
>    - Touchscreen
> 
> Add human-readable comments for devices without mainlined driver and
> binding. Such comments can e.g. be help to find testers if someone
> starts to work on the missing drivers.
> 
> Signed-off-by: Andreas Kemnade <andreas@kemnade.info>

...

> +
> +	pmic@4b {
> +		compatible = "rohm,bd71879", "rohm,bd71828";
> +		reg = <0x4b>;
> +		pinctrl-names = "default";
> +		pinctrl-0 = <&pinctrl_bd71828>;
> +
> +		interrupt-parent = <&gpio4>;
> +		interrupts = <19 IRQ_TYPE_LEVEL_LOW>;
> +		system-power-controller;
> +
> +		clocks = <&clks 0>;
> +		#clock-cells = <0>;
> +		clock-output-names = "bd71828-32k-out";
> +
> +		gpio-controller;
> +		#gpio-cells = <2>;
> +		gpio-reserved-ranges = <0 1>, <2 1>;
> +
> +		rohm,charger-sense-resistor-ohms = <30000000>;

I am afraid that this one is _my_ very much terrible brainfart. Yeah, 
pile up the stones and start casting ;)

I am fairly sure the sense resistor is 30 mOhm (0,030 Ohm), not 30 MOhm 
(30 000 000 Ohm). (And I am the one who misinterpreted the M in some 
email/data-sheet in the past - and never questioned the sanity).

In short, AFAICS the sense resistor is added "in series" to the system 
load. Eg:

           --------
       ---| Rsense |-----
      |    --------      |
  ---------           -------
| VSupply |         | Rload |
  ---------           -------
      |                  |
       ------------------

Hence, by measuring the voltage drop on the Rsense gives us the current 
flowing through the system ( good old U = RI ).

I believe having 30 Mohm (30 000 000 Ohm) resistor there would not make 
much of sense... With a Fermi estimate that the system works with 
voltage magnitude of 1V and current magnitude of 1A and then applying 
good old P = UI and U = RI would give us wonderful results :) Quite a 
battery on poor Kobo, right? You'd better to not touch the battery 
termninals ;) Oh, and looking the driver code I've written for handling 
this property... Sometimes I really don't like mirrors :)

Well, now that I got this out - I suppose this could be
rohm,charger-sense-resistor-milli-ohms = <30>;
or
rohm,charger-sense-resistor-micro-ohms = <30000>;

I further guess there is no upstreamn binding doc for this property. I 
think there is also no upstream charger driver for the BD71828/BD71879 - 
only an early RFC and some downstream mess - but stil it'd be nice to 
have the property in place as the size of the sense resistor is needed 
when converting coulomb counter register values to current.


Yours,
	-- Matti