linux-kernel - Re: [PATCH v2 3/5] arm64: dts: freescale: add Ka-Ro Electronics tx8m-1610 COM

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Message-ID: <20251029081138.2161a92a@karo-electronics.de>
Date: Wed, 29 Oct 2025 08:11:38 +0100
From: Lothar Waßmann <LW@...O-electronics.de>
To: Maud Spierings <maudspierings@...ontroll.com>
Cc: Matti Vaittinen <mazziesaccount@...il.com>, Rob Herring
 <robh@...nel.org>, Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley
 <conor+dt@...nel.org>, Shawn Guo <shawnguo@...nel.org>, Sascha Hauer
 <s.hauer@...gutronix.de>, Pengutronix Kernel Team <kernel@...gutronix.de>,
 Fabio Estevam <festevam@...il.com>, devicetree@...r.kernel.org,
 linux-kernel@...r.kernel.org, imx@...ts.linux.dev,
 linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2 3/5] arm64: dts: freescale: add Ka-Ro Electronics
 tx8m-1610 COM

Hi,

On Tue, 28 Oct 2025 14:10:04 +0100 Maud Spierings wrote:
> On 10/28/25 13:42, Maud Spierings wrote:
> > On 10/28/25 13:15, Matti Vaittinen wrote:  
[...]
> >> Could/Should this be described using the:
> >> 'rohm,feedback-pull-up-r1-ohms' and
> >> 'rohm,feedback-pull-up-r2-ohms'? If I understand the comment 
> >> correctly, that might allow the driver to be able to use correctly 
> >> scaled voltages.
> >>
> >> https://elixir.bootlin.com/linux/v6.18-rc1/source/Documentation/ 
> >> devicetree/bindings/regulator/rohm,bd71837-regulator.yaml#L108
> >>  
> > 
> > Ah I didn't know those existed, should've checked the bindings in more 
> > detail, thanks for the hint!
> > 
> > I will have to investigate this carefully, since I don't have access to 
> > the actual design of the COM, so I don't know exactly what is there.
> >   
> 
> So I am not yet entirely sure if this works out, I used the calculation 
> in the driver:
> 
> /*
>   * Setups where regulator (especially the buck8) output voltage is scaled
>   * by adding external connection where some other regulator output is 
> connected
>   * to feedback-pin (over suitable resistors) is getting popular amongst 
> users
>   * of BD71837. (This allows for example scaling down the buck8 voltages 
> to suit
>   * lover GPU voltages for projects where buck8 is (ab)used to supply power
>   * for GPU. Additionally some setups do allow DVS for buck8 but as this do
>   * produce voltage spikes the HW must be evaluated to be able to 
> survive this
>   * - hence I keep the DVS disabled for non DVS bucks by default. I 
> don't want
>   * to help you burn your proto board)
>   *
>   * So we allow describing this external connection from DT and scale the
>   * voltages accordingly. This is what the connection should look like:
>   *
>   * |------------|
>   * |	buck 8  |-------+----->Vout
>   * |		|	|
>   * |------------|	|
>   *	| FB pin	|
>   *	|		|
>   *	+-------+--R2---+
>   *		|
>   *		R1
>   *		|
>   *	V FB-pull-up
>   *
>   *	Here the buck output is sifted according to formula:
>   *
>   * Vout_o = Vo - (Vpu - Vo)*R2/R1
>   * Linear_step = step_orig*(R1+R2)/R1
>   *
>   * where:
>   * Vout_o is adjusted voltage output at vsel reg value 0
>   * Vo is original voltage output at vsel reg value 0
>   * Vpu is the pull-up voltage V FB-pull-up in the picture
>   * R1 and R2 are resistor values.
>   *
>   * As a real world example for buck8 and a specific GPU:
>   * VLDO = 1.6V (used as FB-pull-up)
>   * R1 = 1000ohms
>   * R2 = 150ohms
>   * VSEL 0x0 => 0.8V – (VLDO – 0.8) * R2 / R1 = 0.68V
>   * Linear Step = 10mV * (R1 + R2) / R1 = 11.5mV
>   */
> 
> Because I do not know the pull up voltage, and I am not sure if it is a 
> pull up.
> 
> So:
> Vout_o = 1.35V
> Vo = 1.1V
> Vpu = unknown
> R2 = 499 Ohm
> R1 = 2200 Ohm
> Gives:
> Vpu = ~0V
> 
> And:
> Vout_o = 1.35V
> Vo = 1.1V
> Vpu = unknown
> R2 = 2200 Ohm
> R1 = 499 Ohm
> Gives:
> Vpu = ~1.04V
> 
> I am not quite sure which resistor is R1 and which is R2 but having 
> there be a pull down to 0V seems the most logical answer?
> 
> I am adding Lothar from Ka-Ro to the CC maybe he can shed some light on 
> this setup.
>
R2 is connected to GND, so Vpu = 0.
With:
	regulator-min-microvolt = <1350000>;
	regulator-max-microvolt = <1350000>;
	rohm,fb-pull-up-microvolt = <0>;
	rohm,feedback-pull-up-r1-ohms = <2200>;
	rohm,feedback-pull-up-r2-ohms = <499>;
the correct voltage should be produced on the BUCK8 output, but a quick
test with these parameters led to:
|failed to get the current voltage: -EINVAL
|bd718xx-pmic bd71847-pmic.3.auto: error -EINVAL: failed to register buck6 regulator
|bd718xx-pmic: probe of bd71847-pmic.3.auto failed with error -22

Apparently noone has ever tested this feature in real life.


Lothar Waßmann