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

Lothar Waßmann
2025-10-29T07:21:05Z

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