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

[Maud Spierings]

On 10/29/25 09:42, Matti Vaittinen wrote:
> On 29/10/2025 09:11, Lothar Waßmann wrote:
> > 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.
>
> Thanks for trying it out Lothar. I am positive this was tested - but 
> probably the use-case has been using a pull-up. I assume having the zero 
> pull-up voltage causes the driver to calculate some bogus values. I 
> think fixing the computation in the driver might not be that big of a 
> task(?) The benefit of doing it would be that the correct voltages would 
> be calculated by the driver.
>
> If real voltages aren't matching what is calculated by the driver, then 
> the voltages requested by regulator consumers will cause wrong voltages 
> to be applied. Debug interfaces will also show wrong voltages, and the 
> safety limits set in the device-tree will not be really respected.
>
> I think this would be well worth fixing.

Do you intend to do this Matti?

Otherwise I will give it a try.

Kind regards,
Maud