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

["Lothar Waßmann" <LW@KARO-electronics.de>]

Hi,

On Wed, 29 Oct 2025 10:42:17 +0200 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.
> 
Before doing the real-life test I did the same calculation that's done
in the driver to be sure that it will generate the correct values:
bc 1.07.1
Copyright 1991-1994, 1997, 1998, 2000, 2004, 2006, 2008, 2012-2017 Free Software Foundation, Inc.
This is free software with ABSOLUTELY NO WARRANTY.
For details type `warranty'. 
fb_uv=0
r1=2200
r2=499
min=800000
step=10000
# default voltage without divider
min+30*step
1100000
min=min-(fb_uv-min)*r2/r1
step=step*(r1+r2)/r1
min
981454
step
12268
# default voltage with divider
min+30*step
1349494

Probably we need to use this value rather than the nominal 135000 as
the target voltage in the DTB.


Lothar Waßmann