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

[Maud Spierings]

Hi Matti,

On 10/29/25 11:05, Matti Vaittinen wrote:
> On 29/10/2025 11:48, Lothar Waßmann wrote:
> > 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.
>
> Yes. When the driver calculates the voltages which match the actual 
> voltages, then you should also use the actual voltages in the device-tree.

Think I've got it:

diff --git a/drivers/regulator/bd718x7-regulator.c 
b/drivers/regulator/bd718x7-regulator.c
index 022d98f3c32a2..ea9c4058ee6a5 100644
--- a/drivers/regulator/bd718x7-regulator.c
+++ b/drivers/regulator/bd718x7-regulator.c
@@ -1613,6 +1613,8 @@ static int setup_feedback_loop(struct device *dev, 
struct device_node *np,
                                step /= r1;

                                new[j].min = min;
+                               new[j].min_sel = 
desc->linear_ranges[j].min_sel;
+                               new[j].max_sel = 
desc->linear_ranges[j].max_sel;
                                new[j].step = step;

                                dev_dbg(dev, "%s: old range min %d, 
step %d\n",


the min_sel and max_sel fields were uninitialized in the new 
linear_range, copying them over from the old one (they refer to the 
register range if I understand correctly so they should not change) 
initializes them.

Then setting 1349494 as the actual voltage makes it fully work. 
Otherwise it complains:
buck6: failed to apply 1350000-1350000uV constraint: -EINVAL

Final debug output now:
[    0.327807] rohm-bd718x7 0-004b: buck6: old range min 800000, step 10000
[    0.327813] rohm-bd718x7 0-004b: new range min 981454, step 12268
[    0.327819] rohm-bd718x7 0-004b: regulator 'buck6' has FB pull-up 
configured

I will add this fix to the next version of this patchset and include 
your requested change in the dts.

Kind regards,
Maud