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

Maud Spierings
2025-10-29T09:02:18Z

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