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

[Matti Vaittinen <mazziesaccount@gmail.com>]

On 29/10/2025 11:00, Maud Spierings wrote:
> 
> 
> 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.

It would be great if you had the time and enthusiasm to take a look at it!

Yours,
	-- Matti