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

[Matti Vaittinen <mazziesaccount@gmail.com>]

On 29/10/2025 11:48, 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.

Yours,
	-- Matti

> 
> 
> Lothar Waßmann