Message-ID: <20251030095434.1dc06df2@karo-electronics.de>
Date: Thu, 30 Oct 2025 09:54:34 +0100
From: Lothar Waßmann <LW@...O-electronics.de>
To: Maud Spierings <maudspierings@...ontroll.com>
Cc: Matti Vaittinen <mazziesaccount@...il.com>, Rob Herring
 <robh@...nel.org>, Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley
 <conor+dt@...nel.org>, Shawn Guo <shawnguo@...nel.org>, Sascha Hauer
 <s.hauer@...gutronix.de>, Pengutronix Kernel Team <kernel@...gutronix.de>,
 Fabio Estevam <festevam@...il.com>, devicetree@...r.kernel.org,
 linux-kernel@...r.kernel.org, imx@...ts.linux.dev,
 linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2 3/5] arm64: dts: freescale: add Ka-Ro Electronics
 tx8m-1610 COM

Hi,

On Wed, 29 Oct 2025 16:35:25 +0100 Maud Spierings wrote:
> 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.
> 
Does it also work with min/max settings in the DTS that are taken from
the designated value +/- 5% tolerance margin, so that the DTS contains
reasonable values determined by the HW requirements, rather than some
artificial number that is enforced by the SW behaviour?
E.g.:
	regulator-min-microvolts = <(135000 - 6750)>;
	regulator-min-microvolts = <(135000 + 6750)>;
Thus, the nominal value of the voltage is explicitly shown in the DTS
file.


Lothar Waßmann

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