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

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Message-ID: <0beeacba-c484-43c0-80fb-66b2e0293681@gmail.com>
Date: Fri, 31 Oct 2025 14:07:03 +0200
From: Matti Vaittinen <mazziesaccount@...il.com>
To: Lothar Waßmann <LW@...O-electronics.de>
Cc: Maud Spierings <maudspierings@...ontroll.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

On 30/10/2025 14:00, Lothar Waßmann wrote:
> Hi,
> 
> On Thu, 30 Oct 2025 13:01:52 +0200 Matti Vaittinen wrote:
>> On 30/10/2025 10:54, Lothar Waßmann wrote:
>>> 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
>>>>>>>>>>   
>>
>> // snip
>>
>>>>>>>
>>>>>>> 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.
>>>>>       
>>>>   
>>
>> // snip
>>
>>>>
>>>> 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?
>>
>> I am unsure what you mean by "artificial number that is enforced by the
>> SW behaviour"?
>>
> The nominal voltage that is required by the consumer is 1.35 V. That's
> the voltage I would expect to set as target for the regulator.
> If that voltage cannot be achieved exactly, I would prefer to have the
> intended voltage listed explicitly rather than listing a value that
> accidentally can be achieved by the regulator but has nothing to do with
> the requirements of the consumer.

Ah. Thanks for the explanation. I get it now - sorry for the noise.

-- 
Matti Vaittinen
Linux kernel developer at ROHM Semiconductors
Oulu Finland

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