linux-kernel - Re: [PATCH v2 1/2] hwmon:(pmbus/tda38740a) TDA38740A Voltage Regulator Driver

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Message-ID: <CAJKbuCav7iCdpph7HYW4=QxEmJ5+vuwadRmaQVUxT-TBSBBGVg@mail.gmail.com>
Date: Mon, 9 Feb 2026 10:34:16 +0530
From: ashish yadav <ashishyadav78@...il.com>
To: Guenter Roeck <linux@...ck-us.net>
Cc: Rob Herring <robh@...nel.org>, Krzysztof Kozlowski <krzk+dt@...nel.org>, 
	Conor Dooley <conor+dt@...nel.org>, linux-hwmon@...r.kernel.org, 
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org, 
	ASHISH YADAV <Ashish.Yadav@...ineon.com>
Subject: Re: [PATCH v2 1/2] hwmon:(pmbus/tda38740a) TDA38740A Voltage
 Regulator Driver

Hi Guenter,

I hope you are doing well.

Please find my response inline.

With Best Regards
  Ashish Yadav

On Mon, Feb 2, 2026 at 5:59 AM Guenter Roeck <linux@...ck-us.net> wrote:
>
> On 1/22/26 03:42, ashish yadav wrote:
> > Hi Guenter,
> >
> > Please find my response inline.
> >
> > Thanks & Regards
> >     Ashish Yadav
> >
> > On Tue, Jan 13, 2026 at 8:40 PM Guenter Roeck <linux@...ck-us.net> wrote:
> >>
> >> On 1/12/26 23:24, ashish yadav wrote:
> >>
> >>>> The need for this, especially why it would only be needed for PMBUS_READ_VOUT
> >>>> but not for any other VOUT related commands, is still insufficiently explained
> >>>> (and I failed to understand the rationale provided earlier).
> >>>>
> >>>
> >>> It is specifically needed for READ_VOUT as it is being used by
> >>> external controller to monitor the rail health.
> >>> Other Vout related parameters are used internally in the IC to for
> >>> output voltage related protections and does not impact any external
> >>> decision making.
> >>>
> >>
> >> Sorry, that doesn't really make sense. How would the chip know to match
> >> VOUT with its VOUT limits if both don't use the same scale ?
> >>
> > The chip telemetry would still show Vout as 0.7V as it does not know
> > about the external feedback resistors.
> > Hence, no need to scale internal Vout related parameters.
> > This scale is only for external vendor use to tweak their telemetry
> > output voltage reading.
> >
>
> You fail to explain why VOUT_SCALE_LOOP - which is supposed to handle such
> situations - can not be used, and why it would be acceptable for other VOUT
> related attributes such as VOUT_MIN, VOUT_MAX, VOUT_MARGIN_LOW, VOUT_MARGIN_HIGH,
> and the various VOUT fault limits to show the wrong values.
>
> For reference:
>
> VOUT_SCALE_LOOP:
> "Used to account for any external attenuation network on VOUT sense
>   feedback and provide correct VOUT reporting."
>

TDA38725A/TDA38740A has only two options of vout_scale_loop. These are
1 and 0.5.
If the output voltage is directly connected to the output rail, then
vout_scale_loop = 1 as there is no resistor divider in the feedback
and feedback voltage is equal to the actual output voltage.

If vout_scale_loop = 0.5, it is recommended to use a resistor divider
(top & bottom – 499ohms each) with a ratio of  0.5 in the feedback
path.
In this case, feedback voltage will be 0.5 x actual output voltage.
As the vout_scale_loop is set to 0.5, IC would use this
vout_scale_loop internally to provide the correct telemetry data.

If a customer uses a resistor divider of 2.21k (top) & 22.1k (bottom),
the divider ratio would be (2.21/22.1 + 2.21 = 0.09).
This is not an option available in the IC as it can only identify 1 and 0.5.
In this case, they configure the IC in vout_scale_loop of 1 and use a
multiplier in Linux code to correct the READ_VOUT telemetry voltage.

Why can vout_scale_loop not be used?
Using vout_scale_loop for correction will also impact all the Vout
related parameters and makes it cumbersome.
To simplify the linux code, customers prefer changing only Read_Vout
value and accept that IC would still operate based on vout_scale_loop
value configured to 1.

> Guenter
>