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Sender: Guenter Roeck <groeck7@gmail.com>
Date:   Fri, 19 Aug 2022 04:35:48 -0700
From:   Guenter Roeck <linux@roeck-us.net>
To:     "Farber, Eliav" <farbere@amazon.com>
Cc:     jdelvare@suse.com, robh+dt@kernel.org, mark.rutland@arm.com,
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        amitlavi@amazon.com, almogbs@amazon.com, dwmw@amazon.co.uk,
        rtanwar@maxlinear.com
Subject: Re: [PATCH v2 12/16] hwmon: (mr75203) modify the temperature equation
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On Fri, Aug 19, 2022 at 10:44:06AM +0300, Farber, Eliav wrote:
> On 8/18/2022 11:23 PM, Guenter Roeck wrote:
> > On Wed, Aug 17, 2022 at 05:43:17AM +0000, Eliav Farber wrote:
> > > Modify the equation and coefficients to convert the digital output to
> > > temperature according to series 5 of the Moortec Embedded Temperature
> > > Sensor (METS) datasheet:
> > > T = G + H * (n / cal5 - 0.5) + J * F
> > > 
> > > The G, H and J coefficients are multiplied by 1000 to get the
> > > temperature
> > > in milli-Celsius.
> > > 
> > 
> > This is, at the very least, confusing. It doesn't explain the discrepancy
> > to the old code nor the change in constant values. I have no idea if this
> > change would result in erroneous readings on some other system where
> > the existing calculation may be the correct one.
> 
> When I tested the driver it was also not clear to me why the equation
> and coefficients in the code don't match the specifications in the data
> sheet.
> I reached out to Maxlinear engineers (@rtanwar) and they also couldn't
> explain the discrepancy.
> After further correspondence I aligned both the equation and coefficients
> in the driver code to the equation and coefficients defined in series 5
> of the Moortec Embedded Temperature Sensor (METS) datasheet which they
> provided.
> 

At least some of the discrepancy is because the original code is more
optimized and avoids overflow. Either case, the above needs to be explained
in the commit description.

> > On top of that, it seems overflow-prune in 32 bit systems.
> 
> I'll check if it can overflow, and if it can I'll fix in next version.
> 
> --
> Regards, Eliav
>