linux-kernel - Re: [PATCH v2 12/16] hwmon: (mr75203) modify the temperature equation

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Message-ID: <2cc79934-2280-79e6-6e63-0e3eb7107e1c@amazon.com>
Date:   Fri, 19 Aug 2022 10:44:06 +0300
From:   "Farber, Eliav" <farbere@...zon.com>
To:     Guenter Roeck <linux@...ck-us.net>
CC:     <jdelvare@...e.com>, <robh+dt@...nel.org>, <mark.rutland@....com>,
        <linux-hwmon@...r.kernel.org>, <devicetree@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <talel@...zon.com>,
        <hhhawa@...zon.com>, <jonnyc@...zon.com>, <hanochu@...zon.com>,
        <ronenk@...zon.com>, <itamark@...zon.com>, <shellykz@...zon.com>,
        <shorer@...zon.com>, <amitlavi@...zon.com>, <almogbs@...zon.com>,
        <dwmw@...zon.co.uk>, <rtanwar@...linear.com>,
        "Farber, Eliav" <farbere@...zon.com>
Subject: Re: [PATCH v2 12/16] hwmon: (mr75203) modify the temperature equation

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.

> 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