linux-kernel - Re: [PATCH v2 2/2] iio: humditiy: hdc3020: fix units for thresholds and hysteresis

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Message-ID: <20250902135404.GA145952@legfed1>
Date: Tue, 2 Sep 2025 15:54:04 +0200
From: Dimitri Fedrau <dima.fedrau@...il.com>
To: Andy Shevchenko <andriy.shevchenko@...el.com>
Cc: dimitri.fedrau@...bherr.com,
	Javier Carrasco <javier.carrasco.cruz@...il.com>,
	Li peiyu <579lpy@...il.com>, Jonathan Cameron <jic23@...nel.org>,
	David Lechner <dlechner@...libre.com>,
	Nuno Sá <nuno.sa@...log.com>,
	Andy Shevchenko <andy@...nel.org>,
	Jonathan Cameron <Jonathan.Cameron@...wei.com>,
	linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org,
	Chris Lesiak <chris.lesiak@...orbio.com>
Subject: Re: [PATCH v2 2/2] iio: humditiy: hdc3020: fix units for thresholds
 and hysteresis

Hi Andy,

Am Tue, Sep 02, 2025 at 03:47:52PM +0300 schrieb Andy Shevchenko:
> On Mon, Sep 01, 2025 at 07:51:59PM +0200, Dimitri Fedrau via B4 Relay wrote:
> > From: Dimitri Fedrau <dimitri.fedrau@...bherr.com>
> > 
> > According to the ABI the units after application of scale and offset are
> > milli degree celsius for temperature thresholds and milli percent for
> > relative humidity thresholds. Currently the resulting units are degree
> > celsius for temperature thresholds and hysteresis and percent for relative
> > humidity thresholds and hysteresis. Change scale factor to fix this issue.
> 
> ...
> 
> >  	/*
> > -	 * Get the temperature threshold from 9 LSBs, shift them to get
> > -	 * the truncated temperature threshold representation and
> > -	 * calculate the threshold according to the formula in the
> > -	 * datasheet. Result is degree celsius scaled by 65535.
> > +	 * Get the temperature threshold from 9 LSBs, shift them to get the
> > +	 * truncated temperature threshold representation and calculate the
> > +	 * threshold according to the formula in the datasheet and additionally
> 
> Replace "formula in the datasheet" by explicit formula
> 

Ok.
> > +	 * scale by HDC3020_THRESH_FRACTION to avoid precision loss when
> > +	 * calculating threshold and hysteresis values.
> >  	 */
> >  	temp = FIELD_GET(HDC3020_THRESH_TEMP_MASK, thresh) <<
> >  	       HDC3020_THRESH_TEMP_TRUNC_SHIFT;
> >  
> > -	return -2949075 + (175 * temp);
> > +	return -589815 + (35 * temp);
> 
> TBH, I prefer to have the proper units be mentioned in the comment along with
> 
> 	return -2949075 / 5 + ((175 / 5) * temp);
> 

You are right, will add the units again.

> 5 itself can be a definition
> 
> #define ..._PRE_SCALE	5
> 
> and used everywhere.
> 
> ...
> 

The explicit formula in the datasheet:

T(degree celsius) = -45 + (175 * temp) / 65535

The formula before the patch:

T(degree celsius) * 65525 = -2949075 + (175 * temp)

Adding the PRE_SCALE into the formula doesn't improve readability from
my perspective. I would prefer to just scale the result as it has been
done before.

> >  	/*
> >  	 * Get the humidity threshold from 7 MSBs, shift them to get the
> >  	 * truncated humidity threshold representation and calculate the
> > -	 * threshold according to the formula in the datasheet. Result is
> > -	 * percent scaled by 65535.
> > +	 * threshold according to the formula in the datasheet and additionally
> > +	 * scale by HDC3020_THRESH_FRACTION to avoid precision loss when
> > +	 * calculating threshold and hysteresis values.
> >  	 */
> 
> Ditto. "percent scaled by ..." is much better to understand.
> 

Ok.
> >  	hum = FIELD_GET(HDC3020_THRESH_HUM_MASK, thresh) <<
> >  	      HDC3020_THRESH_HUM_TRUNC_SHIFT;
> >  
> > -	return hum * 100;
> > +	return hum * 20;
> >  }

Best regards,
Dimitri Fedrau