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

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Message-ID: <aLbneKXFd7Nc711T@smile.fi.intel.com>
Date: Tue, 2 Sep 2025 15:47:52 +0300
From: Andy Shevchenko <andriy.shevchenko@...el.com>
To: dimitri.fedrau@...bherr.com
Cc: 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>,
	Dimitri Fedrau <dima.fedrau@...il.com>,
	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

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

> +	 * 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);

5 itself can be a definition

#define ..._PRE_SCALE	5

and used everywhere.

...

>  	/*
>  	 * 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.

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

-- 
With Best Regards,
Andy Shevchenko