linux-kernel - Re: [PATCH v3 2/2] iio: humditiy: hdc3020: fix units for thresholds and hysteresis

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Message-Id: <DDIR6RWVMIRK.2DQB98BRS180Z@gmail.com>
Date: Wed, 15 Oct 2025 16:29:49 +0800
From: "Javier Carrasco" <javier.carrasco.cruz@...il.com>
To: <dimitri.fedrau@...bherr.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>
Cc: "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 v3 2/2] iio: humditiy: hdc3020: fix units for thresholds
 and hysteresis

On Mon Oct 13, 2025 at 4:12 PM CST, 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.
>
> Fixes: 3ad0e7e5f0cb ("iio: humidity: hdc3020: add threshold events support")
> Reported-by: Chris Lesiak <chris.lesiak@...orbio.com>
> Signed-off-by: Dimitri Fedrau <dimitri.fedrau@...bherr.com>
> ---
>  drivers/iio/humidity/hdc3020.c | 67 ++++++++++++++++++++++++------------------
>  1 file changed, 39 insertions(+), 28 deletions(-)
>
> diff --git a/drivers/iio/humidity/hdc3020.c b/drivers/iio/humidity/hdc3020.c
> index 8aa567d9aded9cab461f1f905b6b5ada721ba2f0..5a978801fb09ca49c449028d046a91e0e67c38ea 100644
> --- a/drivers/iio/humidity/hdc3020.c
> +++ b/drivers/iio/humidity/hdc3020.c
> @@ -72,6 +72,9 @@
>  #define HDC3020_MAX_TEMP_HYST_MICRO	164748607
>  #define HDC3020_MAX_HUM_MICRO		99220264
>  
> +/* Divide 65535 from the datasheet by 5 to avoid overflows */
> +#define HDC3020_THRESH_FRACTION		(65535 / 5)
> +
>  struct hdc3020_data {
>  	struct i2c_client *client;
>  	struct gpio_desc *reset_gpio;
> @@ -376,15 +379,17 @@ static int hdc3020_thresh_get_temp(u16 thresh)
>  	int temp;
>  
>  	/*
> -	 * 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 explicit formula in the datasheet and
> +	 * additionally scale by HDC3020_THRESH_FRACTION to avoid precision loss
> +	 * when calculating threshold and hysteresis values. Result is degree
> +	 * celsius scaled by HDC3020_THRESH_FRACTION.

I see that Andy asked you to include the formula, and you accepted that
suggestion, but the formula is still missing. I also think it should be
added to the comment because that would clarify where this kind of
magic numbers in the return value come from.

>  	 */
>  	temp = FIELD_GET(HDC3020_THRESH_TEMP_MASK, thresh) <<
>  	       HDC3020_THRESH_TEMP_TRUNC_SHIFT;
>  
> -	return -2949075 + (175 * temp);
> +	return -589815 + (35 * temp);
>  }

With that modification,

Reviewed-by: Javier Carrasco <javier.carrasco.cruz@...il.com>