Re: [PATCH] hwmon: (ntc_thermistor): Use library interpolation

[Jonathan Cameron <jic23@kernel.org>]

On Sat,  3 Jul 2021 20:01:31 +0200
Linus Walleij <linus.walleij@linaro.org> wrote:

> The kernel has a helper function for linear interpolation so
> use it. It incidentally makes the code easier to read as well.
> 
> Cc: Peter Rosin <peda@axentia.se>
> Cc: Chris Lesiak <chris.lesiak@licor.com>
> Cc: linux-iio@vger.kernel.org
> Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

Potential precision problem because the multiplication by 1000 is now done
post division whereas before it was before it?

I'm only eyeballing the code so no idea if it makes a practical difference.

> ---
>  drivers/hwmon/ntc_thermistor.c | 11 +++++++----
>  1 file changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/hwmon/ntc_thermistor.c b/drivers/hwmon/ntc_thermistor.c
> index 8587189c7f15..61bd0e074ec9 100644
> --- a/drivers/hwmon/ntc_thermistor.c
> +++ b/drivers/hwmon/ntc_thermistor.c
> @@ -14,6 +14,7 @@
>  #include <linux/err.h>
>  #include <linux/of.h>
>  #include <linux/of_device.h>
> +#include <linux/fixp-arith.h>
>  
>  #include <linux/platform_data/ntc_thermistor.h>
>  
> @@ -557,10 +558,12 @@ static int get_temp_mc(struct ntc_data *data, unsigned int ohm)
>  		/* Unable to use linear approximation */
>  		temp = data->comp[low].temp_c * 1000;
>  	} else {
> -		temp = data->comp[low].temp_c * 1000 +
> -			((data->comp[high].temp_c - data->comp[low].temp_c) *
> -			 1000 * ((int)ohm - (int)data->comp[low].ohm)) /
> -			((int)data->comp[high].ohm - (int)data->comp[low].ohm);
> +		temp = fixp_linear_interpolate(data->comp[low].ohm,
> +					       data->comp[low].temp_c,
> +					       data->comp[high].ohm,
> +					       data->comp[high].temp_c,
> +					       ohm);
> +		temp *= 1000;
>  	}
>  	return temp;
>  }