On Mon, Jul 05, 2021 at 12:20:14AM +0200, Linus Walleij wrote:
> The kernel has a helper function for linear interpolation so
> use it. It incidentally makes the code easier to read as well.
>
> Tested on the ST-Ericsson HREFv60plus hardware reference design
> with two thermistors forming a thermal zone.
>
> Cc: Peter Rosin <peda@xxxxxxxxxx>
> Cc: Chris Lesiak <chris.lesiak@xxxxxxxxx>
> Cc: linux-iio@xxxxxxxxxxxxxxx
> Signed-off-by: Linus Walleij <linus.walleij@xxxxxxxxxx>
Applied.
Thanks,
Guenter
> ---
> ChangeLog v1->v2:
> - Drop the check for low == high as the linear interpolation
> library function does this for us.
> - Multiply the temperatures in the table with 1000 *before*
> interpolating instead of *after* so we increase precision
> in the interpolation.
> ---
> drivers/hwmon/ntc_thermistor.c | 20 +++++++++++---------
> 1 file changed, 11 insertions(+), 9 deletions(-)
>
> diff --git a/drivers/hwmon/ntc_thermistor.c b/drivers/hwmon/ntc_thermistor.c
> index 8587189c7f15..4c26db6738f9 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>
>
> @@ -553,15 +554,16 @@ static int get_temp_mc(struct ntc_data *data, unsigned int ohm)
> int temp;
>
> lookup_comp(data, ohm, &low, &high);
> - if (low == high) {
> - /* 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);
> - }
> + /*
> + * First multiplying the table temperatures with 1000 to get to
> + * millicentigrades (which is what we want) and then interpolating
> + * will give the best precision.
> + */
> + temp = fixp_linear_interpolate(data->comp[low].ohm,
> + data->comp[low].temp_c * 1000,
> + data->comp[high].ohm,
> + data->comp[high].temp_c * 1000,
> + ohm);
> return temp;
> }
>
