[PATCH v2] hwmon: (ntc_thermistor): Use library interpolation

[Linus Walleij <linus.walleij@linaro.org>]

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@axentia.se>
Cc: Chris Lesiak <chris.lesiak@licor.com>
Cc: linux-iio@vger.kernel.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
---
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;
 }
 
-- 
2.31.1