Correct interpolation algorithm of thermal values.

Signed-off-by: Ravi Chandra Sadineni <ravisadineni@chromium.org>

BRANCH=none
BUG=chrome-os-partner:56206
TEST=make buildall -j

Change-Id: I22ac65f33b27b8ce8ad0687971f47b82395aa781
Reviewed-on: https://chromium-review.googlesource.com/370402
Commit-Ready: David Hendricks <dhendrix@chromium.org>
Commit-Ready: Ravi Chandra Sadineni <ravisadineni@chromium.org>
Tested-by: Ravi Chandra Sadineni <ravisadineni@chromium.org>
Reviewed-by: David Hendricks <dhendrix@chromium.org>

1 files changed, 18 insertions, 17 deletions

diff --git a/driver/temp_sensor/thermistor_ncp15wb.c b/driver/temp_sensor/thermistor_ncp15wb.c
index 64ba9fea40..dae8251575 100644
--- a/driver/temp_sensor/thermistor_ncp15wb.c
+++ b/driver/temp_sensor/thermistor_ncp15wb.c
@@ -103,8 +103,8 @@ int thermistor_linear_interpolate(uint16_t mv,
 		const struct thermistor_info *info)
 {
 	const struct thermistor_data_pair *data = info->data;
-	int v0, v1, t0, t1, num_steps;
-	int head, tail, mid;
+	int v_high = 0, v_low = 0, t_low, t_high, num_steps;
+	int head, tail, mid = 0;
 
 	/* We need at least two points to form a line. */
 	ASSERT(info->num_pairs >= 2);
@@ -113,39 +113,40 @@ int thermistor_linear_interpolate(uint16_t mv,
 	 * If input value is out of bounds return the lowest or highest
 	 * value in the data sets provided.
 	 */
-	if (mv < data[0].mv * info->scaling_factor)
+	if (mv > data[0].mv * info->scaling_factor)
 		return data[0].temp;
-	else if (mv > data[info->num_pairs - 1].mv * info->scaling_factor)
+	else if (mv < data[info->num_pairs - 1].mv * info->scaling_factor)
 		return data[info->num_pairs - 1].temp;
 
 	head = 0;
 	tail = info->num_pairs - 1;
 	while (head != tail) {
 		mid = (head + tail) / 2;
-		v0 = data[mid].mv * info->scaling_factor;
-		v1 = data[mid + 1].mv * info->scaling_factor;
+		v_high = data[mid].mv * info->scaling_factor;
+		v_low = data[mid + 1].mv * info->scaling_factor;
 
-		if ((mv >= v0) && (mv <= v1))
+		if ((mv <= v_high) && (mv >= v_low))
 			break;
-		else if (mv < v0)
+		else if (mv > v_high)
 			tail = mid;
-		else if (mv > v1)
+		else if (mv < v_low)
 			head = mid + 1;
 	}
 
-	t0 = data[mid].temp;
-	t1 = data[mid + 1].temp;
+	t_low = data[mid].temp;
+	t_high = data[mid + 1].temp;
 
 	/*
 	 * The obvious way of doing this is to figure out how many mV per
 	 * degree are in between the two points (mv_per_deg_c), and then how
-	 * many of those exist between the input voltage and lower voltage (v0):
-	 *   1. mv_per_deg_c = (v1 - v0) / (t1 - t0)
-	 *   2. num_steps = (mv - v0) / mv_per_deg_c
-	 *   3. result = t0 + num_steps
+	 * many of those exist between the input voltage and voltage of
+	 * lower temperature :
+	 *   1. mv_per_deg_c = (v_high - v_low) / (t_high - t_low)
+	 *   2. num_steps = (v_high - mv) / mv_per_deg_c
+	 *   3. result = t_low + num_steps
 	 *
 	 * Combine #1 and #2 to mitigate precision loss due to integer division.
 	 */
-	num_steps = ((mv - v0) * (t1 - t0)) / (v1 - v0);
-	return t0 + num_steps;
+	num_steps = ((v_high - mv) * (t_high - t_low)) / (v_high - v_low);
+	return t_low + num_steps;
 }