[PATCH] thermal: generic-adc: Fix adc to temp interpolation

[PATCH] thermal: generic-adc: Fix adc to temp interpolation

[Bjorn Andersson]

First correct the edge case to return the last element if we're
outside the range, rather than at the last element, so that
interpolation is not omitted for points between the two last entries in
the table.

Then correct the formula to perform linear interpolation based the two
points surrounding the read ADC value. The indices for temp are kept as
"hi" and "lo" to pair with the adc indices, but there's no requirement
that the temperature is provided in descendent order. mult_frac() is
used to prevent issues with overflowing the int.

Cc: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
---
 drivers/thermal/thermal-generic-adc.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/drivers/thermal/thermal-generic-adc.c b/drivers/thermal/thermal-generic-adc.c
index bf1c628d4a7a..e22fc60ad36d 100644
--- a/drivers/thermal/thermal-generic-adc.c
+++ b/drivers/thermal/thermal-generic-adc.c
@@ -26,7 +26,7 @@ struct gadc_thermal_info {
 
 static int gadc_thermal_adc_to_temp(struct gadc_thermal_info *gti, int val)
 {
-	int temp, adc_hi, adc_lo;
+	int temp, temp_hi, temp_lo, adc_hi, adc_lo;
 	int i;
 
 	for (i = 0; i < gti->nlookup_table; i++) {
@@ -36,13 +36,17 @@ static int gadc_thermal_adc_to_temp(struct gadc_thermal_info *gti, int val)
 
 	if (i == 0) {
 		temp = gti->lookup_table[0];
-	} else if (i >= (gti->nlookup_table - 1)) {
+	} else if (i >= gti->nlookup_table) {
 		temp = gti->lookup_table[2 * (gti->nlookup_table - 1)];
 	} else {
 		adc_hi = gti->lookup_table[2 * i - 1];
 		adc_lo = gti->lookup_table[2 * i + 1];
-		temp = gti->lookup_table[2 * i];
-		temp -= ((val - adc_lo) * 1000) / (adc_hi - adc_lo);
+
+		temp_hi = gti->lookup_table[2 * i - 2];
+		temp_lo = gti->lookup_table[2 * i];
+
+		temp = temp_hi + mult_frac(temp_lo - temp_hi, val - adc_hi,
+					   adc_lo - adc_hi);
 	}
 
 	return temp;
-- 
2.18.0