armada thermal issues with negative temperatures

[Damien Thébault]

Hello,

I've been using an armada 370 SoC, and the thermal driver is reporting
erroneous values at negative temperatures.

After looking at what happened, in the armada_get_temp() function,
after applying shift and mask, we get the following values around 0°C:
- 315 (just above 0)
- 316 (just below 0)

If the code after /* Get formula coeficients */ is executed with those
two values, the first one gives the proper value of 216, but the second
one gives 310160 instead of the proper value of -506.

To have the proper final value, a signed division must be performed,
which will only be done if both operands are signed according to the C
standard, otherwise an unsigned division is done, which will provide
wrong results.

As the current coefficients are just fitting in unsigned long values,
the best fix I could find was to just cast each operand to (signed)
long.

With this fix, the temperature is the proper one, even for negative
values.

Best Regards
---
 drivers/thermal/armada_thermal.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/thermal/armada_thermal.c
b/drivers/thermal/armada_thermal.c
index ae75328..ad8b88c 100644
--- a/drivers/thermal/armada_thermal.c
+++ b/drivers/thermal/armada_thermal.c
@@ -177,9 +177,9 @@ static int armada_get_temp(struct
thermal_zone_device *thermal,
 	div = priv->data->coef_div;
 
 	if (priv->data->inverted)
-		*temp = ((m * reg) - b) / div;
+		*temp = (long)((m * reg) - b) / (long)div;
 	else
-		*temp = (b - (m * reg)) / div;
+		*temp = (long)(b - (m * reg)) / (long)div;
 	return 0;
 }
 
-- 
2.8.2

-- 
Damien Thébault