thermal: stm32: Fix low threshold interrupt flood

With the STM32 thermal peripheral, it is not possible to dump the
temperature that has caused the interrupt.
When the temperature reaches the low threshold, we generally read
a temperature that is a little bit higher than the low threshold.
This maybe due to sampling precision, and also because the CPU becomes
hotter when it quits WFI mode.
In that case, the framework does not change the trip points. This leads
to a lot of low threshold interrupts.

The fix is to set the low threshold value 0.5 degrees Celsius
below the actual request.

The problem is not so frequent with the high threshold and it would
no be a good idea to set the threshold value higher than the request.

Signed-off-by: Pascal Paillet <p.paillet@st.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20200110101605.24984-7-p.paillet@st.com
diff --git a/drivers/thermal/st/stm_thermal.c b/drivers/thermal/st/stm_thermal.c
index aaf25ca..1cc5e6c 100644
--- a/drivers/thermal/st/stm_thermal.c
+++ b/drivers/thermal/st/stm_thermal.c
@@ -321,7 +321,8 @@
 
 	if (low > -INT_MAX) {
 		sensor->low_temp_enabled = 1;
-		ret = stm_thermal_calculate_threshold(sensor, low, &th);
+		/* add 0.5 of hysteresis due to measurement error */
+		ret = stm_thermal_calculate_threshold(sensor, low - 500, &th);
 		if (ret)
 			return ret;