thermal/drivers/mediatek/lvts_thermal: Don't leave threshold zeroed

The thermal framework might leave the low threshold unset if there
aren't any lower trip points. This leaves the register zeroed, which
translates to a very high temperature for the low threshold. The
interrupt for this threshold is then immediately triggered, and the
state machine gets stuck, preventing any other temperature monitoring
interrupts to ever trigger.

(The same happens by not setting the Cold or Hot to Normal thresholds
when using those)

Set the unused threshold to a valid low value. This value was chosen so
that for any valid golden temperature read from the efuse, when the
value is converted to raw and back again to milliCelsius, the result
doesn't underflow.

Fixes: f5f633b1 ("thermal/drivers/mediatek: Add the Low Voltage Thermal Sensor driver")
Signed-off-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
Reviewed-by: Alexandre Mergnat <amergnat@baylibre.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20230706153823.201943-6-nfraprado@collabora.com

drivers/thermal/mediatek/lvts_thermal.c

@@ -83,6 +83,8 @@
 
 #define LVTS_HW_SHUTDOWN_MT8195		105000
 
+#define LVTS_MINIMUM_THRESHOLD		20000
+
 static int golden_temp = LVTS_GOLDEN_TEMP_DEFAULT;
 static int coeff_b = LVTS_COEFF_B;
 
@@ -294,7 +296,7 @@ static int lvts_set_trips(struct thermal_zone_device *tz, int low, int high)
 {
 	struct lvts_sensor *lvts_sensor = thermal_zone_device_priv(tz);
 	void __iomem *base = lvts_sensor->base;
-	u32 raw_low = lvts_temp_to_raw(low);
+	u32 raw_low = lvts_temp_to_raw(low != -INT_MAX ? low : LVTS_MINIMUM_THRESHOLD);
 	u32 raw_high = lvts_temp_to_raw(high);
 
 	/*
@@ -306,11 +308,9 @@ static int lvts_set_trips(struct thermal_zone_device *tz, int low, int high)
 	 *
 	 * 14-0 : Raw temperature for threshold
 	 */
-	if (low != -INT_MAX) {
-		pr_debug("%s: Setting low limit temperature interrupt: %d\n",
-			 thermal_zone_device_type(tz), low);
-		writel(raw_low, LVTS_OFFSETL(base));
-	}
+	pr_debug("%s: Setting low limit temperature interrupt: %d\n",
+		 thermal_zone_device_type(tz), low);
+	writel(raw_low, LVTS_OFFSETL(base));
 
 	/*
 	 * High offset temperature threshold