staging: ti-soc-thermal: remove external heat while extrapolating hotspot

For boards that provide a PCB sensor close to SoC junction
temperature, it is possible to remove the cumulative heat
reported by the SoC temperature sensor.

This patch changes the extrapolation computation to consider
an external sensor in the extrapolation equations.

Signed-off-by: Eduardo Valentin <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/staging/ti-soc-thermal/ti-thermal-common.c

···
 /* common data structures */
 struct ti_thermal_data {
 	struct thermal_zone_device *ti_thermal;
+	struct thermal_zone_device *pcb_tz;
 	struct thermal_cooling_device *cool_dev;
 	struct ti_bandgap *bgp;
 	enum thermal_device_mode mode;
···
 static inline int ti_thermal_get_temp(struct thermal_zone_device *thermal,
 				      unsigned long *temp)
 {
+	struct thermal_zone_device *pcb_tz = NULL;
 	struct ti_thermal_data *data = thermal->devdata;
 	struct ti_bandgap *bgp;
 	const struct ti_temp_sensor *s;
-	int ret, tmp, pcb_temp, slope, constant;
+	int ret, tmp, slope, constant;
+	unsigned long pcb_temp;
 
 	if (!data)
 		return 0;
···
 	if (ret)
 		return ret;
 
-	pcb_temp = 0;
-	/* TODO: Introduce pcb temperature lookup */
+	/* Default constants */
+	slope = s->slope;
+	constant = s->constant;
+
+	pcb_tz = data->pcb_tz;
 	/* In case pcb zone is available, use the extrapolation rule with it */
-	if (pcb_temp) {
-		tmp -= pcb_temp;
-		slope = s->slope_pcb;
-		constant = s->constant_pcb;
-	} else {
-		slope = s->slope;
-		constant = s->constant;
+	if (!IS_ERR_OR_NULL(pcb_tz)) {
+		ret = thermal_zone_get_temp(pcb_tz, &pcb_temp);
+		if (!ret) {
+			tmp -= pcb_temp; /* got a valid PCB temp */
+			slope = s->slope_pcb;
+			constant = s->constant_pcb;
+		} else {
+			dev_err(bgp->dev,
+				"Failed to read PCB state. Using defaults\n");
+		}
 	}
 	*temp = ti_thermal_hotspot_temperature(tmp, slope, constant);
 
···
 	data->sensor_id = id;
 	data->bgp = bgp;
 	data->mode = THERMAL_DEVICE_ENABLED;
+	data->pcb_tz = thermal_zone_get_zone_by_name("pcb");
 	INIT_WORK(&data->thermal_wq, ti_thermal_work);
 
 	return data;