[PATCH] thermal: imx: update formula for thermal sensor

[Anson Huang <b20788@freescale.com>]

Thermal sensor used to need two calibration points which are
in fuse map to get a slope for converting thermal sensor's raw
data to real temperature in degree C. Due to the chip calibration
limitation, hardware team provides an universal formula to get
real temperature from internal thermal sensor raw data:

Slope = 0.4297157 - (0.0015976 * 25C fuse);

Update the formula, as there will be no hot point calibration
data in fuse map from now on.

Signed-off-by: Anson Huang <b20788@freescale.com>
---
 drivers/thermal/imx_thermal.c |   39 ++++++++++++++++++++++++++-------------
 1 file changed, 26 insertions(+), 13 deletions(-)

diff --git a/drivers/thermal/imx_thermal.c b/drivers/thermal/imx_thermal.c
index 45af765..a99c631 100644
--- a/drivers/thermal/imx_thermal.c
+++ b/drivers/thermal/imx_thermal.c
@@ -62,12 +62,16 @@ enum imx_thermal_trip {
 #define IMX_POLLING_DELAY		2000 /* millisecond */
 #define IMX_PASSIVE_DELAY		1000
 
+#define FACTOR0				10000000
+#define FACTOR1				15976
+#define FACTOR2				4297157
+
 struct imx_thermal_data {
 	struct thermal_zone_device *tz;
 	struct thermal_cooling_device *cdev;
 	enum thermal_device_mode mode;
 	struct regmap *tempmon;
-	int c1, c2; /* See formula in imx_get_sensor_data() */
+	u32 c1, c2; /* See formula in imx_get_sensor_data() */
 	unsigned long temp_passive;
 	unsigned long temp_critical;
 	unsigned long alarm_temp;
@@ -84,7 +88,7 @@ static void imx_set_alarm_temp(struct imx_thermal_data *data,
 	int alarm_value;
 
 	data->alarm_temp = alarm_temp;
-	alarm_value = (alarm_temp - data->c2) / data->c1;
+	alarm_value = (data->c2 - alarm_temp) / data->c1;
 	regmap_write(map, TEMPSENSE0 + REG_CLR, TEMPSENSE0_ALARM_VALUE_MASK);
 	regmap_write(map, TEMPSENSE0 + REG_SET, alarm_value <<
 			TEMPSENSE0_ALARM_VALUE_SHIFT);
@@ -136,7 +140,7 @@ static int imx_get_temp(struct thermal_zone_device *tz, unsigned long *temp)
 	n_meas = (val & TEMPSENSE0_TEMP_CNT_MASK) >> TEMPSENSE0_TEMP_CNT_SHIFT;
 
 	/* See imx_get_sensor_data() for formula derivation */
-	*temp = data->c2 + data->c1 * n_meas;
+	*temp = data->c2 - n_meas * data->c1;
 
 	/* Update alarm value to next higher trip point */
 	if (data->alarm_temp == data->temp_passive && *temp >= data->temp_passive)
@@ -305,6 +309,7 @@ static int imx_get_sensor_data(struct platform_device *pdev)
 	int t1, t2, n1, n2;
 	int ret;
 	u32 val;
+	u64 temp64;
 
 	map = syscon_regmap_lookup_by_phandle(pdev->dev.of_node,
 					      "fsl,tempmon-data");
@@ -330,6 +335,8 @@ static int imx_get_sensor_data(struct platform_device *pdev)
 	 *   [31:20] - sensor value @ 25C
 	 *    [19:8] - sensor value of hot
 	 *     [7:0] - hot temperature value
+	 * Use universal formula now and only need sensor value @ 25C
+	 * slope = 0.4297157 - (0.0015976 * 25C fuse)
 	 */
 	n1 = val >> 20;
 	n2 = (val & 0xfff00) >> 8;
@@ -337,20 +344,26 @@ static int imx_get_sensor_data(struct platform_device *pdev)
 	t1 = 25; /* t1 always 25C */
 
 	/*
-	 * Derived from linear interpolation,
-	 * Tmeas = T2 + (Nmeas - N2) * (T1 - T2) / (N1 - N2)
+	 * Derived from linear interpolation:
+	 * slope = 0.4297157 - (0.0015976 * 25C fuse)
+	 * slope = (FACTOR2 - FACTOR1 * n1) / FACTOR0
+	 * (Nmeas - n1) / (Tmeas - t1) = slope
 	 * We want to reduce this down to the minimum computation necessary
 	 * for each temperature read.  Also, we want Tmeas in millicelsius
 	 * and we don't want to lose precision from integer division. So...
-	 * milli_Tmeas = 1000 * T2 + 1000 * (Nmeas - N2) * (T1 - T2) / (N1 - N2)
-	 * Let constant c1 = 1000 * (T1 - T2) / (N1 - N2)
-	 * milli_Tmeas = (1000 * T2) + c1 * (Nmeas - N2)
-	 * milli_Tmeas = (1000 * T2) + (c1 * Nmeas) - (c1 * N2)
-	 * Let constant c2 = (1000 * T2) - (c1 * N2)
-	 * milli_Tmeas = c2 + (c1 * Nmeas)
+	 * Tmeas = (Nmeas - n1) / slope + t1
+	 * milli_Tmeas = 1000 * (Nmeas - n1) / slope + 1000 * t1
+	 * milli_Tmeas = -1000 * (n1 - Nmeas) / slope + 1000 * t1
+	 * Let constant c1 = (-1000 / slope)
+	 * milli_Tmeas = (n1 - Nmeas) * c1 + 1000 * t1
+	 * Let constant c2 = n1 *c1 + 1000 * t1
+	 * milli_Tmeas = c2 - Nmeas * c1
 	 */
-	data->c1 = 1000 * (t1 - t2) / (n1 - n2);
-	data->c2 = 1000 * t2 - data->c1 * n2;
+	temp64 = FACTOR0;
+	temp64 *= 1000;
+	do_div(temp64, FACTOR1 * n1 - FACTOR2);
+	data->c1 = temp64;
+	data->c2 = n1 * data->c1 + 1000 * t1;
 
 	/*
 	 * Set the default passive cooling trip point to 20 °C below the
-- 
1.7.9.5