iio: humditiy: hdc3020: fix units for thresholds and hysteresis

According to the ABI the units after application of scale and offset are
milli degree celsius for temperature thresholds and milli percent for
relative humidity thresholds. Currently the resulting units are degree
celsius for temperature thresholds and hysteresis and percent for relative
humidity thresholds and hysteresis. Change scale factor to fix this issue.

Fixes: 3ad0e7e5f0cb ("iio: humidity: hdc3020: add threshold events support")
Reported-by: Chris Lesiak <[e-mail]>
Reviewed-by: Javier Carrasco <[e-mail]>
Signed-off-by: Dimitri Fedrau <[e-mail]>
Cc: <[e-mail]>
Signed-off-by: Jonathan Cameron <[e-mail]>

drivers/iio/humidity/hdc3020.c

···
 #define HDC3020_MAX_TEMP_HYST_MICRO	164748607
 #define HDC3020_MAX_HUM_MICRO		99220264
 
+/* Divide 65535 from the datasheet by 5 to avoid overflows */
+#define HDC3020_THRESH_FRACTION		(65535 / 5)
+
 struct hdc3020_data {
 	struct i2c_client *client;
 	struct gpio_desc *reset_gpio;
···
 	int temp;
 
 	/*
-	 * Get the temperature threshold from 9 LSBs, shift them to get
-	 * the truncated temperature threshold representation and
-	 * calculate the threshold according to the formula in the
-	 * datasheet. Result is degree celsius scaled by 65535.
+	 * Get the temperature threshold from 9 LSBs, shift them to get the
+	 * truncated temperature threshold representation and calculate the
+	 * threshold according to the explicit formula in the datasheet:
+	 * T(C) = -45 + (175 * temp) / 65535.
+	 * Additionally scale by HDC3020_THRESH_FRACTION to avoid precision loss
+	 * when calculating threshold and hysteresis values. Result is degree
+	 * celsius scaled by HDC3020_THRESH_FRACTION.
 	 */
 	temp = FIELD_GET(HDC3020_THRESH_TEMP_MASK, thresh) <<
 	       HDC3020_THRESH_TEMP_TRUNC_SHIFT;
 
-	return -2949075 + (175 * temp);
+	return -2949075 / 5 + (175 / 5 * temp);
 }
 
 static int hdc3020_thresh_get_hum(u16 thresh)
···
 	/*
 	 * Get the humidity threshold from 7 MSBs, shift them to get the
 	 * truncated humidity threshold representation and calculate the
-	 * threshold according to the formula in the datasheet. Result is
-	 * percent scaled by 65535.
+	 * threshold according to the explicit formula in the datasheet:
+	 * RH(%) = 100 * hum / 65535.
+	 * Additionally scale by HDC3020_THRESH_FRACTION to avoid precision loss
+	 * when calculating threshold and hysteresis values. Result is percent
+	 * scaled by HDC3020_THRESH_FRACTION.
 	 */
 	hum = FIELD_GET(HDC3020_THRESH_HUM_MASK, thresh) <<
 	      HDC3020_THRESH_HUM_TRUNC_SHIFT;
 
-	return hum * 100;
+	return hum * 100 / 5;
 }
 
 static u16 hdc3020_thresh_set_temp(int s_temp, u16 curr_thresh)
···
 	else
 		s_clr = s_thresh + s_hyst;
 
-	/* Divide by 65535 to get units of micro */
-	return div_s64(s_clr, 65535);
+	/* Divide by HDC3020_THRESH_FRACTION to get units of micro */
+	return div_s64(s_clr, HDC3020_THRESH_FRACTION);
 }
 
 static int _hdc3020_write_thresh(struct hdc3020_data *data, u16 reg, u16 val)
···
 
 	clr = ret;
 	/* Scale value to include decimal part into calculations */
-	s_val = (val < 0) ? (val * 1000000 - val2) : (val * 1000000 + val2);
+	s_val = (val < 0) ? (val * 1000 - val2) : (val * 1000 + val2);
 	switch (chan->type) {
 	case IIO_TEMP:
 		switch (info) {
···
 			/* Calculate old hysteresis */
 			s_thresh = (s64)hdc3020_thresh_get_temp(thresh) * 1000000;
 			s_clr = (s64)hdc3020_thresh_get_temp(clr) * 1000000;
-			s_hyst = div_s64(abs(s_thresh - s_clr), 65535);
+			s_hyst = div_s64(abs(s_thresh - s_clr),
+					 HDC3020_THRESH_FRACTION);
 			/* Set new threshold */
 			thresh = reg_val;
 			/* Set old hysteresis */
···
 		case IIO_EV_INFO_HYSTERESIS:
 			/*
 			 * Function hdc3020_thresh_get_temp returns temperature
-			 * in degree celsius scaled by 65535. Scale by 1000000
-			 * to be able to subtract scaled hysteresis value.
+			 * in degree celsius scaled by HDC3020_THRESH_FRACTION.
+			 * Scale by 1000000 to be able to subtract scaled
+			 * hysteresis value.
 			 */
 			s_thresh = (s64)hdc3020_thresh_get_temp(thresh) * 1000000;
 			/*
 			 * Units of s_val are in micro degree celsius, scale by
-			 * 65535 to get same units as s_thresh.
+			 * HDC3020_THRESH_FRACTION to get same units as s_thresh.
 			 */
 			s_val = min(abs(s_val), HDC3020_MAX_TEMP_HYST_MICRO);
-			s_hyst = (s64)s_val * 65535;
+			s_hyst = (s64)s_val * HDC3020_THRESH_FRACTION;
 			s_clr = hdc3020_thresh_clr(s_thresh, s_hyst, dir);
 			s_clr = max(s_clr, HDC3020_MIN_TEMP_MICRO);
 			s_clr = min(s_clr, HDC3020_MAX_TEMP_MICRO);
···
 			/* Calculate old hysteresis */
 			s_thresh = (s64)hdc3020_thresh_get_hum(thresh) * 1000000;
 			s_clr = (s64)hdc3020_thresh_get_hum(clr) * 1000000;
-			s_hyst = div_s64(abs(s_thresh - s_clr), 65535);
+			s_hyst = div_s64(abs(s_thresh - s_clr),
+					 HDC3020_THRESH_FRACTION);
 			/* Set new threshold */
 			thresh = reg_val;
 			/* Try to set old hysteresis */
···
 		case IIO_EV_INFO_HYSTERESIS:
 			/*
 			 * Function hdc3020_thresh_get_hum returns relative
-			 * humidity in percent scaled by 65535. Scale by 1000000
-			 * to be able to subtract scaled hysteresis value.
+			 * humidity in percent scaled by HDC3020_THRESH_FRACTION.
+			 * Scale by 1000000 to be able to subtract scaled
+			 * hysteresis value.
 			 */
 			s_thresh = (s64)hdc3020_thresh_get_hum(thresh) * 1000000;
 			/*
-			 * Units of s_val are in micro percent, scale by 65535
-			 * to get same units as s_thresh.
+			 * Units of s_val are in micro percent, scale by
+			 * HDC3020_THRESH_FRACTION to get same units as s_thresh.
 			 */
-			s_hyst = (s64)s_val * 65535;
+			s_hyst = (s64)s_val * HDC3020_THRESH_FRACTION;
 			s_clr = hdc3020_thresh_clr(s_thresh, s_hyst, dir);
 			s_clr = max(s_clr, 0);
 			s_clr = min(s_clr, HDC3020_MAX_HUM_MICRO);
···
 		thresh = hdc3020_thresh_get_temp(ret);
 		switch (info) {
 		case IIO_EV_INFO_VALUE:
-			*val = thresh;
+			*val = thresh * MILLI;
 			break;
 		case IIO_EV_INFO_HYSTERESIS:
 			ret = hdc3020_read_be16(data, reg_clr);
···
 				return ret;
 
 			clr = hdc3020_thresh_get_temp(ret);
-			*val = abs(thresh - clr);
+			*val = abs(thresh - clr) * MILLI;
 			break;
 		default:
 			return -EOPNOTSUPP;
 		}
-		*val2 = 65535;
+		*val2 = HDC3020_THRESH_FRACTION;
 		return IIO_VAL_FRACTIONAL;
 	case IIO_HUMIDITYRELATIVE:
 		thresh = hdc3020_thresh_get_hum(ret);
 		switch (info) {
 		case IIO_EV_INFO_VALUE:
-			*val = thresh;
+			*val = thresh * MILLI;
 			break;
 		case IIO_EV_INFO_HYSTERESIS:
 			ret = hdc3020_read_be16(data, reg_clr);
···
 				return ret;
 
 			clr = hdc3020_thresh_get_hum(ret);
-			*val = abs(thresh - clr);
+			*val = abs(thresh - clr) * MILLI;
 			break;
 		default:
 			return -EOPNOTSUPP;
 		}
-		*val2 = 65535;
+		*val2 = HDC3020_THRESH_FRACTION;
 		return IIO_VAL_FRACTIONAL;
 	default:
 		return -EOPNOTSUPP;