iio: isl29018: Support fractional ALS scaling.

The Industrial IO framework supports scaling ADC values by fractions,
but most drivers default to using whole numbers.
This change turns on fractional scaling in the isl29018 driver.
Signed-off-by: Bryan Freed <bfreed@chromium.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

drivers/staging/iio/light/isl29018.c

@@ -63,6 +63,7 @@ struct isl29018_chip {
 	struct regmap		*regmap;
 	struct mutex		lock;
 	unsigned int		lux_scale;
+	unsigned int		lux_uscale;
 	unsigned int		range;
 	unsigned int		adc_bit;
 	int			prox_scheme;
@@ -145,13 +146,22 @@ static int isl29018_read_sensor_input(struct isl29018_chip *chip, int mode)
 static int isl29018_read_lux(struct isl29018_chip *chip, int *lux)
 {
 	int lux_data;
+	unsigned int data_x_range, lux_unshifted;
 
 	lux_data = isl29018_read_sensor_input(chip, COMMMAND1_OPMODE_ALS_ONCE);
 
 	if (lux_data < 0)
 		return lux_data;
 
-	*lux = (lux_data * chip->range * chip->lux_scale) >> chip->adc_bit;
+	/* To support fractional scaling, separate the unshifted lux
+	 * into two calculations: int scaling and micro-scaling.
+	 * lux_uscale ranges from 0-999999, so about 20 bits.  Split
+	 * the /1,000,000 in two to reduce the risk of over/underflow.
+	 */
+	data_x_range = lux_data * chip->range;
+	lux_unshifted = data_x_range * chip->lux_scale;
+	lux_unshifted += data_x_range / 1000 * chip->lux_uscale / 1000;
+	*lux = lux_unshifted >> chip->adc_bit;
 
 	return 0;
 }
@@ -339,6 +349,8 @@ static int isl29018_write_raw(struct iio_dev *indio_dev,
 	mutex_lock(&chip->lock);
 	if (mask == IIO_CHAN_INFO_CALIBSCALE && chan->type == IIO_LIGHT) {
 		chip->lux_scale = val;
+		/* With no write_raw_get_fmt(), val2 is a MICRO fraction. */
+		chip->lux_uscale = val2;
 		ret = 0;
 	}
 	mutex_unlock(&chip->lock);
@@ -379,7 +391,8 @@ static int isl29018_read_raw(struct iio_dev *indio_dev,
 	case IIO_CHAN_INFO_CALIBSCALE:
 		if (chan->type == IIO_LIGHT) {
 			*val = chip->lux_scale;
-			ret = IIO_VAL_INT;
+			*val2 = chip->lux_uscale;
+			ret = IIO_VAL_INT_PLUS_MICRO;
 		}
 		break;
 	default: