staging:iio:dac:ad5791: Allow asymmetrical reference voltages

The ad5791 currently assumes that the negative and positive supply have the
same absolute value, which is not necessarily true. This patch introduces a
offset attribute which will contain the negative supply voltage scaled
according to the iio spec. The raw attribute now accepts values in the range
of 0 to max instead of -max/2 to max/2.

While we are at it also fix the vref span calculation. Since both positive and
negative reference voltages are specificed as absolute values we need to add
them and not subtract them to get the reference voltage span.

Signed-off-by: Lars-Peter Clausen <[e-mail]>
Acked-by: Jonathan Cameron <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/staging/iio/dac/ad5791.c

···
 	.indexed = 1,					\
 	.address = AD5791_ADDR_DAC0,			\
 	.channel = 0,					\
-	.info_mask = (1 << IIO_CHAN_INFO_SCALE_SHARED),	\
+	.info_mask = (1 << IIO_CHAN_INFO_SCALE_SHARED) | \
+		(1 << IIO_CHAN_INFO_OFFSET_SHARED),	\
 	.scan_type = IIO_ST('u', bits, 24, shift)	\
 }
 
···
 			   long m)
 {
 	struct ad5791_state *st = iio_priv(indio_dev);
+	u64 val64;
 	int ret;
 
 	switch (m) {
···
 			return ret;
 		*val &= AD5791_DAC_MASK;
 		*val >>= chan->scan_type.shift;
-		*val -= (1 << (chan->scan_type.realbits - 1));
 		return IIO_VAL_INT;
 	case (1 << IIO_CHAN_INFO_SCALE_SHARED):
 		*val = 0;
 		*val2 = (st->vref_mv * 1000) >> chan->scan_type.realbits;
 		return IIO_VAL_INT_PLUS_MICRO;
+	case (1 << IIO_CHAN_INFO_OFFSET_SHARED):
+		val64 = (((u64)st->vref_neg_mv) << chan->scan_type.realbits);
+		do_div(val64, st->vref_mv);
+		*val = -val64;
+		return IIO_VAL_INT;
 	default:
 		return -EINVAL;
 	}
···
 
 	switch (mask) {
 	case 0:
-		val += (1 << (chan->scan_type.realbits - 1));
 		val &= AD5791_RES_MASK(chan->scan_type.realbits);
 		val <<= chan->scan_type.shift;
 
···
 	st->pwr_down = true;
 	st->spi = spi;
 
-	if (!IS_ERR(st->reg_vss) && !IS_ERR(st->reg_vdd))
-		st->vref_mv = (pos_voltage_uv - neg_voltage_uv) / 1000;
-	else if (pdata)
-		st->vref_mv = pdata->vref_pos_mv - pdata->vref_neg_mv;
-	else
+	if (!IS_ERR(st->reg_vss) && !IS_ERR(st->reg_vdd)) {
+		st->vref_mv = (pos_voltage_uv + neg_voltage_uv) / 1000;
+		st->vref_neg_mv = neg_voltage_uv / 1000;
+	} else if (pdata) {
+		st->vref_mv = pdata->vref_pos_mv + pdata->vref_neg_mv;
+		st->vref_neg_mv = pdata->vref_neg_mv;
+	} else {
 		dev_warn(&spi->dev, "reference voltage unspecified\n");
+	}
 
 	ret = ad5791_spi_write(spi, AD5791_ADDR_SW_CTRL, AD5791_SWCTRL_RESET);
 	if (ret)

drivers/staging/iio/dac/ad5791.h

···
  * @reg_vss:		negative supply regulator
  * @chip_info:		chip model specific constants
  * @vref_mv:		actual reference voltage used
+ * @vref_neg_mv:	voltage of the negative supply
  * @pwr_down_mode	current power down mode
  */
 
···
 	struct regulator		*reg_vss;
 	const struct ad5791_chip_info	*chip_info;
 	unsigned short			vref_mv;
+	unsigned int			vref_neg_mv;
 	unsigned			ctrl;
 	unsigned			pwr_down_mode;
 	bool				pwr_down;