[PATCH v2] iio: adc: ltc2471: Fix LTC2473 differential offset

[raoxu]

From: Xu Rao <raoxu@xxxxxxxxxxxxx>

The LTC2473 reports its differential conversion result as an unsigned
16-bit value spanning -VREF to +VREF. The IIO scale is consequently
reported as 2 * VREF divided by 2^16.

IIO offsets are expressed in raw codes, but the driver currently returns
-LTC2471_VREF, which is a millivolt value. This makes the processed
reading incorrect: with a 1.25 V reference, the midpoint raw code is
shifted by 1250 instead of 32768 codes.

Return the raw-code offset so that the midpoint maps to 0 mV and the
endpoints map to -VREF and +VREF as intended. The single-ended LTC2471
does not expose the offset attribute and is unaffected.

The supplied fix was reviewed and this commit message prepared with
OpenCode AI assistance.

Fixes: 1d3eacbfa644 ("iio: Add LTC2471/LTC2473 driver")
Signed-off-by: Xu Rao <raoxu@xxxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
---
Changes in v2:
- Replace the raw -32768 literal with the named LTC2473_DIFF_OFFSET
  macro.

 drivers/iio/adc/ltc2471.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/iio/adc/ltc2471.c b/drivers/iio/adc/ltc2471.c
index 369af700969f..eb418c0fa732 100644
--- a/drivers/iio/adc/ltc2471.c
+++ b/drivers/iio/adc/ltc2471.c
@@ -25,6 +25,7 @@ struct ltc2471_data {

 /* Reference voltage is 1.25V */
 #define LTC2471_VREF 1250
+#define LTC2473_DIFF_OFFSET (-32768)

 /* Read two bytes from the I2C bus to obtain the ADC result */
 static int ltc2471_get_value(struct i2c_client *client)
@@ -69,7 +70,7 @@ static int ltc2471_read_raw(struct iio_dev *indio_dev,

 	case IIO_CHAN_INFO_OFFSET:
 		/* Only differential chip has this property */
-		*val = -LTC2471_VREF;
+		*val = LTC2473_DIFF_OFFSET;
 		return IIO_VAL_INT;

 	default:
--
2.50.1