[PATCH v2] iio: adc: max1363: sign-extend bipolar differential channel reads

MAX1363 differential channels are bipolar (scan_type.sign = 's'), but
max1363_read_single_chan() masks the raw value to the ADC resolution
without sign-extending it. Negative differential readings are therefore
reported to userspace as large positive values (e.g. -1 as 4095 on a
12-bit part).

Sign-extend the masked value from the resolution bit for differential
channels. Single-ended channels are unipolar and are left unchanged.

Fixes: d1325cf45077 ("Staging: IIO: max1363 ADC driver")
Cc: [email protected]
Assisted-by: Claude:claude-opus-4
Signed-off-by: Cong Nguyen <[email protected]>
---
Changes in v2:
- Condense the commit message (Andy Shevchenko).
- Fix the Fixes: tag to the original driver commit d1325cf45077 rather than
  the staging->iio move; the bug has been present since the driver was added
  in staging (Andy Shevchenko).

 drivers/iio/adc/max1363.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/drivers/iio/adc/max1363.c b/drivers/iio/adc/max1363.c
index 4d0b79cfeb27..497f5daadcea 100644
--- a/drivers/iio/adc/max1363.c
+++ b/drivers/iio/adc/max1363.c
@@ -405,6 +405,14 @@ static int max1363_read_single_chan(struct iio_dev *indio_dev,
 
 		data = rxbuf[0];
 	}
+
+	/*
+	 * Differential channels are bipolar and the device returns the sample
+	 * in two's complement, so sign-extend it from the resolution bit.
+	 */
+	if (chan->differential)
+		data = sign_extend32(data, st->chip_info->bits - 1);
+
 	*val = data;
 
 	return 0;
-- 
2.25.1