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

The MAX1363 differential channels are bipolar (scan_type.sign = 's') and
the device returns those samples in two's complement. However
max1363_read_single_chan() masks the raw value to the ADC resolution and
stores it as-is, without sign extension. A negative differential reading
is therefore reported to userspace as a large positive value (for a 12-bit
part, -1 reads back as 4095).

These channels expose only IIO_CHAN_INFO_RAW (no offset), so the raw sysfs
value is expected to be the signed result. Sign-extend the masked value
from the resolution bit for differential channels before returning it.
Single-ended channels are unipolar (sign = 'u') and are left unchanged.

Fixes: 168c9d95a940 ("iio:adc:max1363 move from staging.")
Cc: [email protected]
Assisted-by: Claude:claude-opus-4
Signed-off-by: Cong Nguyen <[email protected]>
---
 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