[PATCH v3] iio: ad7949: fix wrong ADC result due to incorrect bit mask

[Wilfried Wessner <wilfried.wessner@gmail.com>]

Fixes a wrong bit mask used for the ADC's result, which was caused by an
improper usage of the GENMASK() macro. The bits higher than ADC's 
resolution are undefined and if not masked out correctly, a wrong result 
can be given. The GENMASK() macro indexing is zero based, so the mask has 
to go from [resolution - 1 , 0].

Fixes: 7f40e0614317f ("iio:adc:ad7949: Add AD7949 ADC driver family")

Signed-off-by: Wilfried Wessner <wilfried.wessner@gmail.com>

---

The issue was found in combination of an AD7682 ADC with an ARM based 
iMX7-CPU. The SPI line was analyzed with a logic analyzer and a 
discrepancy between applied voltage level and the ADC reported value 
in user space was observed. Digging into the driver code revealed an 
improper mask used for the ADC-result.


 drivers/iio/adc/ad7949.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/iio/adc/ad7949.c b/drivers/iio/adc/ad7949.c
index 5d597e5050f6..1b4b3203e428 100644
--- a/drivers/iio/adc/ad7949.c
+++ b/drivers/iio/adc/ad7949.c
@@ -91,7 +91,7 @@ static int ad7949_spi_read_channel(struct ad7949_adc_chip *ad7949_adc, int *val,
 	int ret;
 	int i;
 	int bits_per_word = ad7949_adc->resolution;
-	int mask = GENMASK(ad7949_adc->resolution, 0);
+	int mask = GENMASK(ad7949_adc->resolution - 1, 0);
 	struct spi_message msg;
 	struct spi_transfer tx[] = {
 		{
-- 
2.25.1