[PATCH v3] iio: adc: ad4030: fix invalid oversampling_ratio validation

ad4030_set_avg_frame_len() computes avg_log2 = ilog2(avg_val) before
validating avg_val. The existing check (avg_val < 0) allows avg_val == 0
to pass through, resulting in ilog2(0) being called with undefined/garbage
behavior.

Reject non-positive values (avg_val <= 0) before computing log2.

This issue was identified with assistance from Claude AI and manually
verified against the code.

Fixes: 949abd1ca5a4 ("iio: adc: ad4030: add averaging support")
Signed-off-by: Salah Triki <[email protected]>
---
Changes in v3:
   - Dropped the !is_power_of_2() check to preserve standard IIO attribute
   rounding behavior, per feedback from David Lechner and Jonathan Cameron.

Changes since v1:
   - Added note stating the issue was identified with assistance from
   Claude AI and verified manually.
   - Removed initialization of avg_log2 at declaration.

 drivers/iio/adc/ad4030.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/iio/adc/ad4030.c b/drivers/iio/adc/ad4030.c
index 9c5f19321e3b..5a5e69dfb5d4 100644
--- a/drivers/iio/adc/ad4030.c
+++ b/drivers/iio/adc/ad4030.c
@@ -746,14 +746,16 @@ static int ad4030_set_chan_calibbias(struct iio_dev *indio_dev,
 static int ad4030_set_avg_frame_len(struct iio_dev *dev, int avg_val)
 {
 	struct ad4030_state *st = iio_priv(dev);
-	unsigned int avg_log2 = ilog2(avg_val);
+	unsigned int avg_log2;
 	unsigned int last_avg_idx = ARRAY_SIZE(ad4030_average_modes) - 1;
 	int freq_hz;
 	int ret;
 
-	if (avg_val < 0 || avg_val > ad4030_average_modes[last_avg_idx])
+	if (avg_val <= 0 || avg_val > ad4030_average_modes[last_avg_idx])
 		return -EINVAL;
 
+	avg_log2 = ilog2(avg_val);
+
 	if (st->offload_trigger) {
 		/*
 		 * The sample averaging and sampling frequency configurations
-- 
2.43.0