[PATCH v2] iio: light: veml6075: fix UV index reported at half value

[Shardul Deshpande <iamsharduld@gmail.com>]

veml6075_get_uvi_micro() normalises the UV index for the configured
integration time. The raw UVA/UVB counts scale linearly with the
integration time, so the responsivity-weighted sum must be divided by the
integration-time scale factor relative to the 50 ms base case (which is
returned undivided).

Per the VEML6075 datasheet the UV_IT field selects the integration time
as 50, 100, 200, 400 and 800 ms for field values 0..4, i.e.
(50 << int_index) ms: each step doubles the integration time and hence
the accumulated counts. The correct scale factor relative to the 50 ms
base is therefore 2^int_index == (1 << int_index). (See also the Vishay
application note "Designing the VEML6075 Into an Application" for the
UV-index responsivity calculation that these constants implement.)

The code instead divided by (2 << int_index) == 2^(int_index + 1), i.e.
twice the correct value, so the reported UV index was half of the true
value for every integration time except 50 ms. As the driver powers up
with VEML6075_IT_100_MS, the UV index was reported at half value out of
the box.

Divide by (1 << int_index) instead. int_index is already bounded to 0..4
by veml6075_read_int_time_index(), and (1 << 0) == 1 reproduces the
undivided 50 ms case, so the per-integration-time switch collapses to a
single expression.

Fixes: 3b82f43238ae ("iio: light: add VEML6075 UVA and UVB light sensor driver")
Cc: stable@vger.kernel.org
Signed-off-by: Shardul Deshpande <iamsharduld@gmail.com>
---
Changes in v2:
- Collapse the per-integration-time switch into a single divide now that
  all cases share the same expression -- int_index is bounded to 0..4 by
  veml6075_read_int_time_index() and (1 << 0) reproduces the 50 ms case.
  (Javier Carrasco)
- Add VEML6075 datasheet (UV_IT integration-time table) and application-
  note references for the integration-time scaling in the changelog.

Link to v1:
https://lore.kernel.org/linux-iio/20260626110400.68885-1-iamsharduld@gmail.com/

 drivers/iio/light/veml6075.c | 18 +++++++-----------
 1 file changed, 7 insertions(+), 11 deletions(-)

diff --git a/drivers/iio/light/veml6075.c b/drivers/iio/light/veml6075.c
index 59187244a..d0ec06eeb 100644
--- a/drivers/iio/light/veml6075.c
+++ b/drivers/iio/light/veml6075.c
@@ -237,17 +237,13 @@ static int veml6075_get_uvi_micro(struct veml6075_data *data, int uva_comp,
 	if (int_index < 0)
 		return int_index;
 
-	switch (int_index) {
-	case VEML6075_IT_50_MS:
-		return uvia_micro + uvib_micro;
-	case VEML6075_IT_100_MS:
-	case VEML6075_IT_200_MS:
-	case VEML6075_IT_400_MS:
-	case VEML6075_IT_800_MS:
-		return (uvia_micro + uvib_micro) / (2 << int_index);
-	default:
-		return -EINVAL;
-	}
+	/*
+	 * The raw counts scale linearly with the integration time, which
+	 * doubles at each step (50, 100, 200, 400, 800 ms == 50 << int_index
+	 * ms; int_index is bounded to 0..4 above). Normalise to the 50 ms
+	 * base by dividing by 2^int_index; (1 << 0) == 1 leaves 50 ms as-is.
+	 */
+	return (uvia_micro + uvib_micro) / (1 << int_index);
 }
 
 static int veml6075_read_uvi(struct veml6075_data *data, int *val, int *val2)
-- 
2.50.1 (Apple Git-155)