[PATCH v3 1/3] iio: light: isl29018: fix overflow and precision in isl29018_read_lux()

[Herman van Hazendonk <github.com@herrie.org>]

The intermediate calculations in isl29018_read_lux() use 32-bit
arithmetic throughout, which overflows in two distinct ways:

1. lux_data * chip->scale.uscale — at 16-bit integration time and the
   1000 fc range, scale.uscale is 976562.  A full-scale 16-bit reading
   (65535) gives 65535 * 976562 ≈ 64 billion, far beyond UINT_MAX.
   The value wraps before the /1000000 division can save it, producing
   a wildly wrong data_x_range.

2. data_x_range * chip->calibscale — even after a correct data_x_range,
   multiplying by a calibscale of a few hundred (reasonable for a deeply
   tinted cover glass) pushes the product past INT_MAX, causing *lux to
   wrap negative.

Additionally, dividing lux_data * scale.uscale by 1000000 before
applying calibscale discards the fractional-lux remainder.  For low
ranges where scale.scale is zero, any reading below 1000000/scale.uscale
counts truncates to a data_x_range of zero, so the calibscale
multiplication cannot rescue it.  This creates a dead-band at low light
levels that is especially visible when a large cover-glass compensation
gain is in use.

Fix the overflows by widening the intermediate variables to u64 and
using div_u64() for the divisions (plain 64-bit division emits
__aeabi_uldivmod on ARM32, which is not available in kernel builds).
Preserve the uscale remainder across the first division so that the
calibscale multiplication captures the sub-lux contribution.  Clamp
the final result to INT_MAX before storing it in the signed int *lux
out parameter.

Signed-off-by: Herman van Hazendonk <github.com@herrie.org>
---
 drivers/iio/light/isl29018.c | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/drivers/iio/light/isl29018.c b/drivers/iio/light/isl29018.c
index b6ab726d1dae..f3312ad670d9 100644
--- a/drivers/iio/light/isl29018.c
+++ b/drivers/iio/light/isl29018.c
@@ -14,6 +14,7 @@
 #include <linux/module.h>
 #include <linux/mutex.h>
 #include <linux/delay.h>
+#include <linux/math64.h>
 #include <linux/regmap.h>
 #include <linux/regulator/consumer.h>
 #include <linux/slab.h>
@@ -193,17 +194,22 @@ static int isl29018_read_sensor_input(struct isl29018_chip *chip, int mode)
 static int isl29018_read_lux(struct isl29018_chip *chip, int *lux)
 {
 	int lux_data;
-	unsigned int data_x_range;
+	u32 uscale_rem;
+	u64 uscale_term, data_x_range, result;
 
 	lux_data = isl29018_read_sensor_input(chip,
 					      ISL29018_CMD1_OPMODE_ALS_ONCE);
 	if (lux_data < 0)
 		return lux_data;
 
-	data_x_range = lux_data * chip->scale.scale +
-		       lux_data * chip->scale.uscale / 1000000;
-	*lux = data_x_range * chip->calibscale +
-	       data_x_range * chip->ucalibscale / 1000000;
+	/* Retain the uscale remainder so calibscale captures sub-lux precision. */
+	uscale_term = (u64)lux_data * chip->scale.uscale;
+	data_x_range = (u64)lux_data * chip->scale.scale +
+		       div_u64_rem(uscale_term, 1000000, &uscale_rem);
+	result = data_x_range * chip->calibscale +
+		 div_u64((u64)uscale_rem * chip->calibscale, 1000000) +
+		 div_u64(data_x_range * chip->ucalibscale, 1000000);
+	*lux = (int)min_t(u64, result, INT_MAX);
 
 	return 0;
 }
-- 
2.43.0