[PATCH 4/4] iio: light: tsl2591: improve lux calculation precision

[Jan Remes <ciakval@gmail.com>]

From: Jan Remes <jan.remes@mycroftmind.com>

The lux calculation computes the ratio CH1 / CH0 as a truncated integer
(CH1 * 1000 / CH0) before using it. When CH1 is close to CH0, the
truncation makes the result off by up to ~35%, e.g. CH0 = 56097,
CH1 = 55973 at LOW gain and 100ms gives 1.517 lux instead of 1.118 lux.

The formula (CH0 - CH1) * (1 - CH1 / CH0) / CPL is equal to

   (CH0 - CH1)^2 * DF / (CH0 * ATIME_ms * AGAIN)

which needs no intermediate ratio. Compute it like that, with a single
division at the end.

Also report the result in micro-lux instead of milli-lux. At MAX gain
and 600ms one count is well below 1 mlx, so the milli-lux resolution
throws away information. The numerator still fits into u64.

Assisted-by: LLM
Signed-off-by: Jan Remes <jan.remes@mycroftmind.com>
---
 drivers/iio/light/tsl2591.c | 27 ++++++++++++---------------
 1 file changed, 12 insertions(+), 15 deletions(-)

diff --git a/drivers/iio/light/tsl2591.c b/drivers/iio/light/tsl2591.c
index b780b236897c..82085bd06850 100644
--- a/drivers/iio/light/tsl2591.c
+++ b/drivers/iio/light/tsl2591.c
@@ -21,6 +21,7 @@
 #include <linux/mutex.h>
 #include <linux/pm_runtime.h>
 #include <linux/sysfs.h>
+#include <linux/units.h>
 
 #include <linux/unaligned.h>
 
@@ -362,12 +363,12 @@ static int tsl2591_wait_adc_complete(struct tsl2591_chip *chip)
  * Counts Per Lux (CPL) = (ATIME_ms * AGAIN) / LUX DF
  * lux = ((C0DATA - C1DATA) * (1 - (C1DATA / C0DATA))) / CPL
  *
- * Scale values to get more representative value of lux i.e.
- * lux = ((C0DATA - C1DATA) * (1000 - ((C1DATA * 1000) / C0DATA))) / CPL
+ * Since (C0DATA - C1DATA) * (1 - (C1DATA / C0DATA)) equals
+ * (C0DATA - C1DATA)^2 / C0DATA, this is computed in micro-lux as
+ * lux = ((C0DATA - C1DATA)^2 * LUX DF * 10^6) / (C0DATA * ATIME_ms * AGAIN)
  *
  * CPL is not computed on its own: at 1x gain it is below 1 for integration
- * times under 500 ms, so the division by LUX DF is folded into the final
- * division instead.
+ * times under 500 ms, so all divisions are done at the end.
  *
  * Channel 0 = IR + Visible
  * Channel 1 = IR only
@@ -383,7 +384,7 @@ static int tsl2591_read_channel_data(struct iio_dev *indio_dev,
 	int int_time_fval, gain_multi;
 	u16 als_ch0, als_ch1, als_max;
 	u64 lux;
-	u32 rem;
+	u32 diff, rem;
 	int ret;
 
 	ret = tsl2591_wait_adc_complete(chip);
@@ -436,19 +437,15 @@ static int tsl2591_read_channel_data(struct iio_dev *indio_dev,
 
 		int_time_fval = TSL2591_FVAL_TO_MSEC(settings->als_int_time);
 
-		/* Calculate lux value */
-		lux = (u64)(als_ch0 - als_ch1) *
-		      (1000 - ((als_ch1 * 1000) / als_ch0)) *
-		      TSL2591_LUX_COEFFICIENT;
-		lux = div_u64(lux, int_time_fval * gain_multi);
+		/* Calculate lux value in micro-lux */
+		diff = als_ch0 - als_ch1;
+		lux = (u64)diff * diff * TSL2591_LUX_COEFFICIENT * MICRO;
+		lux = div64_u64(lux, (u64)als_ch0 * int_time_fval * gain_multi);
 
 		dev_dbg(&client->dev, "Raw lux calculation: %llu\n", lux);
 
-		/* Divide by 1000 to get real lux value before scaling */
-		*val = div_u64_rem(lux, 1000, &rem);
-
-		/* Get the decimal part of lux reading */
-		*val2 = rem * 1000;
+		*val = div_u64_rem(lux, MICRO, &rem);
+		*val2 = rem;
 
 		break;
 	default:
-- 
2.53.0