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Subject[PATCH] iio: light sensor: Improve granularity of tsl2583 lux values.
Date
When illuminance0_calibbias gets 4000 (for a 4x multiplier), we see lux
granularity of 4. Reversing the order of the right shift and multiplication
retains the precision of the unadjusted lux value.

Signed-off-by: Bryan Freed <bfreed@chromium.org>
---
drivers/staging/iio/light/tsl2583.c | 17 ++++++++++++++---
1 files changed, 14 insertions(+), 3 deletions(-)
diff --git a/drivers/staging/iio/light/tsl2583.c b/drivers/staging/iio/light/tsl2583.c
index 80f77cf..186a3ec 100644
--- a/drivers/staging/iio/light/tsl2583.c
+++ b/drivers/staging/iio/light/tsl2583.c
@@ -194,6 +194,7 @@ static int taos_get_lux(struct iio_dev *indio_dev)
{
u16 ch0, ch1; /* separated ch0/ch1 data from device */
u32 lux; /* raw lux calculated from device data */
+ u64 lux64;
u32 ratio;
u8 buf[5];
struct taos_lux *p;
@@ -297,9 +298,19 @@ static int taos_get_lux(struct iio_dev *indio_dev)
lux = (lux + (chip->als_time_scale >> 1)) /
chip->als_time_scale;

- /* adjust for active gain scale */
- lux >>= 13; /* tables have factor of 8192 builtin for accuracy */
- lux = (lux * chip->taos_settings.als_gain_trim + 500) / 1000;
+ /* Adjust for active gain scale.
+ * The taos_device_lux tables above have a factor of 8192 built in,
+ * so we need to shift right.
+ * User-specified gain provides a multiplier.
+ * Apply user-specified gain before shifting right to retain precision.
+ * Use 64 bits to avoid overflow on multiplication.
+ * Then go back to 32 bits before division to avoid using div_u64().
+ */
+ lux64 = lux;
+ lux64 = lux64 * chip->taos_settings.als_gain_trim + 500;
+ lux64 >>= 13;
+ lux = lux64;
+ lux /= 1000;
if (lux > TSL258X_LUX_CALC_OVER_FLOW) { /* check for overflow */
return_max:
lux = TSL258X_LUX_CALC_OVER_FLOW;
--
1.7.3.1


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