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iio: light sensor: Improve granularity of tsl2583 lux values.

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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>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
This commit is contained in:
Bryan Freed 2011-12-02 14:19:30 -08:00 committed by Greg Kroah-Hartman
parent d666c0d490
commit 014fcb1db5
1 changed files with 14 additions and 3 deletions
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17
drivers/staging/iio/light/tsl2583.c
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@ -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;
lux64 >>= 13;
lux = lux64;
lux = (lux + 500) / 1000;
if (lux > TSL258X_LUX_CALC_OVER_FLOW) { /* check for overflow */
return_max:
lux = TSL258X_LUX_CALC_OVER_FLOW;