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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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