On 12/02/2011 10:19 PM, Bryan Freed wrote:
> 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.
Looks sensible to me, but having said that, Greg, please leave a little
while for Jon Brenner to get to it as he clearly knows this driver
a lot better than me!
>
> Signed-off-by: Bryan Freed <bfreed@xxxxxxxxxxxx>
Acked-by: Jonathan Cameron <jic23@xxxxxxxxxx>
> ---
> 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..25f4c1d 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;
> + 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;
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