Re: [PATCH] iio: light: veml6030: fix microlux value calculation

[Jonathan Cameron <jic23@kernel.org>]

On Wed, 16 Oct 2024 19:04:31 +0200
Javier Carrasco <javier.carrasco.cruz@gmail.com> wrote:

> The raw value conversion to obtain a measurement in lux as
> INT_PLUS_MICRO does not calculate the decimal part properly to display
> it as micro (in this case microlux). It only calculates the module to
> obtain the decimal part from a resolution that is 10000 times the
> provided in the datasheet (0.5376 lux/cnt for the veml6030). The
> resulting value must still be multiplied by 100 to make it micro.
> 
> This bug was introduced with the original implementation of the driver.
> 
> Cc: stable@vger.kernel.org
> Fixes: 7b779f573c48 ("iio: light: add driver for veml6030 ambient light sensor")
> Signed-off-by: Javier Carrasco <javier.carrasco.cruz@gmail.com>
Applied to the fixes-togreg branch of iio.git.

Thanks,

Jonathan

> ---
> I found this almost by chance while testing new supported devices. The
> decimal part was always suspiciously small, and when I compared samples
> to the expected value according to the datasheet, it became clear what was
> going on.
> 
> Example with a veml7700 (same resolution as the veml6030):
> 
> Resolution for gain = 1/8, IT = 100 ms: 0.5736 lux/cnt.
> 
> cat in_illuminance_raw in_illuminance_input
> 40
> 21.005040 -> wrong! 40 * 0.5736 is 21.504.
> 
> Tested with a veml6035 and a veml7700, the same will happen with the
> original veml6030, as the operation is identical for all devices.
> ---
>  drivers/iio/light/veml6030.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/iio/light/veml6030.c b/drivers/iio/light/veml6030.c
> index d6f3b104b0e6..a0bf03e37df7 100644
> --- a/drivers/iio/light/veml6030.c
> +++ b/drivers/iio/light/veml6030.c
> @@ -691,7 +691,7 @@ static int veml6030_read_raw(struct iio_dev *indio_dev,
>  			}
>  			if (mask == IIO_CHAN_INFO_PROCESSED) {
>  				*val = (reg * data->cur_resolution) / 10000;
> -				*val2 = (reg * data->cur_resolution) % 10000;
> +				*val2 = (reg * data->cur_resolution) % 10000 * 100;
>  				return IIO_VAL_INT_PLUS_MICRO;
>  			}
>  			*val = reg;
> 
> ---
> base-commit: 15e7d45e786a62a211dd0098fee7c57f84f8c681
> change-id: 20241016-veml6030-fix-processed-micro-616d00d555dc
> 
> Best regards,