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

[Jonathan Cameron <jic23@kernel.org>]

On Sat, 19 Oct 2024 15:33:59 +0100
Jonathan Cameron <jic23@kernel.org> wrote:

> 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.

As per the linux-next merge conflict email that I've just +CC linux-iio on,
I messed this up and it landed on the wrong code block.

Tried again...

Jonathan

> 
> 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,  
> 
>