Re: [PATCH] iio: pressure: bmp280: fix relative humidity unit

[Tomasz Duszynski <tduszyns@gmail.com>]

On Wed, May 30, 2018 at 03:20:37PM +0800, Matt Ranostay wrote:
> On Wed, May 30, 2018 at 1:05 PM, Matt Ranostay
> <matt.ranostay@konsulko.com> wrote:
> > On Wed, May 30, 2018 at 9:17 AM, Phil Reid <preid@electromag.com.au> wrote:
> >> On 30/05/2018 08:44, Matt Ranostay wrote:
> >>>
> >>> On Mon, May 28, 2018 at 11:38 PM, Tomasz Duszynski <tduszyns@gmail.com>
> >>> wrote:
> >>>>
> >>>> According to IIO ABI relative humidity reading should be
> >>>> returned in milli percent.
> >>>>
> >>>> This patch addresses that by applying proper scaling and
> >>>> returning integer instead of fractional format type specifier.
> >>>>
> >>>
> >>> *sigh* seems this is my mistake, but good catch.  Slight nitpick
> >>> below.. otherwise looks good
> >>>
> >>>> Signed-off-by: Tomasz Duszynski <tduszyns@gmail.com>
> >>>> ---
> >>>>   drivers/iio/pressure/bmp280-core.c | 5 ++---
> >>>>   1 file changed, 2 insertions(+), 3 deletions(-)
> >>>>
> >>>> diff --git a/drivers/iio/pressure/bmp280-core.c
> >>>> b/drivers/iio/pressure/bmp280-core.c
> >>>> index 5ec3e41b65f2..fe87d27779d9 100644
> >>>> --- a/drivers/iio/pressure/bmp280-core.c
> >>>> +++ b/drivers/iio/pressure/bmp280-core.c
> >>>> @@ -415,10 +415,9 @@ static int bmp280_read_humid(struct bmp280_data
> >>>> *data, int *val, int *val2)
> >>>>          }
> >>>>          comp_humidity = bmp280_compensate_humidity(data, adc_humidity);
> >>>>
> >>>> -       *val = comp_humidity;
> >>>> -       *val2 = 1024;
> >>>> +       *val = comp_humidity * 1000 / 1024;
> >>>
> >>>
> >>> Minor nitpick  that it would look cleaner as:   (comp_humidity / 1024) *
> >>> 1000
> >>>
> >> be careful of integer division.
> >>
> >
> > Ah yes good point. You will have to check if comp_humidity isn't zero
> > or it is possible to have a divide-by-zero.
>
> D'oh actually divide by zero would be an issue. But you'll want to be
> sure of possible overflows (doubt that would be an issue here).

What kind of division-by-zero are you meaning? When would that happen
in this case?

>
> However it may be better to just add the scaling factor of 1000 with
> IIO_CHAN_INFO_SCALE  and make the processed value now a raw one.

Is measurement precision your concern here? I would not bother with that
since the sensor is not a top-notch anyway. Even datasheet itself specifies
error margin of +/-3% for relative humidity.

>
> >
> > - Matt
> >
> >>
> >>>>
> >>>> -       return IIO_VAL_FRACTIONAL;
> >>>> +       return IIO_VAL_INT;
> >>>>   }
> >>>>
> >>>>   static int bmp280_read_raw(struct iio_dev *indio_dev,
> >>>> --
> >>>> 2.17.0
> >>
> >>
> >>>
> >>
> >>
> >> --
> >> Regards
> >> Phil Reid
> >>