Re: [PATCH] iio: chemical: ccs811: Fix output of IIO_CONCENTRATION channels

[Jonathan Cameron <jic23@kernel.org>]

On Sun, 10 Dec 2017 13:11:51 -0800
Matt Ranostay <mranostay@gmail.com> wrote:

> On Sun, Dec 10, 2017 at 10:24 AM, Jonathan Cameron <jic23@kernel.org> wrote:
> > On Wed,  6 Dec 2017 18:57:58 +0200
> > Narcisa Ana Maria Vasile <narcisaanamaria12@gmail.com> wrote:
> > +CC Matt  
> >> in_concentration_raw should report, according to sysfs-bus-iio documentation,
> >> a "Raw (unscaled no offset etc.) percentage reading of a substance."
> >>
> >> Modify scale to convert from ppm/ppb to percentage:
> >> 1 ppm = 0.0001%
> >> 1 ppb = 0.0000001%
> >>
> >> There is no offset needed to convert the ppm/ppb to percentage,
> >> so remove offset from IIO_CONCENTRATION (IIO_MOD_CO2) channel.
> >>
> >> Signed-off-by: Narcisa Ana Maria Vasile <narcisaanamaria12@gmail.com>  
> 
> Reviewed-by: Matt Ranostay <matt.ranostay@konsulko.com>
Applied to the fixes-togreg branch of iio.git and marked for stable
so hopefully we can avoid too much userspace breakage...

Thanks,

Jonathan
> 
> >> ---
> >> Details:
> >> The sensors reports data in parts per million for CO2 and parts per
> >> billion for VOC. The current scaling and offset values were meant to
> >> convert the ppm/ppb value into a percentage value with respect to
> >> the sensor's measurement range:
> >> For example:
> >>     For eCO2, possible raw values range from 400ppm to 8192ppm.
> >>     If the value reported is, let's say 500ppm, then, after scaling and
> >>     offset are applied, the percentage, with reference to the interval
> >>     [400, 8192] is 1.28%, using this interval mapping formula:
> >>     (500 - 400) * (100 - 0) / (8192 - 400) + 0. Instead, the value should
> >>     be 500 * 0.0001 = 0.05% (from ppm to percentage).
> >>
> >> Note:
> >> In the docs, the in_concentration_raw value is said to be a
> >> "raw percentage reading of a substance". The raw value from the sensor
> >> is a ppm/ppb value, not a percentage value. It becomes a percentage value
> >> after applying a scale. Is this the correct behaviour (after applying scale,
> >> a percentage value is obtained) or did I misunderstood the docs?  
> >
> > I 'think' you are right on this, but would like some input from
> > someone more familiar with chemical sensors in general.
> >
> > Matt, any comment on this?
> >  
> 
> Yes the IIO_CONCENTRATION is suppose to be in ppm/ppb units that are
> converted to percent with the scaling value.
> Basically since someday we may have a sensor that actually with a
> digit on the other the decimal point, but hopefully not really 1.28%
> CO2 since that is well into toxic zone :)
> 
> - Matt
> 
> > Thanks,
> >
> > Jonathan  
> >>
> >>  drivers/iio/chemical/ccs811.c | 13 +++----------
> >>  1 file changed, 3 insertions(+), 10 deletions(-)
> >>
> >> diff --git a/drivers/iio/chemical/ccs811.c b/drivers/iio/chemical/ccs811.c
> >> index 97bce83..fbe2431 100644
> >> --- a/drivers/iio/chemical/ccs811.c
> >> +++ b/drivers/iio/chemical/ccs811.c
> >> @@ -96,7 +96,6 @@ struct ccs811_data {
> >>               .channel2 = IIO_MOD_CO2,
> >>               .modified = 1,
> >>               .info_mask_separate = BIT(IIO_CHAN_INFO_RAW) |
> >> -                                   BIT(IIO_CHAN_INFO_OFFSET) |
> >>                                     BIT(IIO_CHAN_INFO_SCALE),
> >>               .scan_index = 0,
> >>               .scan_type = {
> >> @@ -255,24 +254,18 @@ static int ccs811_read_raw(struct iio_dev *indio_dev,
> >>                       switch (chan->channel2) {
> >>                       case IIO_MOD_CO2:
> >>                               *val = 0;
> >> -                             *val2 = 12834;
> >> +                             *val2 = 100;
> >>                               return IIO_VAL_INT_PLUS_MICRO;
> >>                       case IIO_MOD_VOC:
> >>                               *val = 0;
> >> -                             *val2 = 84246;
> >> -                             return IIO_VAL_INT_PLUS_MICRO;
> >> +                             *val2 = 100;
> >> +                             return IIO_VAL_INT_PLUS_NANO;
> >>                       default:
> >>                               return -EINVAL;
> >>                       }
> >>               default:
> >>                       return -EINVAL;
> >>               }
> >> -     case IIO_CHAN_INFO_OFFSET:
> >> -             if (!(chan->type == IIO_CONCENTRATION &&
> >> -                   chan->channel2 == IIO_MOD_CO2))
> >> -                     return -EINVAL;
> >> -             *val = -400;
> >> -             return IIO_VAL_INT;
> >>       default:
> >>               return -EINVAL;
> >>       }
> >> --
> >> 1.9.1
> >>  
> >  
> --
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