On Thu Jul 15, 2021 at 5:48 AM EDT, Peter Rosin wrote:
>
> On 2021-07-15 05:12, Liam Beguin wrote:
> > From: Liam Beguin <lvb@xxxxxxxxxx>
> >
> > Some ADCs use IIO_VAL_INT_PLUS_{NANO,MICRO} scale types.
> > Add support for these to allow using the iio-rescaler with them.
> >
> > Signed-off-by: Liam Beguin <lvb@xxxxxxxxxx>
> > ---
> > drivers/iio/afe/iio-rescale.c | 15 +++++++++++++++
> > 1 file changed, 15 insertions(+)
> >
> > diff --git a/drivers/iio/afe/iio-rescale.c b/drivers/iio/afe/iio-rescale.c
> > index 4c3cfd4d5181..a2b220b5ba86 100644
> > --- a/drivers/iio/afe/iio-rescale.c
> > +++ b/drivers/iio/afe/iio-rescale.c
> > @@ -92,7 +92,22 @@ static int rescale_read_raw(struct iio_dev *indio_dev,
> > do_div(tmp, 1000000000LL);
> > *val = tmp;
> > return ret;
> > + case IIO_VAL_INT_PLUS_NANO:
> > + tmp = ((s64)*val * 1000000000LL + *val2) * rescale->numerator;
> > + do_div(tmp, rescale->denominator);
> > +
> > + *val = div_s64(tmp, 1000000000LL);
> > + *val2 = tmp - *val * 1000000000LL;
> > + return ret;
>
> This is too simplistic and prone to overflow. We need something like
> this
> (untested)
>
> tmp = (s64)*val * rescale->numerator;
> rem = do_div(tmp, rescale->denominator);
> *val = tmp;
> tmp = ((s64)rem * 1000000000LL + (s64)*val2) * rescale->numerator;
> do_div(tmp, rescale->denominator);
> *val2 = tmp;
>
> Still not very safe with numerator and denominator both "large", but
> much
> better. And then we need normalizing the fraction part after the above,
> of
> course.
>
Understood, I'll test that.
> And, of course, I'm not sure what *val == -1 and *val2 == 500000000
> really
> means. Is that -1.5 or -0.5? The above may very well need adjusting for
> negative values...
>
I would've assumed the correct answer is -1 + 500000000e-9 = -0.5
but adding a test case to iio-test-format.c seems to return -1.5...
I believe that's a bug but we can work around if for now by moving the
integer part of *val2 to *val.
Liam
> Cheers,
> Peter
>
> > + case IIO_VAL_INT_PLUS_MICRO:
> > + tmp = ((s64)*val * 1000000LL + *val2) * rescale->numerator;
> > + do_div(tmp, rescale->denominator);
> > +
> > + *val = div_s64(tmp, 1000000LL);
> > + *val2 = tmp - *val * 1000000LL;
> > + return ret;
> > default:
> > + dev_err(&indio_dev->dev, "unsupported type %d\n", ret);
> > return -EOPNOTSUPP;
> > }
> > default:
> >
