Messages in this thread | | | Date | Fri, 16 Jul 2021 15:18:33 -0400 | Subject | Re: [PATCH v5 05/10] iio: afe: rescale: add INT_PLUS_{MICRO,NANO} support | From | "Liam Beguin" <> |
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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@xiphos.com> > > > > 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@xiphos.com> > > --- > > 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: > >
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