Messages in this thread | | | Subject | Re: [PATCH v4 05/10] iio: afe: rescale: add INT_PLUS_{MICRO,NANO} support | From | Peter Rosin <> | Date | Sat, 10 Jul 2021 10:14:20 +0200 |
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On 2021-07-09 21:30, Liam Beguin wrote: > On Fri Jul 9, 2021 at 12:29 PM EDT, Peter Rosin wrote: >> >> >> On 2021-07-06 18:09, Liam Beguin wrote: >>> From: Liam Beguin <lvb@xiphos.com> >>> >>> Add IIO_VAL_INT_PLUS_{NANO,MICRO} scaling support. >>> Scale the integer part and the decimal parts individually and keep the >>> original scaling type. >>> >>> Signed-off-by: Liam Beguin <lvb@xiphos.com> >>> --- >>> drivers/iio/afe/iio-rescale.c | 8 ++++++++ >>> 1 file changed, 8 insertions(+) >>> >>> diff --git a/drivers/iio/afe/iio-rescale.c b/drivers/iio/afe/iio-rescale.c >>> index ba3bdcc69b16..1d0e24145d87 100644 >>> --- a/drivers/iio/afe/iio-rescale.c >>> +++ b/drivers/iio/afe/iio-rescale.c >>> @@ -89,7 +89,15 @@ static int rescale_read_raw(struct iio_dev *indio_dev, >>> do_div(tmp, 1000000000LL); >>> *val = tmp; >>> return ret; >>> + case IIO_VAL_INT_PLUS_NANO: >>> + case IIO_VAL_INT_PLUS_MICRO: >>> + tmp = (s64)*val * rescale->numerator; >>> + *val = div_s64(tmp, rescale->denominator); >>> + tmp = (s64)*val2 * rescale->numerator; >>> + *val2 = div_s64(tmp, rescale->denominator); >> > > Hi Peter, > >> Hi! >> >> You are losing precision, and you are not mormalising after the >> calculation. > > Can you elaborate a little on what you mean here? > > Do you mean that I should make sure that *val2, the PLUS_{NANO,MICRO} > part, doesn't contain an integer part? And if so transfer that part back > to *val?
Yes. On 32-bit, you will easily wrap, especially for PLUS_NANO. You'd only need a scale factor of 10 or so and a fractional part above .5 to hit the roof (10 * 500000000 > 2^32).
But I also mean that you are losing precision when you are scaling the integer part and the fractional part separately. That deserves at least a comment, but ideally it should be handled correctly.
>> I think it's better to not even attempt this given that the results can >> be >> really poor. > > Unfortunatelly, I'm kinda stuck with this as some of my ADC use these > types.
Ok. Crap. :-)
Cheers, Peter
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