Re: [PATCH v6 09/13] iio: afe: rescale: fix precision on fractional log scale

["Liam Beguin" <liambeguin@gmail.com>]

On Wed Jul 28, 2021 at 3:58 AM EDT, Peter Rosin wrote:
> On 2021-07-28 02:26, Liam Beguin wrote:
> > On Fri Jul 23, 2021 at 5:20 PM EDT, Peter Rosin wrote:
> >> On 2021-07-21 05:06, Liam Beguin wrote:
> >>> From: Liam Beguin <lvb@xiphos.com>
> >>>
> >>> The IIO_VAL_FRACTIONAL_LOG2 scale type doesn't return the expected
> >>> scale. Update the case so that the rescaler returns a fractional type
> >>> and a more precise scale.
> >>>
> >>> Signed-off-by: Liam Beguin <lvb@xiphos.com>
> >>> ---
> >>>  drivers/iio/afe/iio-rescale.c | 9 +++------
> >>>  1 file changed, 3 insertions(+), 6 deletions(-)
> >>>
> >>> diff --git a/drivers/iio/afe/iio-rescale.c b/drivers/iio/afe/iio-rescale.c
> >>> index 35fa3b4e53e0..47cd4a6d9aca 100644
> >>> --- a/drivers/iio/afe/iio-rescale.c
> >>> +++ b/drivers/iio/afe/iio-rescale.c
> >>> @@ -44,12 +44,9 @@ int rescale_process_scale(struct rescale *rescale, int scale_type,
> >>>  		*val2 = rescale->denominator;
> >>>  		return IIO_VAL_FRACTIONAL;
> >>>  	case IIO_VAL_FRACTIONAL_LOG2:
> >>> -		tmp = *val * 1000000000LL;
> >>> -		do_div(tmp, rescale->denominator);
> >>> -		tmp *= rescale->numerator;
> >>> -		do_div(tmp, 1000000000LL);
> >>> -		*val = tmp;
> >>> -		return scale_type;
> >>> +		*val = rescale->numerator * *val;
> >>> +		*val2 = rescale->denominator * (1 << *val2);
> >>> +		return IIO_VAL_FRACTIONAL;
> >>
> >> Hi!
> > 
> > Hi Peter,
> > 
> >>
> >> I do not think this is an uncontested improvement. You have broken the
> >> case
> >> where *val2 is "large" before the scale factor is applied.
> > 
> > I was a little reluctant to add this change as I keep increasing the
> > scope of this series, but since I added tests for all cases, I didn't
> > want to leave this one out.
>
> > Would you rather I drop this patch and the test cases associated to it?
>
> Why drop the tests? Are they doing any harm? Or are they testing exactly
> the problem situation that fail without this patch?

They are testing this problem and fail without the patch.

>
> In that case, I guess fix the tests to pass and preferably add tests
> for the *val2 is "large" situation (that this patch breaks) so that the
> next person trying to improve precision is made aware of the overflow
> problem. Does that make sense?

To handle large values of *val2, I could use the same logic as in
IIO_VAL_FRACTIONAL with check_mul_overflow() and gcd().

would that be okay?

Thanks,
Liam

>
> Cheers,
> Peter
>
> > Thanks,
> > Liam
> > 
> >>
> >> Cheers,
> >> Peter
> >>
> >>>  	case IIO_VAL_INT_PLUS_NANO:
> >>>  		tmp = ((s64)*val * 1000000000LL + *val2) * rescale->numerator;
> >>>  		tmp = div_s64(tmp, rescale->denominator);
> >>>
> >