Re: [PATCH v8 08/14] iio: afe: rescale: reduce risk of integer overflow

[Liam Beguin <liambeguin@gmail.com>]

On Thu, Aug 26, 2021 at 11:13:38AM +0200, Peter Rosin wrote:
> On 2021-08-20 21:17, Liam Beguin wrote:
> > From: Liam Beguin <lvb@xiphos.com>
> > 
> > Reduce the risk of integer overflow by doing the scale calculation on
> > a 64-bit integer. Since the rescaling is only performed on *val, reuse
> > the IIO_VAL_FRACTIONAL_LOG2 case.
> 
> While this patch certainly helps with overflow problems, it also
> potentially kills precision in some cases where there currently are
> no overflow issues.
> 
> E.g. this patch transforms 5/32768 scaled by 3/10000 from the exact
> 
> 15 / 327680000 (0.0000000457763671875)
> 
> to the heavily truncated plain old sorry "zero".
> 
> Sure, 9/14 improves the situation, but patch 9/14 simply cannot
> make this example any better than returning 2 significant digits
> since the value is so small.

The 100 ppm check introduced in 09/14 is really objective and might not
be the best choice. Changing it to

	- if (abs(rem) > 10000000 && abs(div64_s64(*val, tmp)) < 100) {
	+ if (abs(rem)) {

Helps with the precision issues you brought up here, and in 09/14.
I was originally trying to keep the original scale as much as possible,
I'll continue the rest of the discussion on the 09/14 thread we already
have.

> 
> Side note, there is also the same type of risk of overflow for
> IIO_VAL_INT. Why does that case not get the same treatment as
> IIO_VAL_FRACTIONAL?
> 

Being totally honest, I noticed we have the same issue with IIO_VAL_INT,
but since I didn't run into the issue on my setup I left it out to focus
on getting the rest cleaned up.

I guess it couldn't hurt to fix that too while we're at it.
I'll work on it!

> But again, I see no elegant solution. The best I can think of is the
> inelegant solution to provide extra info on the input range, the
> exact desired scaling method, the desired output type, some mix of
> all of the above or something else that helps determining the
> appropriate scaling method w/o looking at the individual number.

I don't really like having to add a range parameter.
If changing the scale type dynamically isn't an issue, I think we can
get away with not adding a parameter.
If it is an issue, we might have to look into it...

Thanks,
Liam

> 
> Cheers,
> Peter
> 
> > Signed-off-by: Liam Beguin <lvb@xiphos.com>
> > ---
> >  drivers/iio/afe/iio-rescale.c | 5 +----
> >  1 file changed, 1 insertion(+), 4 deletions(-)
> > 
> > diff --git a/drivers/iio/afe/iio-rescale.c b/drivers/iio/afe/iio-rescale.c
> > index 809e966f7058..c408c4057c08 100644
> > --- a/drivers/iio/afe/iio-rescale.c
> > +++ b/drivers/iio/afe/iio-rescale.c
> > @@ -27,16 +27,13 @@ int rescale_process_scale(struct rescale *rescale, int scale_type,
> >  	u32 neg;
> >  
> >  	switch (scale_type) {
> > -	case IIO_VAL_FRACTIONAL:
> > -		*val *= rescale->numerator;
> > -		*val2 *= rescale->denominator;
> > -		return scale_type;
> >  	case IIO_VAL_INT:
> >  		*val *= rescale->numerator;
> >  		if (rescale->denominator == 1)
> >  			return scale_type;
> >  		*val2 = rescale->denominator;
> >  		return IIO_VAL_FRACTIONAL;
> > +	case IIO_VAL_FRACTIONAL:
> >  	case IIO_VAL_FRACTIONAL_LOG2:
> >  		tmp = (s64)*val * 1000000000LL;
> >  		tmp = div_s64(tmp, rescale->denominator);
> > 
>