Re: [PATCH v6 05/13] iio: afe: rescale: add INT_PLUS_{MICRO,NANO} support

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

On Fri Jul 30, 2021 at 2:49 AM EDT, Peter Rosin wrote:
> On 2021-07-29 17:56, Liam Beguin wrote:
> > On Wed Jul 28, 2021 at 3:19 AM EDT, Peter Rosin wrote:
> >> On 2021-07-28 02:21, Liam Beguin wrote:
> >>> On Fri Jul 23, 2021 at 5:16 PM EDT, Peter Rosin wrote:
> >>>> On 2021-07-21 05:06, 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 | 14 ++++++++++++++
> >>>>>  1 file changed, 14 insertions(+)
> >>>>>
> >>>>> diff --git a/drivers/iio/afe/iio-rescale.c b/drivers/iio/afe/iio-rescale.c
> >>>>> index d0669fd8eac5..2b73047365cc 100644
> >>>>> --- a/drivers/iio/afe/iio-rescale.c
> >>>>> +++ b/drivers/iio/afe/iio-rescale.c
> >>>>> @@ -41,6 +41,20 @@ int rescale_process_scale(struct rescale *rescale, int scale_type,
> >>>>>  		do_div(tmp, 1000000000LL);
> >>>>>  		*val = tmp;
> >>>>>  		return scale_type;
> >>>>> +	case IIO_VAL_INT_PLUS_NANO:
> >>>>> +		tmp = ((s64)*val * 1000000000LL + *val2) * rescale->numerator;
> >>>>> +		tmp = div_s64(tmp, rescale->denominator);
> >>>>> +
> >>>>> +		*val = div_s64(tmp, 1000000000LL);
> >>>>> +		*val2 = tmp - *val * 1000000000LL;
> >>>>> +		return scale_type;
> >>>
> >>> Hi Peter,
> >>>
> >>>>
> >>>> Hi!
> >>>>
> >>>> My objection from v5 still stands. Did you forget or did you simply send
> >>>> the
> >>>> wrong patch?
> >>>
> >>> Apologies, again I didn't mean to make it seem like I ignored your comments.
> >>> I tried your suggestion, but had issues when *val2 would overflow into
> >>> the integer part.
> > 
> > Hi Peter,
> > 
> >>
> >> Not saying anything about it not working does indeed make it seem like
> >> you
> >> ignored it :-) Or did I just miss where you said this? Anyway, no
> >> problem,
> >> it can be a mess dealing with a string of commits when there are
> >> numerous
> >> things to take care of between each iteration. And it's very easy to
> >> burn
> >> out and just back away. Please don't do that!
> > 
> > It was my mistake. Thanks for the encouragement :-)
> > 
> >>
> >>> Even though what I has was more prone to integer overflow with the first
> >>> multiplication, I thought it was still a valid solution as it passed the
> >>> tests.
> >>
> >> I did state that you'd need to add overflow handling from the fraction
> >> calculation and handling for negative values, so it was no surprise that
> >> my original sketchy suggestion didn't work as-is.
> >>
> >>>
> >>>>
> >>>> Untested suggestion, this time handling negative values and
> >>>> canonicalizing any
> >>>> overflow from the fraction calculation.
> >>>>
> >>>> neg = *val < 0 || *val2 < 0;
> >>>> tmp = (s64)abs(*val) * rescale->numerator;
> >>>> rem = do_div(tmp, rescale->denominator);
> >>>> *val = tmp;
> >>>> tmp = rem * 1000000000LL + (s64)abs(*val2) * rescale->numerator;
> >>>> do_div(tmp, rescale->denominator);
> >>>> *val2 = do_div(tmp, 1000000000LL);
> >>>> *val += tmp;
> >>>> if (neg) {
> >>>> if (*val < 0)
> >>>> *val = -*val;
> >>>> else
> >>>> *val2 = -*val;
> >>
> >> This last line should of course be *val2 = -*val2;
> >> Sorry.
> >>
> >>>
> >>> I'll look into this suggestion.
> >>
> >> Thanks!
> >>
> > 
> > Starting from what you suggested, here's what I came up with.
> > I also added a few test cases to cover corner cases.
> > 
> > 	if (scale_type == IIO_VAL_INT_PLUS_NANO)
> > 		mult = 1000000000LL;
> > 	else
> > 		mult = 1000000LL;
> > 	/*
> > 	 * For IIO_VAL_INT_PLUS_{MICRO,NANO} scale types if *val OR
> > 	 * *val2 is negative the schan scale is negative
> > 	 */
> > 	neg = *val < 0 || *val2 < 0;
> > 
> > 	tmp = (s64)abs(*val) * (s32)abs(rescale->numerator);
>
> Small nit, but I think abs() returns a signed type compatible
> with the argument type. I.e. (s32)abs(rescale->...) where both
> numerator and denominator are already s32 could just as well
> be written without the cast as plain old abs(rescale->...)

Understood, I'll get rid of the redundant typecasts

>
>
> > 	*val = div_s64_rem(tmp, (s32)abs(rescale->denominator), &rem);
> > 
> > 	tmp = (s64)rem * mult +
> > 		(s64)abs(*val2) * (s32)abs(rescale->numerator);
> > 	tmp = div_s64(tmp, (s32)abs(rescale->denominator));
> > 
> > 	*val += div_s64_rem(tmp, mult, val2);
> > 
> > 	/*
> > 	 * If the schan scale or only one of the rescaler elements is
> > 	 * negative, the combined scale is negative.
> > 	 */
> > 	if (neg || ((rescale->numerator < 0) ^ (rescale->denominator < 0)))
> > 		*val = -*val;
>
> Unconditionally negating *val doesn't negate the combined value when
> *val is zero and *val2 isn't. My test "if (*val < 0)" above attempting
> to take care of this case is clearly not right. It should of course be
> "if (*val > 0)" since *val is not yet negated. Duh!

Oh I see, thanks for pointing that out. Since at that point *val can't
be negative because of all the abs() calls, we could also just check
that *val is not zero.

>
> In fact, I think a few tests scaling to/from the [-1,1] interval
> would be benefitial for this exact reason.

Sounds good, I'll add a few more cases for this.
Thanks,
Liam

>
> Cheers,
> Peter