linux-kernel - Re: [PATCH v6 05/13] iio: afe: rescale: add INT_PLUS

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Message-Id: <CD6QQPL9EHY9.YIM3VQZ89I9P@shaak>
Date:   Fri, 30 Jul 2021 16:01:47 -0400
From:   "Liam Beguin" <liambeguin@...il.com>
To:     "Peter Rosin" <peda@...ntia.se>, <jic23@...nel.org>,
        <lars@...afoo.de>, <pmeerw@...erw.net>
Cc:     <linux-kernel@...r.kernel.org>, <linux-iio@...r.kernel.org>,
        <devicetree@...r.kernel.org>, <robh+dt@...nel.org>
Subject: Re: [PATCH v6 05/13] iio: afe: rescale: add INT_PLUS_{MICRO,NANO}
 support

On Fri Jul 30, 2021 at 3:01 AM EDT, Peter Rosin wrote:
> On 2021-07-30 08:49, 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@...hos.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@...hos.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->...)
> > 
> > 
> >> 	*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)))
>
> Hang on, that's not right. If the value and only one of the rescaler
> elements is negative, the result is positive. || is not the correct
> logical operation.
>
> >> 		*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!
> > 
> > In fact, I think a few tests scaling to/from the [-1,1] interval
> > would be benefitial for this exact reason.
>
> So, with both these issues taken care of:
>
> if (neg ^ ((rescale->numerator < 0) ^ (rescale->denominator < 0))) {
> if (*val > 0)
> *val = -*val;
> else
> *val2 = -*val2;
> }
>
> (bitwise ^ is safe since all operands come from logical operations, i.e.
> they are either zero or one and nothing else)

You're right, this should've been a ^ from the start.

Thanks,
Liam

>
> Cheers,
> Peter