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

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Date:   Tue, 27 Jul 2021 20:21:48 -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 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.
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.

>
> 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;

I'll look into this suggestion.

> }
>
> > +	case IIO_VAL_INT_PLUS_MICRO:
> > +		tmp = ((s64)*val * 1000000LL + *val2) * rescale->numerator;
> > +		tmp = div_s64(tmp, rescale->denominator);
> > +
> > +		*val = div_s64(tmp, 1000000);
>
> Why do you not have the LL suffix here?

Doesnt' LL make it into a 64 bit integer?
I left it out because the second parameter of div_s64() should be s32.

Thanks,
Liam

>
> Cheers,
> Peter
>
> > +		*val2 = tmp - *val * 1000000;
> > +		return scale_type;
> >  	default:
> >  		return -EOPNOTSUPP;
> >  	}
> >