linux-kernel - Re: [PATCH v5 05/10] iio: afe: rescale: add INT_PLUS

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Date:   Mon, 19 Jul 2021 11:15:29 -0400
From:   "Liam Beguin" <liambeguin@...il.com>
To:     "Peter Rosin" <peda@...ntia.se>,
        "Jonathan Cameron" <jic23@...nel.org>
Cc:     <lars@...afoo.de>, <pmeerw@...erw.net>,
        <linux-kernel@...r.kernel.org>, <linux-iio@...r.kernel.org>,
        <devicetree@...r.kernel.org>, <robh+dt@...nel.org>
Subject: Re: [PATCH v5 05/10] iio: afe: rescale: add INT_PLUS_{MICRO,NANO}
 support

On Mon Jul 19, 2021 at 4:31 AM EDT, Peter Rosin wrote:
>
>
> On 2021-07-19 01:44, Liam Beguin wrote:
> > On Sat Jul 17, 2021 at 12:55 PM EDT, Jonathan Cameron wrote:
> >> On Fri, 16 Jul 2021 15:18:33 -0400
> >> "Liam Beguin" <liambeguin@...il.com> wrote:
> >>
> >>> On Thu Jul 15, 2021 at 5:48 AM EDT, Peter Rosin wrote:
> >>>>
> >>>> On 2021-07-15 05:12, 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 | 15 +++++++++++++++
> >>>>>  1 file changed, 15 insertions(+)
> >>>>>
> >>>>> diff --git a/drivers/iio/afe/iio-rescale.c b/drivers/iio/afe/iio-rescale.c
> >>>>> index 4c3cfd4d5181..a2b220b5ba86 100644
> >>>>> --- a/drivers/iio/afe/iio-rescale.c
> >>>>> +++ b/drivers/iio/afe/iio-rescale.c
> >>>>> @@ -92,7 +92,22 @@ static int rescale_read_raw(struct iio_dev *indio_dev,
> >>>>>  			do_div(tmp, 1000000000LL);
> >>>>>  			*val = tmp;
> >>>>>  			return ret;
> >>>>> +		case IIO_VAL_INT_PLUS_NANO:
> >>>>> +			tmp = ((s64)*val * 1000000000LL + *val2) * rescale->numerator;
> >>>>> +			do_div(tmp, rescale->denominator);
> >>>>> +
> >>>>> +			*val = div_s64(tmp, 1000000000LL);
> >>>>> +			*val2 = tmp - *val * 1000000000LL;
> >>>>> +			return ret;  
> >>>>
> >>>> This is too simplistic and prone to overflow. We need something like
> >>>> this
> >>>> (untested)
> >>>>
> >>>> tmp = (s64)*val * rescale->numerator;
> >>>> rem = do_div(tmp, rescale->denominator);
> >>>> *val = tmp;
> >>>> tmp = ((s64)rem * 1000000000LL + (s64)*val2) * rescale->numerator;
> >>>> do_div(tmp, rescale->denominator);
> >>>> *val2 = tmp;
> >>>>
> >>>> Still not very safe with numerator and denominator both "large", but
> >>>> much
> >>>> better. And then we need normalizing the fraction part after the above,
> >>>> of
> >>>> course.
> >>>>  
> >>>
> >>> Understood, I'll test that.
> >>>
> >>>> And, of course, I'm not sure what *val == -1 and *val2 == 500000000
> >>>> really
> >>>> means. Is that -1.5 or -0.5? The above may very well need adjusting for
> >>>> negative values...
> >>>>  
> >>>
> >>> I would've assumed the correct answer is -1 + 500000000e-9 = -0.5
> >>> but adding a test case to iio-test-format.c seems to return -1.5...
> >>
> > 
> > Hi Jonathan,
> > 
> >> No. -1.5 is as intended, though the IIO_VAL_PLUS_MICRO is rather
> >> confusing
> >> naming :( We should perhaps add more documentation for that. Signs were
> >> always a bit of a pain with this two integer scheme for fixed point.
> >>
> >> The intent is to have moderately readable look up tables with the
> >> problem that
> >> we don't have a signed 0 available. Meh, maybe this decision a long time
> >> back wasn't a the right one, but it may be a pain to change now as too
> >> many
> >> drivers to check!
> >>
> >> 1, 0000000 == 1
> >> 0, 5000000 == 0.5
> >> 0, 0000000 == 0
> >> 0, -5000000 == -0.5
> >> -1, 5000000 == -1.5
> >>
> > 
> > Understood, thanks for clearing that out.

Hi Peter,

>
> I just realized that do_div assumes unsigned operands...
>
> :-(

I noticed the same thing after adding the kunit tests.
I added patches for that.

For IIO_VAL_PLUS_{MICRO,NANO} specifically, I have something working but
I like your approach better so I'll work on it a little more.

Thanks,
Liam

>
> Cheers,
> Peter