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Date:   Fri, 30 Jul 2021 15:57:40 -0400
Message-Id: <CD6QNK3LBY59.DKBWN8531DNM@shaak>
Cc:     <linux-kernel@vger.kernel.org>, <linux-iio@vger.kernel.org>,
        <devicetree@vger.kernel.org>, <robh+dt@kernel.org>
Subject: Re: [PATCH v6 05/13] iio: afe: rescale: add INT_PLUS_{MICRO,NANO}
 support
From:   "Liam Beguin" <liambeguin@gmail.com>
To:     "Peter Rosin" <peda@axentia.se>, <jic23@kernel.org>,
        <lars@metafoo.de>, <pmeerw@pmeerw.net>
References: <20210721030613.3105327-1-liambeguin@gmail.com>
 <20210721030613.3105327-6-liambeguin@gmail.com>
 <c9d77dc0-7f4c-0df0-cce1-8cb30074e115@axentia.se>
 <CD4CE5OQT5TJ.2BFPBRYK7FCOW@shaak>
 <18f749be-284f-3342-a6d2-b42aa39fc13a@axentia.se>
 <CD5QWB9MW1H5.3SSPWGD5DR6J5@shaak>
 <a77e7a18-67dc-5bc5-427b-89d6b1e82b85@axentia.se>
In-Reply-To: <a77e7a18-67dc-5bc5-427b-89d6b1e82b85@axentia.se>
Precedence: bulk

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-re=
scale.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 *rescal=
e, int scale_type,
> >>>>>  		do_div(tmp, 1000000000LL);
> >>>>>  		*val =3D tmp;
> >>>>>  		return scale_type;
> >>>>> +	case IIO_VAL_INT_PLUS_NANO:
> >>>>> +		tmp =3D ((s64)*val * 1000000000LL + *val2) * rescale->numerator;
> >>>>> +		tmp =3D div_s64(tmp, rescale->denominator);
> >>>>> +
> >>>>> +		*val =3D div_s64(tmp, 1000000000LL);
> >>>>> +		*val2 =3D 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 co=
mments.
> >>> I tried your suggestion, but had issues when *val2 would overflow int=
o
> >>> the integer part.
> >=20
> > Hi Peter,
> >=20
> >>
> >> 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!
> >=20
> > It was my mistake. Thanks for the encouragement :-)
> >=20
> >>
> >>> Even though what I has was more prone to integer overflow with the fi=
rst
> >>> 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 th=
at
> >> 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 =3D *val < 0 || *val2 < 0;
> >>>> tmp =3D (s64)abs(*val) * rescale->numerator;
> >>>> rem =3D do_div(tmp, rescale->denominator);
> >>>> *val =3D tmp;
> >>>> tmp =3D rem * 1000000000LL + (s64)abs(*val2) * rescale->numerator;
> >>>> do_div(tmp, rescale->denominator);
> >>>> *val2 =3D do_div(tmp, 1000000000LL);
> >>>> *val +=3D tmp;
> >>>> if (neg) {
> >>>> if (*val < 0)
> >>>> *val =3D -*val;
> >>>> else
> >>>> *val2 =3D -*val;
> >>
> >> This last line should of course be *val2 =3D -*val2;
> >> Sorry.
> >>
> >>>
> >>> I'll look into this suggestion.
> >>
> >> Thanks!
> >>
> >=20
> > Starting from what you suggested, here's what I came up with.
> > I also added a few test cases to cover corner cases.
> >=20
> > 	if (scale_type =3D=3D IIO_VAL_INT_PLUS_NANO)
> > 		mult =3D 1000000000LL;
> > 	else
> > 		mult =3D 1000000LL;
> > 	/*
> > 	 * For IIO_VAL_INT_PLUS_{MICRO,NANO} scale types if *val OR
> > 	 * *val2 is negative the schan scale is negative
> > 	 */
> > 	neg =3D *val < 0 || *val2 < 0;
> >=20
> > 	tmp =3D (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 =3D div_s64_rem(tmp, (s32)abs(rescale->denominator), &rem);
> >=20
> > 	tmp =3D (s64)rem * mult +
> > 		(s64)abs(*val2) * (s32)abs(rescale->numerator);
> > 	tmp =3D div_s64(tmp, (s32)abs(rescale->denominator));
> >=20
> > 	*val +=3D div_s64_rem(tmp, mult, val2);
> >=20
> > 	/*
> > 	 * 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 =3D -*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