> From: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
> Sent: Wednesday, January 5, 2022 2:39 PM
> To: Muhammad Usama Anjum <usama.anjum@xxxxxxxxxxxxx>
> Cc: Lars-Peter Clausen <lars@xxxxxxxxxx>; Hennerich, Michael
> <Michael.Hennerich@xxxxxxxxxx>; Jonathan Cameron
> <jic23@xxxxxxxxxx>; Chindris, Mihail <Mihail.Chindris@xxxxxxxxxx>;
> open list:IIO SUBSYSTEM AND DRIVERS <linux-iio@xxxxxxxxxxxxxxx>;
> open list <linux-kernel@xxxxxxxxxxxxxxx>; kernel@xxxxxxxxxxxxx;
> kernel-janitors@xxxxxxxxxxxxxxx
> Subject: Re: [PATCH] drivers:iio:dac make expression evaluation 64-bit
>
> [External]
>
> On Wed, Dec 22, 2021 at 12:20:32AM +0500, Muhammad Usama Anjum
> wrote:
> > Two 32-bit values are being evaluated using 32-bit arithmetic and
> then
> > passed to s64 type. It is wrong. Expression should be evaluated using
> > 64-bit arithmetic and then passed.
> >
> > Fixes: 8f2b54824b ("drivers:iio:dac: Add AD3552R driver support")
> > Signed-off-by: Muhammad Usama Anjum
> <usama.anjum@xxxxxxxxxxxxx>
> > ---
> > drivers/iio/dac/ad3552r.c | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/iio/dac/ad3552r.c b/drivers/iio/dac/ad3552r.c
> > index 97f13c0b9631..b03d3c7cd4c4 100644
> > --- a/drivers/iio/dac/ad3552r.c
> > +++ b/drivers/iio/dac/ad3552r.c
> > @@ -770,7 +770,7 @@ static void
> ad3552r_calc_gain_and_offset(struct ad3552r_desc *dac, s32 ch)
> > dac->ch_data[ch].scale_dec = DIV_ROUND_CLOSEST((s64)rem
> * 1000000,
> > 65536);
> >
> > - dac->ch_data[ch].offset_int = div_s64_rem(v_min * 65536,
> span, &rem);
> > + dac->ch_data[ch].offset_int = div_s64_rem(v_min * 65536L,
> span, &rem);
>
> "v_min" is relatively close to zero on a number line so this can't
> overflow. There is no way that this change affects anything at runtime
> (except making the code a tiny tiny bit slower).
>
> And it should be 65536LL for 32 bit systems?
>
If I'm not missing nothing obvious, 65536LL is the right thing to do...
I did not really checked, but if v_min * 65536 can never overflow,
then yeah, this is not really "fixing" nothing.
- Nuno Sá
