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 <[email protected]>
---
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);
tmp = (s64)rem * 1000000;
dac->ch_data[ch].offset_dec = div_s64(tmp, span);
}
--
2.30.2
On Wed, Dec 22, 2021 at 5:59 PM Muhammad Usama Anjum
<[email protected]> 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.
...
> dac->ch_data[ch].scale_dec = DIV_ROUND_CLOSEST((s64)rem * 1000000,
> 65536);
Shouldn't the above be fixed as well? Has anybody tried to compile on
32-bit arch this?
> - 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);
> tmp = (s64)rem * 1000000;
> dac->ch_data[ch].offset_dec = div_s64(tmp, span);
> }
--
With Best Regards,
Andy Shevchenko
On 12/22/21 11:49 PM, Andy Shevchenko wrote:
> On Wed, Dec 22, 2021 at 5:59 PM Muhammad Usama Anjum
> <[email protected]> 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.
>
> ...
>
>> dac->ch_data[ch].scale_dec = DIV_ROUND_CLOSEST((s64)rem * 1000000,
>> 65536);
>
> Shouldn't the above be fixed as well? Has anybody tried to compile on
> 32-bit arch this?
No, it correct already. In this case, rem is being typecasted to s64 and
then multiplied with a 32-bit number, 1000000. Thus 64-bit arithmetic is
being performed here.
On 12/23/21 5:34 PM, Muhammad Usama Anjum wrote:
> On 12/22/21 11:49 PM, Andy Shevchenko wrote:
>> On Wed, Dec 22, 2021 at 5:59 PM Muhammad Usama Anjum
>> <[email protected]> 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.
>> ...
>>
>>> dac->ch_data[ch].scale_dec = DIV_ROUND_CLOSEST((s64)rem * 1000000,
>>> 65536);
>> Shouldn't the above be fixed as well? Has anybody tried to compile on
>> 32-bit arch this?
> No, it correct already. In this case, rem is being typecasted to s64 and
> then multiplied with a 32-bit number, 1000000. Thus 64-bit arithmetic is
> being performed here.
What Andy means is that this needs to be DIV_S64_ROUND_CLOSEST() to work
on 32-bit platforms. But it is clearly unrelated to your change and
should be in its own patch.
> >>> 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.
> >> ...
> >>
> >>> dac->ch_data[ch].scale_dec = DIV_ROUND_CLOSEST((s64)rem *
> 1000000,
> >>> 65536);
> >> Shouldn't the above be fixed as well? Has anybody tried to compile on
> >> 32-bit arch this?
> > No, it correct already. In this case, rem is being typecasted to s64
> > and then multiplied with a 32-bit number, 1000000. Thus 64-bit
> > arithmetic is being performed here.
>
> What Andy means is that this needs to be DIV_S64_ROUND_CLOSEST() to
> work on 32-bit platforms. But it is clearly unrelated to your change and should
> be in its own patch.
Indeed, I didn't test it on 32 bits.
But both changes make sense to me.
Regards,
Mihail
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 <[email protected]>
> ---
> 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?
But I just don't see the point of this change. Presumably it is to make
a static analyzer happy?
regards,
dan carpenter
> From: Dan Carpenter <[email protected]>
> Sent: Wednesday, January 5, 2022 2:39 PM
> To: Muhammad Usama Anjum <[email protected]>
> Cc: Lars-Peter Clausen <[email protected]>; Hennerich, Michael
> <[email protected]>; Jonathan Cameron
> <[email protected]>; Chindris, Mihail <[email protected]>;
> open list:IIO SUBSYSTEM AND DRIVERS <[email protected]>;
> open list <[email protected]>; [email protected];
> [email protected]
> 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
> <[email protected]>
> > ---
> > 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 S?