staging: iio: ad5933: Use div64_ul instead of do_div

  do_div() does a 64-by-32 division. Here the divisor is an unsigned long
which on some platforms is 64 bit wide. So use div64_ul instead of do_div
to avoid a possible truncation. Issue was identified using the
coccicheck tool.

Signed-off-by: Deepak R Varma <drv@mailo.com>
---
 drivers/staging/iio/impedance-analyzer/ad5933.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--
2.34.1
  

> -----Original Message-----
> From: Deepak R Varma <drv@mailo.com>
> Sent: Thursday, October 27, 2022 11:29 PM
> To: outreachy@lists.linux.dev; Lars-Peter Clausen <lars@metafoo.de>;
> Hennerich, Michael <Michael.Hennerich@analog.com>; Jonathan Cameron
> <jic23@kernel.org>; Greg Kroah-Hartman <gregkh@linuxfoundation.org>;
> linux-iio@vger.kernel.org; linux-staging@lists.linux.dev; linux-
> kernel@vger.kernel.org
> Subject: [PATCH] staging: iio: ad5933: Use div64_ul instead of do_div
> 
> [External]
> 
> do_div() does a 64-by-32 division. Here the divisor is an unsigned long
> which on some platforms is 64 bit wide. So use div64_ul instead of do_div
> to avoid a possible truncation. Issue was identified using the
> coccicheck tool.
> 
> Signed-off-by: Deepak R Varma <drv@mailo.com>
> ---

Reviewed-by: Nuno Sá <nuno.sa@analog.com>
  

On Fri, Oct 28, 2022 at 02:58:49AM +0530, Deepak R Varma wrote:
> do_div() does a 64-by-32 division. Here the divisor is an unsigned long
> which on some platforms is 64 bit wide. So use div64_ul instead of do_div
> to avoid a possible truncation. Issue was identified using the
> coccicheck tool.
> 
> Signed-off-by: Deepak R Varma <drv@mailo.com>
> ---
>  drivers/staging/iio/impedance-analyzer/ad5933.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/staging/iio/impedance-analyzer/ad5933.c b/drivers/staging/iio/impedance-analyzer/ad5933.c
> index f177b20f0f2d..730bb31a20d8 100644
> --- a/drivers/staging/iio/impedance-analyzer/ad5933.c
> +++ b/drivers/staging/iio/impedance-analyzer/ad5933.c
> @@ -196,7 +196,7 @@ static int ad5933_set_freq(struct ad5933_state *st,
>  	} dat;
> 
>  	freqreg = (u64)freq * (u64)(1 << 27);
> -	do_div(freqreg, st->mclk_hz / 4);
> +	freqreg = div64_ul(freqreg, st->mclk_hz / 4);
> 
>  	switch (reg) {
>  	case AD5933_REG_FREQ_START:
> --
> 2.34.1

No, this isn't ok, please read the mailing list archives for why these
changes are not going to be accepted:
	https://lore.kernel.org/r/e2ec77060cc84a33b49d5fd11d7867f6@AcuMS.aculab.com

Please always at least look at the archives of the past few weeks as to
if changes like this are able to be accepted or not.

thanks,

greg k-h
  

> > -----Original Message-----
> > From: Deepak R Varma <drv@mailo.com>
> >
> > [External]
> >
> > do_div() does a 64-by-32 division. Here the divisor is an unsigned long
> > which on some platforms is 64 bit wide. So use div64_ul instead of do_div
> > to avoid a possible truncation. Issue was identified using the
> > coccicheck tool.

These changes should all get nacked unless the domain of the values
can be shown to be out of range.

The entire point of do_div() is that because division is expensive
using a limited range division is significantly faster.

Even on Intel 64 bit cpu the 64 by 32 divide is significantly
faster then a full 64 bit divide for the same input values.

One might also question why the divisor is actually 'unsigned long'
at all. The code is almost certainly expected to compile for 32bit
so the domain of the value should fit in 32 bits.
So either the type could be unsigned int, or it really doesn't matter
that the value is truncated to 32bit because it can never be larger.

	David


> >
> > Signed-off-by: Deepak R Varma <drv@mailo.com>
> > ---
> 
> Reviewed-by: Nuno Sá <nuno.sa@analog.com>

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)
  

On Sat, Oct 29, 2022 at 09:52:40AM +0200, Greg Kroah-Hartman wrote:
> On Fri, Oct 28, 2022 at 02:58:49AM +0530, Deepak R Varma wrote:
> > do_div() does a 64-by-32 division. Here the divisor is an unsigned long
> > which on some platforms is 64 bit wide. So use div64_ul instead of do_div
> > to avoid a possible truncation. Issue was identified using the
> > coccicheck tool.
> >
> > Signed-off-by: Deepak R Varma <drv@mailo.com>
> > ---
> >  drivers/staging/iio/impedance-analyzer/ad5933.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/staging/iio/impedance-analyzer/ad5933.c b/drivers/staging/iio/impedance-analyzer/ad5933.c
> > index f177b20f0f2d..730bb31a20d8 100644
> > --- a/drivers/staging/iio/impedance-analyzer/ad5933.c
> > +++ b/drivers/staging/iio/impedance-analyzer/ad5933.c
> > @@ -196,7 +196,7 @@ static int ad5933_set_freq(struct ad5933_state *st,
> >  	} dat;
> >
> >  	freqreg = (u64)freq * (u64)(1 << 27);
> > -	do_div(freqreg, st->mclk_hz / 4);
> > +	freqreg = div64_ul(freqreg, st->mclk_hz / 4);
> >
> >  	switch (reg) {
> >  	case AD5933_REG_FREQ_START:
> > --
> > 2.34.1
>
> No, this isn't ok, please read the mailing list archives for why these
> changes are not going to be accepted:
> 	https://lore.kernel.org/r/e2ec77060cc84a33b49d5fd11d7867f6@AcuMS.aculab.com
>
> Please always at least look at the archives of the past few weeks as to
> if changes like this are able to be accepted or not.

Hello Greg,
My apologies for not looking at the lore mailing archive. I only looked at the
past git commits and found a few similar changes accepted in the past. My bad. I
will always look at the mailing archive as well going forward.

There are other review comments from the experts on similar patches. I
appreciate everyone's time and comment. I will look at those and revert
accordingly.

Thank you,
./drv
>
> thanks,
>
> greg k-h
  

On Sat, Oct 29, 2022 at 11:23:06AM +0000, David Laight wrote:
> > > -----Original Message-----
> > > From: Deepak R Varma <drv@mailo.com>
> > >
> > > [External]
> > >
> > > do_div() does a 64-by-32 division. Here the divisor is an unsigned long
> > > which on some platforms is 64 bit wide. So use div64_ul instead of do_div
> > > to avoid a possible truncation. Issue was identified using the
> > > coccicheck tool.
>
> These changes should all get nacked unless the domain of the values
> can be shown to be out of range.

Hello David,
I looked through the data sheets for these hardware [ad983*] and believe the
divisor max can attain 75MHz which fits well in a 32 bit size. Hence the
proposed changes for these drivers to switch to div64_ul may actually slowdown
the division. Please correct if I said something incorrect.

Requesting to kindly ignore my proposed changes.

>
> The entire point of do_div() is that because division is expensive
> using a limited range division is significantly faster.
>
> Even on Intel 64 bit cpu the 64 by 32 divide is significantly
> faster then a full 64 bit divide for the same input values.
>
> One might also question why the divisor is actually 'unsigned long'
> at all. The code is almost certainly expected to compile for 32bit
> so the domain of the value should fit in 32 bits.
> So either the type could be unsigned int, or it really doesn't matter
> that the value is truncated to 32bit because it can never be larger.

Thank you for the detailed explanation. This is very helpful.



Thank you for the detailed explanation. This is very helpful.

./drv

>
> 	David
>
>
> > >
> > > Signed-off-by: Deepak R Varma <drv@mailo.com>
> > > ---
> >
> > Reviewed-by: Nuno Sá <nuno.sa@analog.com>
>
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
> Registration No: 1397386 (Wales)
>