linux-kernel - Re: [PATCH] power: supply: da9150: use div64_u64() instead of do

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Message-ID: <20220211214029.syw76upbqthasgwb@mercury.elektranox.org>
Date:   Fri, 11 Feb 2022 22:40:29 +0100
From:   Sebastian Reichel <sebastian.reichel@...labora.com>
To:     Christophe JAILLET <christophe.jaillet@...adoo.fr>
Cc:     Qing Wang <wangqing@...o.com>,
        Support Opensource <support.opensource@...semi.com>,
        linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] power: supply: da9150: use div64_u64() instead of
 do_div()

Hi,

On Fri, Feb 11, 2022 at 09:53:08PM +0100, Christophe JAILLET wrote:
> Le 11/02/2022 à 21:50, Sebastian Reichel a écrit :
> > Hi,
> > 
> > On Wed, Feb 09, 2022 at 12:39:46AM -0800, Qing Wang wrote:
> > > From: Wang Qing <wangqing@...o.com>
> > > 
> > > do_div() does a 64-by-32 division.
> > > When the divisor is u64, do_div() truncates it to 32 bits, this means it
> > > can test non-zero and be truncated to zero for division.
> > > 
> > > fix do_div.cocci warning:
> > > do_div() does a 64-by-32 division, please consider using div64_u64 instead.
> > > 
> > > Signed-off-by: Wang Qing <wangqing@...o.com>
> > > ---
> > 
> > Thanks, queued.
> 
> All these patches are broken and should be NACKed, fixed or reverted if
> applied.
> 
> See [1].
> 
> CJ
> 
> [1]: https://lore.kernel.org/linux-kernel/19b96972-cee7-937f-21ce-c78982ed2048@linaro.org/

Oops, thanks for the hint. Patch dropped. I actually missed the
do_div(div, 1000000) during my review (probably should go sleeping),
so div should be within safe range in the second do_div() anyways.

-- Sebastian

> 
> > 
> > -- Sebastian
> > 
> > >   drivers/power/supply/da9150-fg.c | 2 +-
> > >   1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/drivers/power/supply/da9150-fg.c b/drivers/power/supply/da9150-fg.c
> > > index 6e36782..896491a
> > > --- a/drivers/power/supply/da9150-fg.c
> > > +++ b/drivers/power/supply/da9150-fg.c
> > > @@ -250,7 +250,7 @@ static int da9150_fg_current_avg(struct da9150_fg *fg,
> > >   	div = (u64) (sd_gain * shunt_val * 65536ULL);
> > >   	do_div(div, 1000000);
> > >   	res = (u64) (iavg * 1000000ULL);
> > > -	do_div(res, div);
> > > +	div64_u64(res, div);
> > >   	val->intval = (int) res;
> > > -- 
> > > 2.7.4
> > > 
> 

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