linux-kernel - RE: [PATCH -next] ethernet: renesas: Use div64_ul instead of do

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Message-ID: <ca35a5ba3970462d8eba69ab440da1b3@AcuMS.aculab.com>
Date:   Thu, 18 Nov 2021 11:08:19 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Sergey Shtylyov' <s.shtylyov@....ru>,
        Yang Li <yang.lee@...ux.alibaba.com>,
        "davem@...emloft.net" <davem@...emloft.net>
CC:     "kuba@...nel.org" <kuba@...nel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-renesas-soc@...r.kernel.org" 
        <linux-renesas-soc@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH -next] ethernet: renesas: Use div64_ul instead of do_div

From: Sergey Shtylyov
> Sent: 18 November 2021 09:08
> On 18.11.2021 5:50, Yang Li 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.
> >
> > Eliminate the following coccicheck warning:
> > ./drivers/net/ethernet/marvell/mvpp2/mvpp2_main.c:2742:1-7: WARNING:
> > do_div() does a 64-by-32 division, please consider using div64_ul
> > instead.
> >
> > Reported-by: Abaci Robot <abaci@...ux.alibaba.com>
> > Signed-off-by: Yang Li <yang.lee@...ux.alibaba.com>
> > ---
> >   drivers/net/ethernet/renesas/ravb_main.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/net/ethernet/renesas/ravb_main.c b/drivers/net/ethernet/renesas/ravb_main.c
> > index b4c597f..2b89710 100644
> > --- a/drivers/net/ethernet/renesas/ravb_main.c
> > +++ b/drivers/net/ethernet/renesas/ravb_main.c
> > @@ -2489,7 +2489,7 @@ static int ravb_set_gti(struct net_device *ndev)
> >   		return -EINVAL;
> >
> >   	inc = 1000000000ULL << 20;
> > -	do_div(inc, rate);
> > +	inc = div64_ul(inc, rate);
> 
>     Why not just:
> 
> 	inc = div64_ul(1000000000ULL << 20, rate);
> 
> >   	if (inc < GTI_TIV_MIN || inc > GTI_TIV_MAX) {
> >   		dev_err(dev, "gti.tiv increment 0x%llx is outside the range 0x%x - 0x%x\n",

Even with hardware divide a 64/32 divide is likely to be faster that a 64/64 one.

Maybe the coccicheck warning message should suggest checking the domain
of the divisor and then changing the type??

	David

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