linux-kernel - Re: [PATCH] mmc: mtk-sd: use div64_u64() instead of do

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Date:   Mon, 28 Feb 2022 13:03:47 +0100
From:   Ulf Hansson <ulf.hansson@...aro.org>
To:     Qing Wang <wangqing@...o.com>,
        Christophe JAILLET <christophe.jaillet@...adoo.fr>
Cc:     Chaotian Jing <chaotian.jing@...iatek.com>,
        Matthias Brugger <matthias.bgg@...il.com>,
        linux-mmc@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-mediatek@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mmc: mtk-sd: use div64_u64() instead of do_div()

On Thu, 17 Feb 2022 at 21:00, Christophe JAILLET
<christophe.jaillet@...adoo.fr> wrote:
>
> Le 17/02/2022 à 16:39, Ulf Hansson a écrit :
> > On Wed, 9 Feb 2022 at 09:39, Qing Wang <wangqing@...o.com> 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>
> >
> > Applied for next, thanks!
>
>
> This is wrong.
>
> See [1].

Thanks for reporting this, I am dropping the patch from my next branch!

See more comments below.

>
>
> Wang Qing, you should really warn all the people you have sent such patches.
>
> CJ
>
> [1]:
> https://lore.kernel.org/linux-kernel/19b96972-cee7-937f-21ce-c78982ed2048@linaro.org/
>
>
> >
> > Kind regards
> > Uffe
> >
> >
> >> ---
> >>   drivers/mmc/host/mtk-sd.c | 2 +-
> >>   1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/drivers/mmc/host/mtk-sd.c b/drivers/mmc/host/mtk-sd.c
> >> index 65037e1..777c9a8
> >> --- a/drivers/mmc/host/mtk-sd.c
> >> +++ b/drivers/mmc/host/mtk-sd.c
> >> @@ -766,7 +766,7 @@ static u64 msdc_timeout_cal(struct msdc_host *host, u64 ns, u64 clks)
> >>                  clk_ns  = 1000000000ULL;
> >>                  do_div(clk_ns, mmc->actual_clock);
> >>                  timeout = ns + clk_ns - 1;
> >> -               do_div(timeout, clk_ns);
> >> +               div64_u64(timeout, clk_ns);

I guess a proper patch would be to convert clk_ns into an u32 instead!?

> >>                  timeout += clks;
> >>                  /* in 1048576 sclk cycle unit */
> >>                  timeout = DIV_ROUND_UP(timeout, BIT(20));
> >> --

Kind regards
Uffe