linux-kernel - RE: [PATCH] dma: dmatest: Use div64

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Date:   Thu, 17 Aug 2023 11:25:32 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     "'coolrrsh@...il.com'" <coolrrsh@...il.com>,
        "vkoul@...nel.org" <vkoul@...nel.org>,
        "dmaengine@...r.kernel.org" <dmaengine@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:     "linux-kernel-mentees@...ts.linuxfoundation.org" 
        <linux-kernel-mentees@...ts.linuxfoundation.org>
Subject: RE: [PATCH] dma: dmatest: Use div64_s64

From: coolrrsh@...il.com
> Sent: Wednesday, August 16, 2023 7:04 AM
> 
> From: Rajeshwar R Shinde <coolrrsh@...il.com>
> 
> In the function do_div, the dividend is evaluated multiple times
> so it can cause side effects. Therefore replace it with div64_s64.

Nope, and even if it did it wouldn't matter here.

> This fixes warning such as:
> drivers/dma/dmatest.c:496:1-7:
> WARNING: do_div() does a 64-by-32 division,
> please consider using div64_s64 instead.

What you should really do is look carefully at the domain
of the input values to see if the division can actually
overflow.

That message is basically incorrect.

The full 64x64 divide is horribly slow on all 32bit archs.
It is even about twice as slow as the 64x32 divide on Intel
x86 cpu in 64bit mode (regardless of the values).

	David

> 
> Signed-off-by: Rajeshwar R Shinde <coolrrsh@...il.com>
> ---
>  drivers/dma/dmatest.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/dma/dmatest.c b/drivers/dma/dmatest.c
> index ffe621695e47..07042f239db8 100644
> --- a/drivers/dma/dmatest.c
> +++ b/drivers/dma/dmatest.c
> @@ -9,6 +9,7 @@
> 
>  #include <linux/err.h>
>  #include <linux/delay.h>
> +#include <linux/math64.h>
>  #include <linux/dma-mapping.h>
>  #include <linux/dmaengine.h>
>  #include <linux/freezer.h>
> @@ -493,7 +494,7 @@ static unsigned long long dmatest_persec(s64 runtime, unsigned int val)
> 
>  	per_sec *= val;
>  	per_sec = INT_TO_FIXPT(per_sec);
> -	do_div(per_sec, runtime);
> +	per_sec=div64_s64(per_sec, runtime);
> 
>  	return per_sec;
>  }
> --
> 2.25.1

-
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