linux-kernel - Re: [PATCH] ARC: Fix _

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Date:	Wed, 28 Aug 2013 11:45:39 -0700
From:	Joe Perches <joe@...ches.com>
To:	Mischa Jonker <Mischa.Jonker@...opsys.com>
Cc:	Vineet.Gupta1@...opsys.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ARC: Fix __udelay parentheses

On Wed, 2013-08-28 at 20:29 +0200, Mischa Jonker wrote:
> Make sure that usecs is casted to long long, to ensure that the
> (usecs * 4295 * HZ) multiplication is 64 bit.
> 
> Initially, the (usecs * 4295 * HZ) part was done as a 32 bit
> multiplication, with the result casted to 64 bit. This led to some bits
> falling off.
> 
> Signed-off-by: Mischa Jonker <mjonker@...opsys.com>
> ---
>  arch/arc/include/asm/delay.h |    4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arc/include/asm/delay.h b/arch/arc/include/asm/delay.h
> index 442ce5d..8d35fe1 100644
> --- a/arch/arc/include/asm/delay.h
> +++ b/arch/arc/include/asm/delay.h
> @@ -56,8 +56,8 @@ static inline void __udelay(unsigned long usecs)
>  	/* (long long) cast ensures 64 bit MPY - real or emulated
>  	 * HZ * 4295 is pre-evaluated by gcc - hence only 2 mpy ops
>  	 */
> -	loops = ((long long)(usecs * 4295 * HZ) *
> -		 (long long)(loops_per_jiffy)) >> 32;
> +	loops = (((long long) usecs) * 4295 * HZ *
> +		  (long long) loops_per_jiffy) >> 32;

Shouldn't this be unsigned long long or u64?

Why is it >> 32 again?

The comment above it doesn't seem to match the code.


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