Re: [PATCH] ARC: Fix __udelay parentheses

[Vineet Gupta]

On 08/29/2013 12:00 AM, 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@xxxxxxxxxxxx>
> ---
>  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;
>  
>  	__delay(loops);
>  }

The intent of writing orig code was to generate only 1 MPYHU insn (32*32 =
high-part-64) for the whole math, at any optimization level whatsoever. If the
first MPY is overflowing, u r likely spinning for > 10,000 usec (10ms) which is 1
scheduling tick on ARC - not good - presumably for hardware debug. It would be
better to use a tight loop there and throw it out later.

The API abuse would only be caught for const @usecs case. Maybe we need to add a
WARN_ON() there.

OTOH, if we really want to fix this, it would be cleaner to rewrite this as
    loops = ((u64)usecs * 4295 * HZ * loops_per_jiffy) >> 32;

Since one factor is upcasted, all are promoted to 64 bit. And we leave the
optimizations to whims of gcc.

@Joern, I would assume that long long vs u64 (or unsigned long long) doesn't
matter in this particular case.

-Vineet
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