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

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Message-ID: <E6BF041E97966E4DB955F0DF883AD2D684F1F601@de02wembxa.internal.synopsys.com>
Date:	Wed, 28 Aug 2013 18:53:44 +0000
From:	Mischa Jonker <Mischa.Jonker@...opsys.com>
To:	Joe Perches <joe@...ches.com>,
	Mischa Jonker <Mischa.Jonker@...opsys.com>
CC:	"Vineet.Gupta1@...opsys.com" <Vineet.Gupta1@...opsys.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] ARC: Fix __udelay parentheses

> > 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?
Yes, it should, but that is not directly related to the issue:) 

> Why is it >> 32 again?
> 
> The comment above it doesn't seem to match the code.
> 
The original code explains about the >> 32:

/*
 * Normal Math for computing loops in "N" usecs
 *  -we have precomputed @loops_per_jiffy
 *  -1 sec has HZ jiffies
 * loops per "N" usecs = ((loops_per_jiffy * HZ / 1000000) * N)
 *
 * Approximate Division by multiplication:
 *  -Mathematically if we multiply and divide a number by same value the
 *   result remains unchanged:  In this case, we use 2^32
 *  -> (loops_per_N_usec * 2^32 ) / 2^32
 *  -> (((loops_per_jiffy * HZ / 1000000) * N) * 2^32) / 2^32
 *  -> (loops_per_jiffy * HZ * N * 4295) / 2^32
 *
 *  -Divide by 2^32 is very simply right shift by 32
 *  -We simply need to ensure that the multiply per above eqn happens in
 *   64-bit precision (if CPU doesn't support it - gcc can emaulate it)
 */

The problem is that the original code already _tried_ to cast to 64 bits (to ensure a 64 bit multiply), but only did so after the multiply (so 32 bit multiply).

Mischa
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