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

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Date:	Wed, 28 Aug 2013 18:43:50 -0700
From:	Joe Perches <joe@...ches.com>
To:	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

On Wed, 2013-08-28 at 19:12 +0000, Mischa Jonker wrote:
> Hello Joe,
> 
> > I don't see the loops_per_jiffy initial shift << 32.
> 
> loops_per_jiffy * HZ = loops_per_second
> loops_per_jiffy * HZ = 1,000,000 * loops_per_us
> loops_per_jiffy * HZ * 4295 = 4,295,000 * loops_per_us
> 
> loops_per_jiffy * HZ * 4294.967296 = 2^32 * loops_per_us
> 
> > > > > -	loops = ((long long)(usecs * 4295 * HZ) *
> > > > > -		 (long long)(loops_per_jiffy)) >> 32;
> > > > > +	loops = (((long long) usecs) * 4295 * HZ *
> > > > > +		  (long long) loops_per_jiffy) >> 32;
> 
> > 
> > I know that.  It's the use of a signed long long vs the unsigned long long
> > that I think wrong.
> 
> Yes that is wrong too.
> 
> > 
> > Why cast a unsigned to a signed?
> 
> I don't know, this was in the original file. The issue that I was trying to solve, was that usleep didn't sleep long enough, and that is fixed by this patch. 
> 
> Wrt signed/unsigned: would you like me to update this patch or create a separate one?

I think the whole thing is odd and it should simply be

	loops = loops_per_jiffy * usecs_to_jiffies(usecs);

and if it's really necessary to have a u64 delay
then __delay should be rewritten to take an one
as an argument and this calc should be:

	u64 loops = (u64)loops_per_jiffy * usecs_to_jiffies(usecs);



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