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Subject: Re: [PATCH 1/2] kernel.h: Add DIV_ROUND_UP_ULL usage
From: Joe Perches <joe@perches.com>
To: "Nicholas A. Bellinger" <nab@linux-iscsi.org>
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Date: Tue, 26 Jul 2011 09:54:35 -0700
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On Tue, 2011-07-26 at 01:07 -0700, Nicholas A. Bellinger wrote:
> On Tue, 2011-07-26 at 00:53 -0700, Joe Perches wrote:
> > On Tue, 2011-07-26 at 07:10 +0000, Nicholas A. Bellinger wrote:
> > > Add new DIV_ROUND_UP_ULL macro usage for 32-bit architectures requiring
> > > unsigned long long division of sectors * dev_max_sectors.
> > []
> > > diff --git a/include/linux/kernel.h b/include/linux/kernel.h
> > []
> > > +#define DIV_ROUND_UP_ULL(ll,d) \
> > > +	({ unsigned long long _tmp = (ll)+(d)-1; do_div(_tmp, d); _tmp; })
> > Maybe use uint64_t and a temporary for d?
> > #define DIV_ROUND_UP_ULL(ll, d)				\
> > ({							\
> > 	typeof(d) _d = d;				\
> > 	uint64_t _tmp = (uint64_t)(ll) + _d - 1;	\
> > 	do_div(_tmp, _d);				\
> > 	_tmp;						\
> > })

> Hi Joe,

Hi Nicholas.

> Not sure on this one myself..  Would this case ever be strictly required
> for proper 32-bit operation with unsigned long long division with
> sector_t..?

It's just for correctness.

For 32 bit uses, if ll is a unsigned long and not an
unsigned long long then without the cast the addition
will be done as a 32 bit value.

e.g.:

$ cat t.c
#include <stdio.h>
#include <stdlib.h>

int main(int argc, char** argv)
{
	unsigned long l = 0xFFFFFFFFUL;
	unsigned long long ull = l + 1;
	unsigned long long ull2 = (unsigned long long)l + 1;

	printf("l: %lu, ull:%llu, ull2: %llu\n", l, ull, ull2);
}
$ gcc t.c
$ ./a.out
l: 4294967295, ull:0, ull2: 4294967296