Messages in this thread Patch in this message | | | Date | Sun, 3 Aug 2003 19:29:19 -0700 | From | Andrew Morton <> | Subject | Re: do_div considered harmful |
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Andries.Brouwer@cwi.nl wrote: > > Writing this ide capacity patch an hour ago or so > I split off a helper sectors_to_MB() since Erik's recent > patch uses this also. > Now that I compare, he wrote > nativeMb = do_div(nativeMb, 1000000); > to divide nativeMb by 1000000. > Similarly, I find in fs/cifs/inode.c > inode->i_blocks = do_div(findData.NumOfBytes, inode->i_blksize);
This should be
int blocksize = 1 << inode->i_blkbits;
inode->i_blocks = (findData.NumOfBytes + blocksize - 1) >> inode->i_blkbits;
and inode.i_blksize should probably be removed from the kernel.
> So, it seems natural to expect that do_div() gives the quotient. > But it gives the remainder. > (Strange, Erik showed correct output.) > > Since the semantics of this object are very unlike that of a C function, > I wonder whether we should write DO_DIV instead, or DO_DIV_AND_REM > to show that a remainder is returned.
Sometimes the slash-star operator comes in handy.
--- 25/include/asm-i386/div64.h~do_div-comment 2003-08-03 19:20:58.000000000 -0700 +++ 25-akpm/include/asm-i386/div64.h 2003-08-03 19:21:11.000000000 -0700 @@ -1,6 +1,16 @@ #ifndef __I386_DIV64 #define __I386_DIV64 +/* + * The semantics of do_div() are: + * + * uint32_t do_div(uint64_t *n, uint32_t base) + * { + * uint32_t remainder = *n % base; + * *n = *n / base; + * return remainder; + * } + */ #define do_div(n,base) ({ \ unsigned long __upper, __low, __high, __mod; \ asm("":"=a" (__low), "=d" (__high):"A" (n)); \ _
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