about 64-bits division in kernel

[Dave Hylands]

Hi loody,

On Thu, May 19, 2011 at 10:51 PM, loody <miloody at gmail.com> wrote:
> hi Dave:
> Thanks for your kind reply.
> 2011/5/20 Dave Hylands <dhylands at gmail.com>:
>> Hi lody,
>>
>> On Thu, May 19, 2011 at 8:34 PM, loody <miloody at gmail.com> wrote:
>>> hi all:
>>> My platform is 32-bits cpu and I need following calculation in my driver.
>>> #define longdiv(sr1, sr2, div)      (unsigned long )((((unsigned long
>>> long)(sr1) << 32) ^ (sr2)) / (div))
>>>
>>> my question are:
>>> 1. why "__udivdi3" has any relationship with above calculation?
>>
>> Because you're doing 64 bit arithmetic (unsigned long long) and 64 bit
>> division is not supported in all kernels.
>
> why the name "__udivdi3" has relation to 64-bits arighmetic?
> Why linker ask for "__udivdi3", it seems there is a common sense for
> linker that when doing 64-bits calculation it will try to find
> "__udivdi3", am i right?

Well, since the CPU doesn't directly have support for 64-bit division,
the compiler uses helper functions. The helper function __udivdi3 is
the one for the particular operation you're providing.

The functions in question are part of a library called libgcc and this
library is not linked into the kernel.

>>> 2. I know the above calculation is implemented in clibc, but why
>>> kernel still implement itself?
>>>     why kernel try to make another wheel instead of including what
>>> clib provided ?
>>
>> The kernel doesn't use anything from the C runtime  library at all.
>>
>> 64-bit division and floating point are 2 things not supported in the
>> kernel, although they do happen to word on some platforms, they aren't
>> portable operations.
>>
> the 64-bit division seem supported in gcc toolchain, and gcc will take
> care the platform issue when we cross-compile the gcc, right?
> It should be safe to static link the 64bits division in gcc.

You need to use the do_div macro available in linux/div64.h to perform
64-bit division in the kernel.

-- 
Dave Hylands
Shuswap, BC, Canada
http://www.davehylands.com