Hi loody, On Thu, May 19, 2011 at 10:51 PM, loody <miloody@xxxxxxxxx> wrote: > hi Dave: > Thanks for your kind reply. > 2011/5/20 Dave Hylands <dhylands@xxxxxxxxx>: >> Hi lody, >> >> On Thu, May 19, 2011 at 8:34 PM, loody <miloody@xxxxxxxxx> 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 _______________________________________________ Kernelnewbies mailing list Kernelnewbies@xxxxxxxxxxxxxxxxx http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
