* about 64-bits division in kernel
@ 2011-05-20 3:34 loody
2011-05-20 4:15 ` Dave Hylands
0 siblings, 1 reply; 6+ messages in thread
From: loody @ 2011-05-20 3:34 UTC (permalink / raw)
To: kernelnewbies
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?
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 ?
--
Regards,
^ permalink raw reply [flat|nested] 6+ messages in thread
* about 64-bits division in kernel
2011-05-20 3:34 about 64-bits division in kernel loody
@ 2011-05-20 4:15 ` Dave Hylands
2011-05-20 5:51 ` loody
0 siblings, 1 reply; 6+ messages in thread
From: Dave Hylands @ 2011-05-20 4:15 UTC (permalink / raw)
To: kernelnewbies
Hi lody,
On Thu, May 19, 2011 at 8:34 PM, loody <miloody@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.
> 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.
--
Dave Hylands
Shuswap, BC, Canada
http://www.davehylands.com
^ permalink raw reply [flat|nested] 6+ messages in thread
* about 64-bits division in kernel
2011-05-20 4:15 ` Dave Hylands
@ 2011-05-20 5:51 ` loody
2011-05-20 6:47 ` Dave Hylands
0 siblings, 1 reply; 6+ messages in thread
From: loody @ 2011-05-20 5:51 UTC (permalink / raw)
To: kernelnewbies
hi Dave:
Thanks for your kind reply.
2011/5/20 Dave Hylands <dhylands@gmail.com>:
> Hi lody,
>
> On Thu, May 19, 2011 at 8:34 PM, loody <miloody@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?
>
>> 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.
--
Regards,
^ permalink raw reply [flat|nested] 6+ messages in thread
* about 64-bits division in kernel
2011-05-20 5:51 ` loody
@ 2011-05-20 6:47 ` Dave Hylands
2011-05-21 6:57 ` loody
0 siblings, 1 reply; 6+ messages in thread
From: Dave Hylands @ 2011-05-20 6:47 UTC (permalink / raw)
To: kernelnewbies
Hi loody,
On Thu, May 19, 2011 at 10:51 PM, loody <miloody@gmail.com> wrote:
> hi Dave:
> Thanks for your kind reply.
> 2011/5/20 Dave Hylands <dhylands@gmail.com>:
>> Hi lody,
>>
>> On Thu, May 19, 2011 at 8:34 PM, loody <miloody@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
^ permalink raw reply [flat|nested] 6+ messages in thread
* about 64-bits division in kernel
2011-05-20 6:47 ` Dave Hylands
@ 2011-05-21 6:57 ` loody
2011-05-21 7:22 ` Dave Hylands
0 siblings, 1 reply; 6+ messages in thread
From: loody @ 2011-05-21 6:57 UTC (permalink / raw)
To: kernelnewbies
hi dave:
2011/5/20 Dave Hylands <dhylands@gmail.com>:
> Hi loody,
>
> On Thu, May 19, 2011 at 10:51 PM, loody <miloody@gmail.com> wrote:
>> hi Dave:
>> Thanks for your kind reply.
>> 2011/5/20 Dave Hylands <dhylands@gmail.com>:
>>> Hi lody,
>>>
>>> On Thu, May 19, 2011 at 8:34 PM, loody <miloody@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.
I found div64.h is located at includ/asm-generic/div64.h instead of
include/linux/
BTW, I still cannot figure out why kernel doesn't adopt the gcc's solution.
libgcc.a will be compiled by different platform, arm, mips, etc.
And that should be safe for kernel to adopt it.
--
Thanks a lot,
miloody
^ permalink raw reply [flat|nested] 6+ messages in thread
* about 64-bits division in kernel
2011-05-21 6:57 ` loody
@ 2011-05-21 7:22 ` Dave Hylands
0 siblings, 0 replies; 6+ messages in thread
From: Dave Hylands @ 2011-05-21 7:22 UTC (permalink / raw)
To: kernelnewbies
Hi miloody,
...snip...
> I found div64.h is located at includ/asm-generic/div64.h instead of
> include/linux/
> BTW, I still cannot figure out why kernel doesn't adopt the gcc's solution.
> libgcc.a will be compiled by different platform, arm, mips, etc.
> And that should be safe for kernel to adopt it.
I believe that the gcc approach is only safe from user-space, and may
cause traps from within kernel space.
>From the reading I've done, gcc expects a certain environment, and the
kernel provides a different environment.
I believe that the primary reason has to do with the fact that libgcc
also provides floating point support, which doesn't work in the
kernel, so they don't allow any of libgcc to be used. I believe that
there are people working on getting support for the integer stuff, but
I'm not sure what the status of this is.
--
Dave Hylands
Shuswap, BC, Canada
http://www.davehylands.com
^ permalink raw reply [flat|nested] 6+ messages in thread
end of thread, other threads:[~2011-05-21 7:22 UTC | newest]
Thread overview: 6+ messages (download: mbox.gz / follow: Atom feed)
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2011-05-20 3:34 about 64-bits division in kernel loody
2011-05-20 4:15 ` Dave Hylands
2011-05-20 5:51 ` loody
2011-05-20 6:47 ` Dave Hylands
2011-05-21 6:57 ` loody
2011-05-21 7:22 ` Dave Hylands
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