[Xbox-linux] Xbox Linux kernel for 2.6.21-rc2 working too

[Ronald C.F. A. <rc...@cu...>]

Subject says it all.

One question, though, that I don't get. In the kernel configuration's  
"Processor type and features" section, the "Math emulation" help  
states this:


> CONFIG_MATH_EMULATION:
>
> Linux can emulate a math coprocessor (used for floating point  
> operations) if you don't have one. 486DX and Pentium processors  
> have a math coprocessor built in, 486SX and 386 do not, unless you  
> added a 487DX or 387, respectively. (The messages during boot time  
> can give you some hints here ["man dmesg"].) Everyone needs either  
> a coprocessor or this emulation.
>
> If you don't have a math coprocessor, you need to say Y here; if  
> you say Y here even though you have a coprocessor, the coprocessor  
> will be used nevertheless. (This behavior can be changed with the  
> kernel command line option "no387", which comes handy if your  
> coprocessor is broken. Try "man bootparam" or see the documentation  
> of your boot loader (lilo or loadlin) about how to pass options to  
> the kernel at boot time.) This means that it is a good idea to say  
> Y here if you intend to use this kernel on different machines.
>
> More information about the internals of the Linux math coprocessor  
> emulation can be found in <file:arch/i386/math-emu/README>.
>
> If you are not sure, say Y; apart from resulting in a 66 KB  
> biggerkernel, it won't hurt.
>
> Symbol: MATH_EMULATION [=y]
> Prompt: Math emulation
> 	Defined at arch/i386/Kconfig:676
> 	Location:
> 		-> Processor type and features

So if Linux automatically uses a math processor, if present, unless  
told to do so otherwise, then why on the one hand does the arch/i386/ 
Makefile specify in the standard kernel the -msoft-float compiler  
flag, and why do we bother removing it for the xbox if the standard  
kernel decides it's important to have that option?

I mean, either the kernel does the right thing, even with this  
option, and uses the coprocessor if present, or it doesn't. Somehow,  
this seems somewhat contradictory to me.

I then googled on the subject, and found this here:

http://uwsg.iu.edu/hypermail/linux/kernel/0409.1/2266.html

> On Wed, Sep 15, 2004 at 02:14:18AM +0600, Denis Zaitsev wrote:
>> Why this kernel is always compiled with the FP emulation for x86?
>> This is the line from the beginning of arch/i386/Makefile:
>>
>>  CFLAGS += -pipe -msoft-float
>>
>>  And it's hardcoded, it does not depend on CONFIG_MATH_EMULATION. So,
>>  is this just a typo or not?
>
> It catches bogus attempts to use floating point in the kernel by
> making them error at link-time.

So does this mean the kernel is not supposed to use floating point at  
all, and the -msoft-float forces the compiled to create references to  
libraries, which then aren't linked and hence cause a link error to  
catch this problem?

If so, shouldn't we stop patching this, and leave the -msoft-float  
compiler flag where it is and reduce the size of the xbox-linux  
kernel patch a bit?

Ronald