[linuxsh-dev] Re: FPU in kernel (SH-4)

[Masahiro A. <m-...@aa...>]

On Wed, 8 Aug 2001 09:37:32 +0900 (JST)
NIIBE Yutaka <gn...@m1...> wrote:

> Masahiro Abe wrote:
>  > RTLinux for SH4 uses FPU. Or more precisely, real-time task running
>  > under rtlinux can use fpu. There can be more than one task running
>  > simultaneously.
> 
> How do you use it?  Save all FPU registers at context switch or
> interrupts?

Well, I did have wrong thing in my mind, and I didn't clearly state the
situation. Sorry.

RTLinux-SH4 scheduler saves/restores FPU registers at its own context
switch time. It has its own mechanism for switching tasks. (It's not
lazy FPU save/restore, because I'm lazy.)

So, now I think my point stated above is worthless. From RTLinux
standpoint, I think Niibe-san's proposal is OK, with one note below:

Please don't force to use -m4-nofpu option for compiling kernel. We've
had a discussion in Japanese ML that "-m4-nofpu" and "-m4" are not
compatible in terms of passing arguments to functions. "-m4-nofpu" does
that in SH3 way, while "-m4" does that in SH4 way, which is different. 
(I can't show you details now, but Kojima-san may remember this
discussion.) RTLinux core module and other Realtime task module must use
"-m4" to be able to use FPU. "-m4-nofpu" kernel and "-m4" module can't
work properly together.

So, please consider introducing "-fno-fpudiv" or "-fidiv" for "-m4", as
Niibe-san proposed, and make "-m4 -fno-fpudiv" kernel and "-m4" module
work properly.

Browsing the branch of this thread, my concern may be already addressed.
I just wanted to add one additional information.

-- 
Masahiro ABE, A&D Co., Ltd.