[linuxsh-dev] floating point usage in SH4 linux

[NIIBE Y. <gn...@m1...>]

Current 2.5.3-pre5 (or our CVS) doesn't support FPU usage in kernel.
Once I've tried to support it (and I think it worked well at that
time), but eventually, I gave up.

You can check, say, 2.4.0 implementation, if you really have
interests.

It goes like this:
    We have FPU lazy switching mechanism.
    Wnen kernel asks FPU use, we pretend as if the task "init" use FPU.
    On Kernel -> Kernel transition, we save FPU registers on kernel stack.

This works well _provided_ the use of FPU in kernel can be seen as if
"init" use FPU.

However, the kernel thread softirqd has been introduced in 2.4.X.
With it, the bottom-half of the driver is invoked by that thread.  If
bottom-half uses FPU, it results quite a mess, because it breaks
the condition (of it's "init" who use FPU).
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