Re: Toshiba TX3927 board boot problem.

[Carsten Langgaard <carstenl@mips.com>]

This doesn't look right, you still need to enable the CU1 bit in the status register to let the FP
emulator kick-in.
FPU-less CPUs should take a coprocessor unusable exception on any floating-point instructions.
I have been running this on several FPU-less CPUs, and it works fine for me.

Actually there is a problem with this code on CPUs, which have a FPU. The problem is that a lot of
CPUs have a CP0 hazard of 4 nops, between setting the CU1 bit in the status register and executing
the first floating point instruction thereafter. It probably only a performance issue, because if
the setting of CU1 hasn't taken effect yet, then we get a coprocessor unusable exception and the
the exception handler will also set the CU1 bit.

/Carsten

Atsushi Nemoto wrote:

> >>>>> On Fri, 26 Oct 2001 22:58:06 +0900 (JST), Atsushi Nemoto <nemoto@toshiba-tops.co.jp> said:
> nemoto> I have seen TX39 dead on "cfc1" insturuction if STATUS.CU1 bit
> nemoto> enabled.  Such codes were in arch/mips/kernel/process.c.
>
> So, please apply this patch to CVS for TX39XX support.
>
> I use CONFIG_CPU_TX39XX in this patch, but I suppose other FPU-less
> CPUs may need this also.
>
> Does anybody know how about on other CPUs?
>
> diff -u linux-sgi-cvs/arch/mips/kernel/process.c linux.new/arch/mips/kernel/
> --- linux-sgi-cvs/arch/mips/kernel/process.c    Mon Oct 22 10:29:56 2001
> +++ linux.new/arch/mips/kernel/process.c        Mon Oct 29 15:49:37 2001
> @@ -57,6 +57,12 @@
>  {
>         /* Forget lazy fpu state */
>         if (last_task_used_math == current) {
> +#ifdef CONFIG_CPU_TX39XX
> +               if (!(mips_cpu.options & MIPS_CPU_FPU)) {
> +                       last_task_used_math = NULL;
> +                       return;
> +               }
> +#endif
>                 set_cp0_status(ST0_CU1);
>                 __asm__ __volatile__("cfc1\t$0,$31");
>                 last_task_used_math = NULL;
> @@ -67,6 +73,12 @@
>  {
>         /* Forget lazy fpu state */
>         if (last_task_used_math == current) {
> +#ifdef CONFIG_CPU_TX39XX
> +               if (!(mips_cpu.options & MIPS_CPU_FPU)) {
> +                       last_task_used_math = NULL;
> +                       return;
> +               }
> +#endif
>                 set_cp0_status(ST0_CU1);
>                 __asm__ __volatile__("cfc1\t$0,$31");
>                 last_task_used_math = NULL;
> ---
> Atsushi Nemoto

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