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Re: Toshiba TX3927 board boot problem.

To: Atsushi Nemoto <nemoto@toshiba-tops.co.jp>
Subject: Re: Toshiba TX3927 board boot problem.
From: Carsten Langgaard <carstenl@mips.com>
Date: Mon, 29 Oct 2001 09:32:14 +0100
Cc: ralf@oss.sgi.com, ajob4me@21cn.com, linux-mips@oss.sgi.com
References: <20011026095319.1C4BBB474@topsms.toshiba-tops.co.jp> <20011026.225806.63990588.nemoto@toshiba-tops.co.jp> <20011029.160225.59648095.nemoto@toshiba-tops.co.jp>
Sender: owner-linux-mips@oss.sgi.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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