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Re: Bug in the _save_fp_context.

To: "Carsten Langgaard" <>, <>
Subject: Re: Bug in the _save_fp_context.
From: "Kevin D. Kissell" <>
Date: Mon, 19 Mar 2001 16:43:07 +0100
References: <>
> I think there is a bug in the _save_fp_context function in
> arch/mips/kernel/r4k_fpu.S
> The problem is the following piece of code:
>  jr ra
>  .set nomacro
>  EX(sw t0,SC_FPC_EIR(a0))
>  nop
>  .set macro
> First of all what should the ".set nomacro" do?
> If it means that the EX macro shouldn't be used then this entry wouldn't
> get into __ex_table, which would be wrong.
> But it look like it uses the macro anyway, regardless of the ".set
> nomacro", at least with the compiler I use.

Not surprising, really.  "EX" is presumably a cpp macro
that gets expanded by gcc from the .S file, based on
some include file.  .set directives affect only the assembler,
and would inhibit assembler-level macros only.  I'm not
sure just what the definition of an assembler macro
would be - it may or may not include pseudo-instructions
like "la" or "li 32_bit_constant".  I *think* that what the
author was trying to do here was to ensure that the
"sw" instruction in the EX expansion was really and
truly a single instruction.

> Never the less we do not handle entries in the __ex_table which is
> located in a branch delay.
> So we need to handle the situation where we take a page fault on an
> instruction which is located in a brach delay slot, or we don't put the
> "potential" faulting instruction in a delay slot.
> Any ideas, how we should handle this in a nice and clean way?

Is the __ex_table really ending up in the delay slot?
Just looking at the source, I have the impression
that the "sw t0,..." instruction should be in the delay
slot, followed by the __ex_table.

On another topic, now that I've patched the kernel to
turn off the stupid stuck interrupt on my Malta board,
I've realized that I can't just connect my old Atlas SCSI
disk.  I'm torn between ordering a Tekram 390 PCI
SCSI card, which should be able to use our "MIPS
safe" NCR driver as-is (I hope) and buying an IDE
disk and going through the network install ritual.
Which do you recommend?  One thing I really never
knew was just what kernel config options I need to
select to build a kernel that can do the NFS-root
bootstrap.  Can you help me there?


            Kevin K.

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