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Re: Can't build a CONFIG_CPU_NEVADA kernel

To: "Kevin D. Kissell" <kevink@mips.com>
Subject: Re: Can't build a CONFIG_CPU_NEVADA kernel
From: Ralf Baechle <ralf@oss.sgi.com>
Date: Fri, 16 Mar 2001 15:04:23 +0100
Cc: <linux-mips@oss.sgi.com>
In-reply-to: <00fc01c0acd3$c881ca80$0deca8c0@Ulysses>; from kevink@mips.com on Wed, Mar 14, 2001 at 11:11:47PM +0100
References: <20010314084633.A25674@nevyn.them.org> <20010314195919.A1911@bacchus.dhis.org> <20010314140529.A29525@nevyn.them.org> <20010314202058.B1911@bacchus.dhis.org> <00fc01c0acd3$c881ca80$0deca8c0@Ulysses>
Sender: owner-linux-mips@oss.sgi.com
User-agent: Mutt/1.2.5i
On Wed, Mar 14, 2001 at 11:11:47PM +0100, Kevin D. Kissell wrote:

> > Don't think of the r8000; the kernel only uses the -mcpu=r8000 option
> > because the Nevada CPUs have _somewhat_ similar scheduling properties
> > to the R8000.  This of it as an independant ISA expension which can
> > be used with an arbitrary MIPS processor - even a R3000 processor.
> 
> In the interests of historical accuracy and general pedantry,
> let me point out that -mcpu=r8000 is in effect a rather klugy
> way of saying "-mips4" to compilers that predate official
> MIPS IV ISA support.  The R8000 was the first MIPS IV
> CPU, followed by the R10000 and the R5000.  The "Nevada"
> processors added three implementation specific instructions
> to the MIPS IV ISA: MAD, MADU, and MUL (targeted multiply).
> "Correct" usage would be to enable those three instructions
> with a "-mcpu=nevada", or better still, "-mcpu=r5200" (for 
> consistency), and to enable the rest of the MIPS IV ISA 
> with "-mips4" instead of the archaic r8000 hack.

Your historic facts may be right but the GCC fact aren't.  -mcpu=xxx tell
GCC to schedule instructions for a certain processor xxx.  This does not
enable the full use of it's instruction set.  Back in time when I choose
these options I choose because GCC didn't know -mcpu=r5000 but the R8000
was supported and it was the closest fit.  Gcc 1.1.2 knows this option
so I just changed all instances of -mcpu=r8000 into -mcpu=r5000.

> Note, furthermore, that -mmad needs to be handled with care: 
> Prior to MIPS standardizing the instruction as part of 
> MIPS32, there were four or five subtly (or not so subltly) 
> different definitions of integer multiply-accumulate for MIPS.  
> Most use the same opcode, but even those can differ in side 
> effects (is the rd field interpreted, etc.). A R4650 madd operation
> will probably behave equivalently on a Nevada CPU, 
> but certainly not on a Vr41xx part, for example.

Unfortunately true but there is a reason that QED's manual marks it as an
proprietary extension ...

  Ralf

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