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Re: [RFC] Optimize swab operations on mips_r2 cpu

To: "Franck" <vagabon.xyz@gmail.com>
Subject: Re: [RFC] Optimize swab operations on mips_r2 cpu
From: "Kevin D. Kissell" <kevink@mips.com>
Date: Thu, 26 Jan 2006 10:17:25 +0100
Cc: "Ralf Baechle" <ralf@linux-mips.org>, <linux-mips@linux-mips.org>
Original-recipient: rfc822;linux-mips@linux-mips.org
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> > Franck wrote:
> > > Let's say that the 4KSC has "wsbh" instruction which is part of
> > > MIPS32R2 instructrion set (I haven't checked it). The question is how
> > > the 4KSC would use the SWAB optimizations since it doesn't define
> > > CONFIG_CPU_MIPS32_R2  ? The 4KSC might not be the only one case...
> >
> > The 4KSc happens not to have the MIPS32R2 WSBH (is that pronounced
> > "wasabi"? ;o) instruction, but it does have the MIPS32R2 ROTR, because
> > it's part of the SmartMIPS ASE.  Our options here include:
> >
> > * Say "to heck with it" and deny the 4KSc use of the ROTR, and stay
> >    with a "#ifdef CONFIG_CPU_MIPS32R2" conditional.
> >
> > * Define CONFIG_CPU_MIPS4KSC as an additional oddball CPU flag, and
> >    make it "#if defined(CONFIG_CPU_MIPS32R2) || defined(CONFIG_CPU_MIPS4KSC)
> >
> > * Have an ASE-support flag, CONFIG_CPU_SMARTMIPS, which would cover both
> >    the 4KSc and 4KSd.  In that case code using ROTR could be conditional on
> >    #if defined(CPU_CONFIG_MIPS32R2) || defined(CONFIG_CPU_SMARTMIPS).
> >
> > I personally think that the third option is the cleanest and most 
> > conceptually
> > correct, but I'm not the guy operationally responsible for maintaining
> > that code.
> 
> I think we will have to use second _and_ third options. I can't find
> out an example, but since 4KSC has some MIPS32_R2 instructions it will
> need to use some specific MIPS32_R2 code sometimes.

You don't understand. There is nothing in the 4KSc that is not in the SmartMIPS 
ASE.
The 4KSc implements MIPS32+SmartMIPS.  The 4KSd implementes MIPS32R2+SmartMIPS.
You're getting confused because some elements of SmartMIPS made it into 
MIPS32R2.
If we have a CONFIG_CPU_SMARTMIPS flag, there would be no need for a
CONFIG_CPU_MIPS4KSC flag.

        Regards,

        Kevin K.

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