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Re: [MIPS] FPU emulator: allow Cause bits of FCSR to be writeable by ctc

To: kevink@paralogos.com
Subject: Re: [MIPS] FPU emulator: allow Cause bits of FCSR to be writeable by ctc1
From: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Date: Thu, 06 May 2010 01:22:40 +0900 (JST)
Cc: ralf@linux-mips.org, mcdonald.shane@gmail.com, linux-mips@linux-mips.org
In-reply-to: <4BE19214.4010209@paralogos.com>
Original-recipient: rfc822;linux-mips@linux-mips.org
References: <4BE122D1.3000609@paralogos.com> <20100505091159.GA4016@linux-mips.org> <4BE19214.4010209@paralogos.com>
Sender: linux-mips-bounce@linux-mips.org
On Wed, 05 May 2010 08:43:16 -0700, "Kevin D. Kissell" <kevink@paralogos.com> 
wrote:
> >> I'm glad to hear that the patch is functional.  It was pretty clear that
> >> it would solve your problem, but I was concerned that the inability to
> >> write the Cause bits was done as a way around some other problem. 
> >> Someone went to a certain amount of trouble to not accept them as
> >> inputs, in violation of spec.  I just looked back, and the bug was
> >> introduced as part of a patch of Ralf's from April 2005 entitled "Fix
> >> Preemption and SMP problems in the FP emulator".  It's unlikely that
> >> overriding CTC semantics actually fixed any underlying problems, but it
> >> may have masked symptoms of problems that we can only hope have been
> >> fixed in the mean time. Anyone run this patch on an FPUless SMP?   Oh,
> >> for a 34Kf with a bunch of TCs! ;o)
> >>     
> >
> > That's commit ID 72402ec9efdb2ea7e0ec6223cf20e7301a57da02 and the patch
> > was comitted during the CVS days which only records the comitter but not
> > the author.  The actual author is Atsushi Nemoto.
> >   
> Well,. I certainly understood that you were simply the guy who did the
> commit.  Didn't mean to make it sound like an accusation, but it was
> marginally easier to type your name and date than to find, cut, and
> paste the commit ID.  Sorry if it came off wrong.  It would be cool if
> Atsushi remembered the reasoning behind the change, but empirical proof
> that undoing it doesn't create a subtle problem for current SMP kernels
> would be better still.

Yes, that's my patch.  Though I cannot remember precisely, maybe I
just had (mis)thought Cause bits in FCSR are read-only at that time.
I have never used real SMP MIPS platforms, so there must be no
SMP-related issues.

I'm OK with Kevin's fix.  Thank you very much for reporting and
investigating.

---
Atsushi Nemoto

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