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Re: Linux/MIPS for DECstations...

To: linux-mips@fnet.fr
Subject: Re: Linux/MIPS for DECstations...
From: Ralf Baechle <ralf@cobaltmicro.com>
Date: Tue, 30 Sep 1997 03:42:59 -0700 (PDT)
In-reply-to: <199709300928.AA04028@mailfw1.ford.com> from "Harald Koerfgen" at Sep 30, 97 11:26:23 am
Hi,

> > I'd like to mention that due to popular demand (Some commercial
> > embedded projects just didn't choose Linux/MIPS because there was no
> > "stable" release 2.0 available) I'm working on backporting most of
> > the MIPS stuff implemented for 2.1.x back to 2.0.30+.  I hope to get
> > it stable this week, at least that is the deadline I'm imposing on
> > myself ... That should also give you DECstation people some slowly
> > moving source base to start from.
> 
> That's great.
> 
> > Could you guys tell me, in which routine the kernel actually died? 
> > It might be that I can fix things very easily.
> 
> Well, yesterday evening I spent a few hours hacking my new 240 
> DECstation (39.85 "real BogoMIPS"(tm) BTW). This $§%&/§(/ Box can't 
> boot via tftp, so I had to choose the hard way: boot ultrix, download 
> new kernel, shutdown, boot new kernel via Ultrixboot, turn DECstation 
> off, turn DECstation on, boot ultrix ...
> 
> To make a long story short, I don't have the exact output at hand 
> here at work but the first debug output tells someting about:
> 
> [swapper 0] Illegal instruction at Address <somewhere>.
> 
> The instruction that causes this exception is:
> 
> > ...
> >reload_pgd_entries:
> >#endif /* CONF_DEBUG_TLB */
> >
> >        /* Load missing pair of entries from the pgd and return. */
> >        mfc0    k1,CP0_CONTEXT 
> >        nop
> >        lw      k0,(k1)           #Never causes nested exception
> >        ^^^^^^^^^^^^^^^
> >        mfc0    k1,CP0_EPC        # get the return PC
> > ...
> 
> in arch/mips/kernel/r2300_misc.S.

Ok, I was suspecting something like that.  That particular TLB exception
handler has to be pretty different from the R4000 version.  It's
doing a very trivial piece of work.  The only thing that makes it
a bit more complicated is the fact that the TLB exception handlers
have to be even more optimized than Microsoft's hype.  In fact the
current handlers are all far to bulky.

I'm not shure which source tree you're using.  Could you post the
TLB exception handler from that tree?

  Ralf

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