8xx v2.6 TLB problems and suggested workaround

Joakim Tjernlund Joakim.Tjernlund at lumentis.se
Thu Apr 7 07:24:46 EST 2005

> On Tue, Apr 05, 2005 at 11:51:42PM +0200, Joakim Tjernlund wrote:
> > Hi Marcelo
> > 
> > Reading your report it doesn't sound likely but I will ask anyway:
> > Is it possible that the problem you are seeing isn't caused by the
> > "famous" CPU bug mentioned here: 
> > http://ozlabs.org/pipermail/linuxppc-embedded/2005-January/016351.html
> > 
> > The DTLB error handler needs DAR to be set correctly and since the
> > dcbX instructions doesn't set DAR in either DTLB Miss nor DTLB Error you
> > may end up trying to fix the wrong address.
> Hi Joakim,
> First of all, thanks your care!

NP, I want to be able to run 8xx on 2.6 in the future.
> Well, I dont think the above issue is exactly what we're hitting because
> DAR is correctly updated on our case with "dcbst".

Are you sure? Cant remeber all details but this looks a bit strange to me
SPR  826 : 0x00001f00         7936
is not 0x00001 supposed to be the physical page?

Also DSISR: C2000000 looks strange and "impossible". Are you sure this value
is correct?

Don't understand why the "tlbie()" call  works around the problem. Can you
explain that a bit more?

> The problem is that it is treated as a write operation, but shouldnt.
> Maybe it is related to dcbst's inability to set DAR?

Could be, but even if it isn't you are in trouble when dcbX instr.
generates DTLB Misses/Errors Sooner or later you will end up with
strange SEGV or hangs.

> BTW, about the CPU15 bug fix, has there been any effort to port/merge 
> it in v2.6 ?

None that I know.

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