Re: CONFIG_MIPS_UNCACHED

[Pete Popov <ppopov@mvista.com>]

Ralf Baechle wrote:
> 
> On Wed, Jan 24, 2001 at 12:10:32PM -0800, Jun Sun wrote:
> 
> > It is really surprising to know this.  It sounds like a CPU bug to me.  Can
> > some MIPS "gods" clarify if such a behaviour is a bug or allowed?
> >
> > BTW, the CPU in EV96100 is QED RM7000, I believe.
> 
> If you want to be strictly correct you have to execute the code that
> disables caching of KSEG0 in uncached space like KSEG1, then flush the
> caches before you can resume execution in KSEG0.  Otherwise you might
> end up with dirty d-caches which when flushed will overwrite more
> uptodate data in memory.  The window is very small but yet exists if
> things are just right.

The EV96100 running Galileo's pmon exhibits exactly this symptom.  Pmon
apparently sets up kseg0 to cache coherency 3;  but eventhough the
kernel also sets it to 3, if I don't flush the caches first I end up
with overwritten data.  A different version of pmon that I have sets
kseg0 to 1 (writethrough). Changing that to 3 isn't a problem -- or at
least it doesn't seem to cause any problems.

Pete