Re: [parisc-linux] PA8800/ZX1 support committed to 2.6.7-rc2-pa2

[James Bottomley <James.Bottomley@steeleye.com>]

On Sat, 2004-06-05 at 16:05, Grant Grundler wrote:
> Well, I can only point at the difference in cache size.

The pa8800 only has a 750k/750k VIPT cache, that's smaller than my
raven.  The 32M L2 cache is PIPT, which doesn't suffer from aliasing or
address remapping effects---in fact, the PA engineers probably arranged
for a fdc not to flush it because there's no point; the only coherency
problems the PIPT cache has is with I/O, which is supposed to be fully
coherent in the ZX1, isn't it.  Thus, we'd only pick up a caching
problems like you describe from the VIPT caches.

> > My money would be on an additional architectural requirement of the
> > PA8800 (maybe even an existing PA one that the <PA8800 just don't need)
> > that we don't respect.
> 
> yes - and we've changed chipsets too.
> 
> Any good ideas on how to prove IO is coherent?

Well, yes, but not without driver magic.  You program a device to take a
piece of data in and rewrite it to a different buffer, then you compare
buffers (making sure the first had a pattern in it and the second was
completely clear).

> It might be the same problem that Naresh described as "SCSI DMA problems".
> I just happen to be using NFS Root instead.
> 
> But I found one bug in Naresh's port that might explain his problem
> (wasn't flushing IO TLB properly). It would be interesting to hear
> if 2.6.7-rc2-pa3 works better for him.

Well, it could be an I/O coherency problem, but if you have one of
those, I'm surprised it boots at all.

James


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