[parisc-linux] Question about parisc machines with little or no consistent memory

[Matthew Wilcox]

On Thu, Dec 05, 2002 at 01:53:28PM -0800, Adam J. Richter wrote:
> 	I have a question about the parisc machines that have little
> or not consistent memory available.  I'm interested in both the
> parisc machines that Linux currently supports and those that it
> has not been ported to.

Machines we support:

Astro/Elroy based: lots of consistent memory available (IOTLB)
CCIO based: lots of consistent memory available (IOTLB)
PCXL/L2 processors: 128? MB available, uncachable.
PCXS/T processors: no consistent memory available.

Machines we don't support:

V class: Can't really comment how much is available, the docs confused me ;-)
T class: No consistent memory available, though there is an IOMMU.

> 	My question is: what kind of inconsistent memory do they have?
> I am most interested in knowing if you can at least allocate memory
> may be read cached but is write through.  If you can select
> write-through and you write a 4-byte word aligned on a 4-byte
> boundary, will you only write those four bytes without writing back
> any potentially stale cached data?  How about if you write a char or a
> short?  If these machines support memory that need read barriers but
> does not have the hazard of potentially writing back incorrect cached
> data, that may simplify adding support for these machines to more
> drivers.

Basically, no.  The memory subsystems on these things can't cope with the
CPU doing a non-cacheline-{sized,aligned} write, so we can't even do
uncachable memory (except on PCX-L and -L2 where the memory controller can
cope with smaller writes).

Caches on PA are always writeback, never writethrough.

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