Re: New API for non cache coherent ppc cpu's

[Paul Mackerras <paulus@samba.org>]

Roman Zippel writes:

> This document only describes DMA _mappings_, it doesn't say anything
> about cache coherency.

It talks about the conditions under which the cpu and the DMA device
will see a consistent view of memory.  The section on the streaming
DMA mappings could be more explicit, but the idea is that between the
pci_map_{single,sg} and the corresponding pci_unmap_*, the device owns
the region and the cpu shouldn't touch it without first calling
pci_dma_sync_*.  (Hmmm, there is possibly a hole here - the API should
maybe require that you call a sync function both before and after
touching the memory.)  At other times the cpu owns the region and
should see a consistent view of that memory.  That implies some cache
flushing in pci_map_* and/or pci_unmap_* on non-cache-coherent
systems.

> That's the other problem, "non-PCI" sounds like ISA there, what about
> other buses?

That is implementation-dependent.  My view is that we should make this
API work for every kind of bus we have, if possible.

Note also that in 2.5 the struct pci_dev is going to transmogrify into
a "struct device" that will be used for I/O devices on any kind of
bus.

Paul.

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