On Fri, 9 Feb 2001, Alan Cox wrote:
> > > For non routing paths its virtually free because the DMA forced the lines
> > > from cache anyway.
> >
> > Are you actually sure about this? I thought DMA from PCI devices reached
> > the main memory without polluting the L2 cache. Otherwise any large DMA
> > transfer would kill the cache (think frame grabbers...)
>
> DMA to main memory normally invalidates those lines in the CPU cache rather
> than the cache snooping and updating its view of them.
In PCI, it is the Memory Write and Invalidate PCI transaction that is
intended to allow core-logics to optimize DMA this way. For normal Memory
Write PCI transactions or when the core-logic is aliasing MWI to MW, the
snooping may well happen. All that stuff, very probably, varies a lot
depending on the core-logic.
As we know, in normal PCI, the target is not told about the transaction
length prior to the bursting of the data. This makes difficult for a core
logic to use cache invalidation rather than dma snooping when a normal MW
is used, thus the invention of MWI.
Gérard.
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This archive was generated by hypermail 2b29 : Thu Feb 15 2001 - 21:00:15 EST