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DMA Hints on parisc/ia64
HP PA-RISC and ia64 IO Controllers support the ability to prefetch cachelines
for IO devices doing DMA. The benefit is same as for CPU: bring the data
closer to the consumer. HP had some historical reasons (PA-RISC) for
carrying this behavior forward to current (ia64) platforms. Since PCI bus
transactions eventually reach the "Front Side Bus" (FSB, aka Central Bus),
either bus can be a bottleneck depending on configuration and workload.
DMA hints is just one means of improving PCI and FSB interactions.
To learn expected characteristics of those interactions, Linux might
consider extending current "Device DMA" mapping interface to specify
a few more DMA behaviors instead of the hacks I've used to test "DMA Hints"
on HP ZX1 platforms.
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