DMA Hints on IA64/PARISC

Modern IO subsystems implement complex DMA transaction parameters, called DMA hints, which are not explicitly supported by the Linux DMA API. This paper investigates benefits of using non-default DMA hints and thus whether such hints should be abstracted into the DMA API. My conclusion is the implementation (ZX1) investigated does not warrant changing the DMA API. Other implementations need to be compared before proposing any changes.

HP PA-RISC and IA64 IO Controllers (ZX1) both support several types of DMA hints and both are commercially available. My primary interest was the ability to prefetch cache lines for PCI devices. The benefit is same as for CPU: bring the data closer to the consumer. But to my surprise, cache line prefetching is not the most important hint since default prefetching works well for all devices. Relaxing the PCI ordering rules turns out to be more important since firmware can't know when it's safe to do so.


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