[parisc-linux] ZONE_DMA

[Grant Grundler]

Matthew Wilcox wrote:
> But, if possible, we want to avoid doing DAC as it cuts down the bandwidth
> (and increases latency by a clock).  obviously, if it's a 64-bit device
> and a 64-bit controller, there's no problem, but we should try to stick
> to 32-bit bus addresses where possible.

That's correct. But then we can bypass the I/O MMU. SBA HW has bits
to support that - but I don't fully understand all the limitations of when
this "mode" can be used. Bypassing the I/O MMU would improve DMA performance
and reduce work CPU has to do to "map" each page of DMA.

Though it's generally a good direction, optimizing the PCI bus isn't
always the right answer.  Ie moving from 32- to 64-bit wide data
transfers is a *much* bigger performance win than DAC.

grant

Grant Grundler
Unix Systems Enablement Lab
+1.408.447.7253