On Tue, 2021-08-31 at 13:39 -0700, David Christensen wrote:

quoted

This series allow Indirect DMA using DDW when available, which
usually
means bigger pagesizes and more TCEs, and so more DMA space.

How is the mapping method selected?  LPAR creation via the HMC, Linux
kernel load parameter, or some other method?

At device/bus probe, if there is enough DMA space available for Direct
DMA, then it's used. If not, it uses indirect DMA.

The hcall overhead doesn't seem too worrisome when mapping 1GB pages
so 
the Indirect DMA method might be best in my situation (DPDK).

Well, it depends on usage.
I mean, the recommended use of IOMMU is to map, transmit and then
unmap, but this will vary on the implementation of the driver.

If, for example, there is some reuse of the DMA mapping, as in a
previous patchset I sent (IOMMU Pagecache), then the hcall overhead can
be reduced drastically.

Dave

Best regards,
Leonardo