Re: iommu dma mapping alignment requirements

[Benjamin Herrenschmidt <benh@au1.ibm.com>]

On Thu, 2007-12-20 at 15:02 -0600, Steve Wise wrote:
> Benjamin Herrenschmidt wrote:
> > Adding A few more people to the discussion. You may well be right and we
> > would have to provide the same alignment, though that sucks a bit as one
> > of the reason we switched to 4K for the IOMMU is that the iommu space
> > available on pSeries is very small and we were running out of it with
> > 64K pages and lots of networking activity.
> > 
> 
> But smarter NIC drivers can resolve this too, I think, but perhaps 
> carving up full pages of mapped buffers instead of just assuming mapping 
> is free...

True, but the problem still happenens today, if we switch back to 64K
iommu page size (which should be possible, I need to fix that), we
-will- run out of iommu space on typical workloads and that is not
acceptable.

So we need to find a compromise.

What I might do is something around the lines of: If size >= PAGE_SIZE,
and vaddr (page_address + offset) is PAGE_SIZE aligned, then I enforce
alignment of the resulting mapping.

That should fix your case. Anything requesting smaller than PAGE_SIZE
mappings would lose that alignment but I -think- it should be safe, and
you still always get 4K alignment anyway (+/- your offset) so at least
small alignment restrictions are still enforced (such as cache line
alignment etc...).

I'll send you a test patch later today.

Ben.