Re: RFC: prepare for struct scatterlist entries without page backing
From: Grant Grundler
Date: Wed Aug 12 2015 - 13:56:37 EST
On Wed, Aug 12, 2015 at 10:00 AM, James Bottomley
> On Wed, 2015-08-12 at 09:05 +0200, Christoph Hellwig wrote:
>> However the ccio (parisc) and sba_iommu (parisc & ia64) IOMMUs seem
>> to be operate mostly on virtual addresses. It's a fairly odd concept
>> that I don't fully grasp, so I'll need some help with those if we want
>> to bring this forward.
James explained the primary function of IOMMUs on parisc (DMA-Cache
coherency) much better than I ever could.
Three more observations:
1) the IOMMU can be bypassed by 64-bit DMA devices on IA64.
2) IOMMU enables 32-bit DMA devices to reach > 32-bit physical memory
and thus avoiding bounce buffers. parisc and older IA-64 have some
32-bit PCI devices - e.g. IDE boot HDD.
3) IOMMU acts as a proxy for IO devices by fetching cachelines of data
for PA-RISC systems whose memory controllers ONLY serve cacheline
sized transactions. ie. 32-bit DMA results in the IOMMU fetching the
cacheline and updating just the 32-bits in a DMA cache coherent
4) IOMMU can improve DMA performance in some cases using "hints"
provided by the OS (e.g. prefetching DMA data or using READ_CURRENT
bus transactions instead of normal memory fetches.)
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