Re: RFC: prepare for struct scatterlist entries without page backing

[Grant Grundler <grantgrundler@xxxxxxxxx>]

On Wed, Aug 12, 2015 at 10:00 AM, James Bottomley
<James.Bottomley@xxxxxxxxxxxxxxxxxxxxx> wrote:
> 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
fashion.

Bonus thought:
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.)

cheers,
grant
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