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Date: Wed, 12 Aug 2015 10:56:26 -0700 From: Grant Grundler <grantgrundler@...il.com> To: James Bottomley <James.Bottomley@...senpartnership.com> Cc: Christoph Hellwig <hch@....de>, Linus Torvalds <torvalds@...ux-foundation.org>, axboe@...nel.dk, dan.j.williams@...el.com, vgupta@...opsys.com, hskinnemoen@...il.com, egtvedt@...fundet.no, realmz6@...il.com, dhowells@...hat.com, monstr@...str.eu, x86@...nel.org, David Woodhouse <dwmw2@...radead.org>, alex.williamson@...hat.com, Grant Grundler <grundler@...isc-linux.org>, open list <linux-kernel@...r.kernel.org>, linux-arch@...r.kernel.org, linux-alpha@...r.kernel.org, linux-ia64@...r.kernel.org, linux-metag@...r.kernel.org, linux-mips@...ux-mips.org, linux-parisc <linux-parisc@...r.kernel.org>, linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org, sparclinux@...r.kernel.org, linux-xtensa@...ux-xtensa.org, linux-nvdimm@...1.01.org, linux-media@...r.kernel.org Subject: Re: RFC: prepare for struct scatterlist entries without page backing On Wed, Aug 12, 2015 at 10:00 AM, James Bottomley <James.Bottomley@...senpartnership.com> 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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