Messages in this thread | | | Subject | Re: [PATCH 03/19] dma-iommu: don't use a scatterlist in iommu_dma_alloc | From | Robin Murphy <> | Date | Wed, 6 Feb 2019 15:28:28 +0000 |
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On 01/02/2019 16:16, Christoph Hellwig wrote: > On Fri, Feb 01, 2019 at 03:24:45PM +0000, Robin Murphy wrote: >> On 14/01/2019 09:41, Christoph Hellwig wrote: >>> Directly iterating over the pages makes the code a bit simpler and >>> prepares for the following changes. >> >> It also defeats the whole purpose of __iommu_dma_alloc_pages(), so I'm not >> really buying the simplification angle - you've *seen* that code, right? ;) > > How does it defeat the purpose of __iommu_dma_alloc_pages?
Because if iommu_map() only gets called at PAGE_SIZE granularity, then the IOMMU PTEs will be created at PAGE_SIZE (or smaller) granularity, so any effort to get higher-order allocations matching larger IOMMU block sizes is wasted, and we may as well have just done this:
for (i = 0; i < count; i++) { struct page *page = alloc_page(gfp); ... iommu_map(..., page_to_phys(page), PAGE_SIZE, ...); }
Really, it's a shame we have to split huge pages for the CPU remap, since in the common case the CPU MMU will have a matching block size, but IIRC there was something in vmap() or thereabouts that explicitly chokes on them.
Robin.
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