Re: [PATCH 03/19] dma-iommu: don't use a scatterlist in iommu_dma_alloc

[Robin Murphy]

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.