Messages in this thread | | | From | Jim Quinlan <> | Date | Fri, 20 Oct 2017 11:27:41 -0400 | Subject | Re: [PATCH 5/9] PCI: host: brcmstb: add dma-ranges for inbound traffic |
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On Fri, Oct 20, 2017 at 10:57 AM, Christoph Hellwig <hch@lst.de> wrote: > On Fri, Oct 20, 2017 at 10:41:56AM -0400, Jim Quinlan wrote: >> I am not sure I understand your comment -- the size of the request >> shouldn't be a factor. Let's look at your example of the DMA request >> of 3fffff00 to 4000000f (physical memory). Lets say it is for 15 >> pages. If we block out the last page [0x3ffff000..0x3fffffff] from >> what is available, there is no 15 page span that can happen across the >> 0x40000000 boundary. For SG, there can be no merge that connects a >> page from one region to another region. Can you give an example of >> the scenario you are thinking of? > > What prevents a merge from say the regions of > 0....3fffffff and 40000000....7fffffff?
Huh? [0x3ffff000...x3ffffff] is not available to be used. Drawing from the original example, we now have to tell Linux that these are now our effective memory regions:
memc0-a@[ 0....3fffefff] <=> pci@[ 0....3fffefff] memc0-b@[100000000...13fffefff] <=> pci@[ 40000000....7fffefff] memc1-a@[ 40000000....7fffefff] <=> pci@[ 80000000....bfffefff] memc1-b@[300000000...33fffefff] <=> pci@[ c0000000....ffffefff] memc2-a@[ 80000000....bfffefff] <=> pci@[100000000...13fffefff] memc2-b@[c00000000...c3fffffff] <=> pci@[140000000...17fffffff]
This leaves a one-page gap between phsyical memory regions which would normally be contiguous. One cannot have a dma alloc that spans any two regions. This is a drastic step, but I don't see an alternative. Perhaps I may be missing what you are saying...
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