Messages in this thread | | | Subject | Re: [PATCH 16/16] dma-mapping: use exact allocation in dma_alloc_contiguous | From | Robin Murphy <> | Date | Fri, 14 Jun 2019 16:05:33 +0100 |
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On 14/06/2019 15:50, 'Christoph Hellwig' wrote: > On Fri, Jun 14, 2019 at 02:15:44PM +0000, David Laight wrote: >> Does this still guarantee that requests for 16k will not cross a 16k boundary? >> It looks like you are losing the alignment parameter. > > The DMA API never gave you alignment guarantees to start with, > and you can get not naturally aligned memory from many of our > current implementations.
Well, apart from the bit in DMA-API-HOWTO which has said this since forever (well, before Git history, at least):
"The CPU virtual address and the DMA address are both guaranteed to be aligned to the smallest PAGE_SIZE order which is greater than or equal to the requested size. This invariant exists (for example) to guarantee that if you allocate a chunk which is smaller than or equal to 64 kilobytes, the extent of the buffer you receive will not cross a 64K boundary."
That said, I don't believe this particular patch should make any appreciable difference - alloc_pages_exact() is still going to give back the same base address as the rounded up over-allocation would, and PAGE_ALIGN()ing the size passed to get_order() already seemed to be pointless.
Robin.
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