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Asked: 2026-06-11T13:02:53+00:00

I want to setup a DMA mapping for a memory buffer allocated outside my

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I want to setup a DMA mapping for a memory buffer allocated outside my control. dma_map_single appears the right API to use but my HW has a restriction due to which the mapping must not cross some power-of-two boundary say for e.g. 1K. The buffer being mapped is of size less than the boundary value always but otherwise variable. So it looks like DMA pools may not work since they need a fixed size even though the “allocation” part is sort of what I need.

Should I just keep doing dma_map_single and check if mapping meets my requirement and release mapping if it does not? Can this cause same mapping to potentially be returned causing a never ending search? If so, I could hang on to the unfit mappings till a fit one is found and then release all the unfit mappings in one shot. These however don’t sound like good ideas.

Does anyone have other/better ideas?

Thanks.

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1 Answer

  1. Editorial Team

    If you can’t guarantee that the buffer you are passed meets your criteria, you may need to allocate an auxiliary buffer and copy to/from that buffer before you DMA. On platforms without an IOMMU or other address translation hardware (eg classic x86, ARM, etc), the DMA mapping operation is really just converting to a physical address. So if you unmap and try again with the same buffer, you’ll always get back the same DMA address.

    On most (all?) other platforms that do have an IOMMU, the translation is still done on chunks >= PAGE_SIZE. In other words, if you’re on a platform with 4K pages, and you do DMA mapping on a buffer at 0xABCDExxx, you’ll always get a DMA address like 0xFGHIJxxx where the low part of the address “xxx” stays the same. (This is because the IOMMU works like a normal MMU and only looks up the page translation, and leaves the low 12 or whatever bits alone)

    So in essentially all cases on all platforms, you can’t use the DMA API to fix up the alignment of the buffer you get passed. As I said, I think the only alternative if the buffers you get passed in don’t meet your alignment requirements is to use a bounce buffer. The DMA pool API is a fine way to allocate these bounce buffers — it’s no problem if you sometimes need a smaller buffer; it’s fine to leave some of the memory you get back unused.

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