CODEWITHSUNDEEP
linux-kernellinux-device-driver
maverick1989
maverick1989

Reputation: 352

DMA cache coherent contiguous allocation

I was looking at allocating a buffer for a DMA transaction. I read that there are two ways of doing this - coherent mappings or streaming mappings.

Now, we want cache coherent buffers. However, we are not doing any scatter/gather so we want the benefits of the dma_map_single call. We have set aside some contiguous memory in the bootargs so we always have enough contiguous memory available.

So I was wondering, can we call dma_alloc_coherent and then dma_map_single after it with the virtual address that dma_alloc_coherent returns? The returned physical address of the map single would then be set as the dma handle that dma_alloc_coherent returned in its call.

Does this make sense? Or is it superfluous/incorrect to do it this way?

Upvotes: 4

Views: 6509

Answers (1)

Peter
Peter

Reputation: 14927

You can't use both. But you're also trying to solve a nonexistent problem. Just use dma_alloc_coherent(), which gives you a contiguous DMA memory buffer with both virtual and physical addresses. DMA to the physical address, access it from the CPU with the virtual address. What's the issue?

Upvotes: 7

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