Messages in this thread | | | From | Masahiro Yamada <> | Date | Thu, 9 Mar 2017 03:06:34 +0900 | Subject | Re: [Question] devm_kmalloc() for DMA ? |
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Hi Robin,
2017-03-08 20:15 GMT+09:00 Robin Murphy <robin.murphy@arm.com>: > On 08/03/17 10:59, Masahiro Yamada wrote: >> Hi experts, >> >> I have a question about >> how to allocate DMA-safe buffer. >> >> >> In my understanding, kmalloc() returns >> memory with DMA safe alignment >> in order to avoid cache-sharing problem when used for DMA. >> >> The alignment is decided by ARCH_DMA_MINALIGN. >> For example, on modern ARM 32bit boards, this value is typically 64. >> So, memory returned by kmalloc() has >> at least 64 byte alignment. >> >> >> On the other hand, devm_kmalloc() does not return >> enough-aligned memory. > > How so? If anything returned by kmalloc() is guaranteed to occupy some > multiple of ARCH_DMA_MINALIGN bytes in order to avoid two allocations > falling into the same cache line, I don't see how stealing the first 16 > bytes *of a single allocation* could make it start sharing cache lines > with another? :/
I just thought of traverse of the linked list of devres_node on a different thread, but it should not happen.
Please forget my stupid question.
> If a particular device has a problem with: > > p = kmalloc(...); > d = dma_map_single(p + 0x10, ...); > do_something_with(d); > > that's a separate issue altogether.
Right. Each device has own requirement for DMA alignment.
Thanks!
-- Best Regards Masahiro Yamada
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