Hi Robin, 2017-03-08 20:15 GMT+09:00 Robin Murphy <robin.murphy@xxxxxxx>: > 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 -- To unsubscribe from this list: send the line "unsubscribe dmaengine" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
