Re: [PATCH V5 3/3] scsi: Align queue to ARCH_DMA_MINALIGN innon-coherent DMA mode

[Christoph Hellwig]

On Mon, Sep 18, 2017 at 03:03:30PM +0800, éåæ wrote:
> I don't think dma_get_cache_alignment is the "absolute minimum alignment" in all cases. At least on MIPS/Loongson, if we use I/O coherent mode (Cached DMA mode), align block queue to 4Bytes is enough. If we align block queue  to dma_get_cache_alignment in I/O coherent mode, there are peformance lost because we cannot use zero-copy in most cases (user buffers are usually not aligned).

If you systems is I/O coherent it should report 1 ARCH_DMA_MINALIGN /
dma_get_cache_alignment().

Note that many drivers only support 512 byte aligned mappings for
block I/O and they've done pretty fine so far.