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Date:   Mon, 18 Sep 2017 08:50:29 -0700
From:   Christoph Hellwig <>
To:     陈华才 <>
Cc:     Christoph Hellwig <>,
        "James E . J . Bottomley" <>,
        "Martin K . Petersen" <>,
        Andrew Morton <>,
        Fuxin Zhang <>,
        linux-scsi <>,
        linux-kernel <>,
        stable <>
Subject: Re: [PATCH V5 3/3] scsi: Align queue to ARCH_DMA_MINALIGN
 innon-coherent DMA mode

On Mon, Sep 18, 2017 at 03:03:30PM +0800, 陈华才 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 /

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

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