linux-kernel - Re: [PATCH 1/2] dma-mapping: introduce relaxed version of dma sync

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Date:   Tue, 18 Aug 2020 09:28:53 +0100
From:   Will Deacon <will@...nel.org>
To:     Cho KyongHo <pullip.cho@...sung.com>
Cc:     joro@...tes.org, catalin.marinas@....com,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, m.szyprowski@...sung.com,
        robin.murphy@....com, janghyuck.kim@...sung.com,
        hyesoo.yu@...sung.com
Subject: Re: [PATCH 1/2] dma-mapping: introduce relaxed version of dma sync

On Tue, Aug 18, 2020 at 04:43:10PM +0900, Cho KyongHo wrote:
> Cache maintenance operations in the most of CPU architectures needs
> memory barrier after the cache maintenance for the DMAs to view the
> region of the memory correctly. The problem is that memory barrier is
> very expensive and dma_[un]map_sg() and dma_sync_sg_for_{device|cpu}()
> involves the memory barrier per every single cache sg entry. In some
> CPU micro-architecture, a single memory barrier consumes more time than
> cache clean on 4KiB. It becomes more serious if the number of CPU cores
> are larger.

Have you got higher-level performance data for this change? It's more likely
that the DSB is what actually forces the prior cache maintenance to
complete, so it's important to look at the bigger picture, not just the
apparent relative cost of these instructions.

Also, it's a miracle that non-coherent DMA even works, so I'm not sure
that we should be complicating the implementation like this to try to
make it "fast".

Will