linux-kernel - Re: [PATCH] swiotlb: sync buffer when mapping FROM

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Message-ID: <20190522130921.GA26874@lst.de>
Date:   Wed, 22 May 2019 15:09:21 +0200
From:   Christoph Hellwig <hch@....de>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Christoph Hellwig <hch@....de>,
        Horia Geantă <horia.geanta@....com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        linux-imx@....com
Subject: Re: [PATCH] swiotlb: sync buffer when mapping FROM_DEVICE

On Wed, May 22, 2019 at 01:50:47PM +0100, Robin Murphy wrote:
> Would that work out any different from the existing DMA_ATTR_SKIP_CPU_SYNC? 
>
> If drivers are prepared to handle this issue from their end, they can 
> already do so for single mappings by using that attr along with explicit 
> partial syncs via dma_sync_single(). For page/sg mappings we'd still have 
> the problem of identifying what part of "partial" actually matters, and 
> probably having to add some additional new sync operations to cope.

Except that the same optimization we are tripping over here is also
present in dma_sync_* - dma_sync_*_to_device with DMA_FROM_DEVICE is a
no-op in swiotlb.