linux-kernel - Re: [PATCH] dma-direct: Set SG_DMA

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Date: Mon, 6 May 2024 09:39:53 -0700
From: "T.J. Mercier" <tjmercier@...gle.com>
To: Christoph Hellwig <hch@....de>
Cc: Marek Szyprowski <m.szyprowski@...sung.com>, Robin Murphy <robin.murphy@....com>, 
	isaacmanjarres@...gle.com, Catalin Marinas <catalin.marinas@....com>, iommu@...ts.linux.dev, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] dma-direct: Set SG_DMA_SWIOTLB flag for dma-direct

On Mon, May 6, 2024 at 9:19 AM Christoph Hellwig <hch@....de> wrote:
>
> On Mon, May 06, 2024 at 09:10:40AM -0700, T.J. Mercier wrote:
> > I want to reject mapping a dma_buf for a device if any of the pages
> > that back the buffer require SWIOTLB. AFAICT there's no way to know
> > whether SWIOTLB is used until after calling dma_map_sg, so afterwards
> > I'm trying to check.
>
> You should not check, you simply must handle it by doing the proper
> DMA API based ownership management.

That doesn't really work for uncached buffers. Since the SWIOTLB
bounces are in the sync functions, and avoiding CMO is the point of
uncached buffers, it doesn't make sense to try to map uncached buffers
that would require SWIOTLB. So unless we can get the DMA API to fail
the map in this case (look for DMA_ATTR_SKIP_CPU_SYNC + SWIOTLB?) I'm
not sure how else this should be done.