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

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Message-ID: <20240506052955.GA4923@lst.de>
Date: Mon, 6 May 2024 07:29:55 +0200
From: Christoph Hellwig <hch@....de>
To: "T.J. Mercier" <tjmercier@...gle.com>
Cc: Christoph Hellwig <hch@....de>,
	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 Fri, May 03, 2024 at 06:37:12PM +0000, T.J. Mercier wrote:
> As of commit 861370f49ce4 ("iommu/dma: force bouncing if the size is
> not cacheline-aligned") sg_dma_mark_swiotlb is called when
> dma_map_sgtable takes the IOMMU path and uses SWIOTLB for some portion
> of a scatterlist. It is never set for the direct path, so drivers
> cannot always rely on sg_dma_is_swiotlb to return correctly after
> calling dma_map_sgtable. Fix this by calling sg_dma_mark_swiotlb in the
> direct path like it is in the IOMMU path.

I don't think this is the right thing to do.  Despite it's name
sg_dma_mark_swiotlb really is dma-iommu specific, and doesn't make sense
in context of dma-direct.  If anything we need to find a better name
for the flag.