linux-kernel - Re: [PATCH] dma: DMA_ATTR_SKIP_CPU

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Date:   Thu, 20 Jul 2023 02:30:04 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Christoph Hellwig <hch@....de>
Cc:     linux-kernel@...r.kernel.org, Jonathan Corbet <corbet@....net>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>, linux-doc@...r.kernel.org,
        iommu@...ts.linux.dev
Subject: Re: [PATCH] dma: DMA_ATTR_SKIP_CPU_SYNC documentation tweaks

On Thu, Jul 20, 2023 at 08:25:25AM +0200, Christoph Hellwig wrote:
> On Thu, Jul 20, 2023 at 02:21:05AM -0400, Michael S. Tsirkin wrote:
> > On Thu, Jul 20, 2023 at 08:07:42AM +0200, Christoph Hellwig wrote:
> > > On Wed, Jul 19, 2023 at 06:15:59AM -0400, Michael S. Tsirkin wrote:
> > > > A recent patchset highlighted to me that DMA_ATTR_SKIP_CPU_SYNC
> > > > might be easily misunderstood.
> > > 
> > > .. just curious: what patchset is that?  DMA_ATTR_SKIP_CPU_SYNC is
> > > often a bad idea and all users probably could use a really good
> > > audit..
> > 
> > Message-Id: <20230710034237.12391-1-xuanzhuo@...ux.alibaba.com>
> 
> Do you have an actual link?

sure, they are not hard to generate ;)

https://lore.kernel.org/all/20230710034237.12391-11-xuanzhuo%40linux.alibaba.com

> > 
> > 
> > Looks like there's really little else can be done: there's a
> > shared page we allow DMA into, so we sync periodically.
> > Then when we unmap we really do not need that data
> > synced again.
> > 
> > What exactly is wrong with this?
> 
> A "shared" page without ownership can't work with the streaming
> DMA API (dma_map_*) at all.  You need to use dma_alloc_coherent
> so that it is mapped uncached.

Hmm confused.  Based on both documentation and code I think this works:

	dma_map
	dma_sync
	dma_sync
	dma_sync
	dma_sync
	dma_unmap(DMA_ATTR_SKIP_CPU_SYNC)

right?

-- 
MST