From: "Michael S. Tsirkin" <mst@redhat.com>
To: Christoph Hellwig <hch@lst.de>
Cc: linux-kernel@vger.kernel.org, Jonathan Corbet <corbet@lwn.net>,
Marek Szyprowski <m.szyprowski@samsung.com>,
Robin Murphy <robin.murphy@arm.com>,
linux-doc@vger.kernel.org, iommu@lists.linux.dev
Subject: Re: [PATCH] dma: DMA_ATTR_SKIP_CPU_SYNC documentation tweaks
Date: Thu, 20 Jul 2023 02:34:41 -0400 [thread overview]
Message-ID: <20230720023359-mutt-send-email-mst@kernel.org> (raw)
In-Reply-To: <20230720022702-mutt-send-email-mst@kernel.org>
On Thu, Jul 20, 2023 at 02:30:08AM -0400, Michael S. Tsirkin wrote:
> 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@linux.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
actually there's a new version
https://lore.kernel.org/all/20230719040422.126357-11-xuanzhuo%40linux.alibaba.com
you can see it does map, sync, unmap
unmap immediately after sync seems to be exactly the use case
for DMA_ATTR_SKIP_CPU_SYNC.
> > >
> > >
> > > 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
next prev parent reply other threads:[~2023-07-20 6:37 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-07-19 10:15 [PATCH] dma: DMA_ATTR_SKIP_CPU_SYNC documentation tweaks Michael S. Tsirkin
2023-07-20 6:07 ` Christoph Hellwig
2023-07-20 6:21 ` Michael S. Tsirkin
2023-07-20 6:25 ` Christoph Hellwig
2023-07-20 6:30 ` Michael S. Tsirkin
2023-07-20 6:34 ` Michael S. Tsirkin [this message]
2023-07-20 6:43 ` Christoph Hellwig
2023-07-20 7:06 ` Michael S. Tsirkin
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