Re: [PATCH] dma: DMA_ATTR_SKIP_CPU_SYNC documentation tweaks

[Michael S. Tsirkin]

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@xxxxxxxxxxxxxxxxx>
> > 
> > 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