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

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Date:   Thu, 20 Jul 2023 03:06:48 -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 08:43:18AM +0200, Christoph Hellwig wrote:
> On Thu, Jul 20, 2023 at 02:30:04AM -0400, Michael S. Tsirkin wrote:
> > sure, they are not hard to generate ;)
> > 
> > https://lore.kernel.org/all/20230710034237.12391-11-xuanzhuo%40linux.alibaba.com
> 
> Thanks, I'll chime in there.
> 
> > > > 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?
> 
> Depends on your definition of "shared".  If there is always a clear
> owner at a given time you can games with lots of syncs that transfer
> ownership.  If there is no clear ownership, and the "device" just
> DMAs into the buffer at random times and the host checks bits in
> there we need to map the buffer uncached.
> 
> I'll chime in in the thread.

Each chunk of that buffer is DMA'd into separately and then sync'd
afterwards.


> > 
> > -- 
> > MST
> ---end quoted text---