Re: [PATCH 1/2] dma-mapping: introduce relaxed version of dma sync

[Cho KyongHo <pullip.cho@samsung.com>]

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On Tue, Aug 18, 2020 at 09:37:20AM +0100, Christoph Hellwig wrote:
> On Tue, Aug 18, 2020 at 09:28:53AM +0100, Will Deacon wrote:
> > On Tue, Aug 18, 2020 at 04:43:10PM +0900, Cho KyongHo wrote:
> > > Cache maintenance operations in the most of CPU architectures needs
> > > memory barrier after the cache maintenance for the DMAs to view the
> > > region of the memory correctly. The problem is that memory barrier is
> > > very expensive and dma_[un]map_sg() and dma_sync_sg_for_{device|cpu}()
> > > involves the memory barrier per every single cache sg entry. In some
> > > CPU micro-architecture, a single memory barrier consumes more time than
> > > cache clean on 4KiB. It becomes more serious if the number of CPU cores
> > > are larger.
> > 
> > Have you got higher-level performance data for this change? It's more likely
> > that the DSB is what actually forces the prior cache maintenance to
> > complete, so it's important to look at the bigger picture, not just the
> > apparent relative cost of these instructions.
> > 
> > Also, it's a miracle that non-coherent DMA even works, so I'm not sure
> > that we should be complicating the implementation like this to try to
> > make it "fast".
> 
> And without not just an important in-tree user but one that actually
> matters and can show how this is correct the whole proposal is complete
> nonstarter.
> 
The patch introduces new kernel configurations
ARCH_HAS_SYNC_DMA_FOR_CPU_RELAXED and ARCH_HAS_SYNC_DMA_FOR_CPU_RELAXED
not to affect the rest of the system. I also confirmed that the patch
does not break some other architectures including arm and x86 which do
not define the new kernel configurations.

Would you let me know some other things to confirm this patch is
correct?

Thank you.

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