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

[Christoph Hellwig]

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.