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

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Date:   Tue, 18 Aug 2020 17:10:06 +0100
From:   Christoph Hellwig <hch@...radead.org>
To:     Will Deacon <will@...nel.org>
Cc:     Cho KyongHo <pullip.cho@...sung.com>, janghyuck.kim@...sung.com,
        catalin.marinas@....com, linux-kernel@...r.kernel.org,
        hyesoo.yu@...sung.com, iommu@...ts.linux-foundation.org,
        robin.murphy@....com, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 1/2] dma-mapping: introduce relaxed version of dma sync

On Tue, Aug 18, 2020 at 11:07:57AM +0100, Will Deacon wrote:
> > > so I'm not sure
> > > that we should be complicating the implementation like this to try to
> > > make it "fast".
> > > 
> > I agree that this patch makes the implementation of dma API a bit more
> > but I don't think this does not impact its complication seriously.
> 
> It's death by a thousand cuts; this patch further fragments the architecture
> backends and leads to arm64-specific behaviour which consequently won't get
> well tested by anybody else. Now, it might be worth it, but there's not
> enough information here to make that call.

So it turns out I misread the series (*cough*, crazy long lines,
*cough*), and it does not actually expose a new API as I thought, but
it still makes a total mess of the internal interface.  It turns out
that on the for cpu side we already have arch_sync_dma_for_cpu_all,
which should do all that is needed.  We could do the equivalent for
the to device side, but only IFF there really is a major benefit for
something that actually is mainstream and matters.