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

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Date:   Wed, 19 Aug 2020 11:01:26 +0900
From:   Cho KyongHo <pullip.cho@...sung.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Will Deacon <will@...nel.org>, 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 05:10:06PM +0100, Christoph Hellwig wrote:
> 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.
> 
Indeed, arch_sync_dma_for_cpu_all() is used where the new internal API
arch_sync_barrier_for_cpu() should be called. I just thought it is a
special hook for MIPS.
In the next version of the patch series, I should consider using
arch_sync_dma_for_cpu_all() and introducting its 'for_dev' version with
some performance data to show the benefit of the change.

Thank you for the proposal.

KyongHo