linux-kernel - Re: [PATCH 01/19] dma-mapping: add a Kconfig symbol to indicated arch_dma_prep

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Date:   Fri, 1 Feb 2019 17:12:02 +0100
From:   Christoph Hellwig <hch@....de>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Christoph Hellwig <hch@....de>, Joerg Roedel <joro@...tes.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Tom Lendacky <thomas.lendacky@....com>,
        iommu@...ts.linux-foundation.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 01/19] dma-mapping: add a Kconfig symbol to indicated
 arch_dma_prep_coherent presence

On Fri, Feb 01, 2019 at 02:22:46PM +0000, Robin Murphy wrote:
> On 14/01/2019 09:41, Christoph Hellwig wrote:
>> Add a Kconfig symbol that indicates an architecture provides a
>> arch_dma_prep_coherent implementation, and provide a stub otherwise.
>>
>> This will allow the generic dma-iommu code to it while still allowing
>> to be built for cache coherent architectures.
>
> I've been pondering this for a while now, and I still can't really come up 
> with a case where arch_dma_prep_coherent() would need to behave differently 
> from arch_sync_dma_for_device(..., DMA_BIDIRECTIONAL). I wonder if we could 
> just save ourselves this little bit of complexity by using that instead...

A lot of architectures do really weird stuff in the dma sync routines.
So my plan would be to consolidate a lot more logic in there first,
and then maybe as a next step we could look into using
arch_sync_dma_for_device eventually.