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From: Arnd Bergmann <arnd@arndb.de>
Date: Thu, 6 Jul 2017 15:56:26 +0200
Message-ID: <CAK8P3a2HS4O9fr2iWK6SjuZ7eiUL37y4SegsLGrf=z6DHtUb2w@mail.gmail.com>
Subject: Re: [RFC PATCH 1/5] base: dma-mapping: Export commonly used symbols
To: Tomasz Figa <tfiga@chromium.org>
Cc: Christoph Hellwig <hch@lst.de>,
        "open list:IOMMU DRIVERS" <iommu@lists.linux-foundation.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Marek Szyprowski <m.szyprowski@samsung.com>,
        Robin Murphy <robin.murphy@arm.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Joerg Roedel <joro@8bytes.org>, Will Deacon <will.deacon@arm.com>,
        Vineet Gupta <vgupta@synopsys.com>,
        Hans-Christian Noren Egtvedt <egtvedt@samfundet.no>,
        Mitchel Humpherys <mitchelh@codeaurora.org>,
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On Thu, Jul 6, 2017 at 3:31 PM, Tomasz Figa <tfiga@chromium.org> wrote:
> On Thu, Jul 6, 2017 at 9:23 PM, Arnd Bergmann <arnd@arndb.de> wrote:
>> On Thu, Jul 6, 2017 at 10:36 AM, Tomasz Figa <tfiga@chromium.org> wrote:
>>> On Thu, Jul 6, 2017 at 5:34 PM, Tomasz Figa <tfiga@chromium.org> wrote:
>>>
>>> Sorry, I intended to mean:
>>>
>>> IMHO re-implementing the code that's already there in videobuf2 again
>>> in the driver, only because, for no good reason mentioned as for now,
>>> having a loadable module providing DMA ops was disliked, would make no
>>> sense.
>>
>> Why would we need to duplicate that code? I would expect that the videobuf2
>> core can simply call the regular dma_mapping interfaces, and you handle the
>> IOPTE generation at the point when the buffer is handed off from the core
>> code to the device driver. Am I missing something?
>
> Well, for example, the iommu-dma helpers already implement all the
> IOVA management, SG iterations, IOMMU API calls, sanity checks and so
> on. There is a significant amount of common code.
>
> On the other hand, if it's strictly about base/dma-mapping, we might
> not need it indeed. The driver could call iommu-dma helpers directly,
> without the need to provide its own DMA ops.

Yes, that's what I meant: if using the IOMMU interface helps, I don't
see anything wrong with that, but using the iommu based
dma_map_ops seems like it may introduce more problems than
it solves.

        Arnd