linux-kernel - Re: [RFC PATCH 1/5] base: dma-mapping: Export commonly used symbols

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Message-ID: <CAK8P3a2HS4O9fr2iWK6SjuZ7eiUL37y4SegsLGrf=z6DHtUb2w@mail.gmail.com>
Date:   Thu, 6 Jul 2017 15:56:26 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Tomasz Figa <tfiga@...omium.org>
Cc:     Christoph Hellwig <hch@....de>,
        "open list:IOMMU DRIVERS" <iommu@...ts.linux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Joerg Roedel <joro@...tes.org>,
        Will Deacon <will.deacon@....com>,
        Vineet Gupta <vgupta@...opsys.com>,
        Hans-Christian Noren Egtvedt <egtvedt@...fundet.no>,
        Mitchel Humpherys <mitchelh@...eaurora.org>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        Hans Verkuil <hverkuil@...all.nl>,
        Sakari Ailus <sakari.ailus@....fi>,
        Pawel Osciak <posciak@...omium.org>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Linux Media Mailing List <linux-media@...r.kernel.org>
Subject: Re: [RFC PATCH 1/5] base: dma-mapping: Export commonly used symbols

On Thu, Jul 6, 2017 at 3:31 PM, Tomasz Figa <tfiga@...omium.org> wrote:
> On Thu, Jul 6, 2017 at 9:23 PM, Arnd Bergmann <arnd@...db.de> wrote:
>> On Thu, Jul 6, 2017 at 10:36 AM, Tomasz Figa <tfiga@...omium.org> wrote:
>>> On Thu, Jul 6, 2017 at 5:34 PM, Tomasz Figa <tfiga@...omium.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