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

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Date:   Thu, 6 Jul 2017 22:49:03 +0900
From:   Tomasz Figa <tfiga@...omium.org>
To:     Arnd Bergmann <arnd@...db.de>
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 10:31 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:
>>>> On Thu, Jul 6, 2017 at 5:26 PM, Arnd Bergmann <arnd@...db.de> wrote:
>>>>> On Thu, Jul 6, 2017 at 3:44 AM, Tomasz Figa <tfiga@...omium.org> wrote:
>>
>>>>
>>>> I'd say that this is something that has been consistently tried to be
>>>> avoided by V4L2 and that's why it's so tightly integrated with DMA
>>>> mapping. 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.
>>>
>>> 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. One caveat, though, we
> are not able to obtain coherent (i.e. uncached) memory with this
> approach, which might have some performance effects and complicates
> the code, that would now need to flush caches even for some small
> internal buffers.

I think I should add a bit of explanation here:
 1) the device is non-coherent with CPU caches, even on x86,
 2) it looks like x86 does not have non-coherent DMA ops, (but it
might be something that could be fixed)
 3) one technically could still use __get_vm_area() and map_vm_area(),
which _are_ exported, to create an uncached mapping. I'll leave it to
you to judge if it would be better than using the already available
generic helpers.

Best regards,
Tomasz