linux-kernel - Re: [RFC PATCH 4/5] iommu/dma: Export non-static functions to use in modules

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Date:   Thu, 6 Jul 2017 16:35:28 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Tomasz Figa <tfiga@...omium.org>
Cc:     Christoph Hellwig <hch@....de>,
        Robin Murphy <robin.murphy@....com>,
        "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>,
        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>
Subject: Re: [RFC PATCH 4/5] iommu/dma: Export non-static functions to use in modules

On Thu, Jul 6, 2017 at 4:24 PM, Tomasz Figa <tfiga@...omium.org> wrote:
> On Thu, Jul 6, 2017 at 11:17 PM, Tomasz Figa <tfiga@...omium.org> wrote:
>> On Thu, Jul 6, 2017 at 11:10 PM, Christoph Hellwig <hch@....de> wrote:
>>> On Thu, Jul 06, 2017 at 12:09:45PM +0100, Robin Murphy wrote:
>>>> I suppose another option is to just make the IOMMU and DMA ops a
>>>> self-contained non-modular driver mirroring the VT-d/AMD-Vi IOMMUs -
>>>> AFAICS it shouldn't have to be all that tightly coupled to the IPU bus
>>>> code, the latter more or less just needs to create the appropriate IOMMU
>>>> device for the driver to find.
>>>
>>> I still haven't seen the driver code, but this seems to be best
>>> solution so far.  Given that it's not a plug in device but part of
>>> an SOC that seems perfectly acceptable to me.
>>
>> I guess that's something that could work. With its caveats of not
>> being able to avoid including the very platform specific code in a
>> generic kernel image or do any quick testing of code changes without a
>> restart, but I guess that's something one could quickly hack in their
>> own downstream (i.e. export the symbols and turn the Kconfig entry
>> into tristate).
>>
>> On the other hand, I'm yet to see any real reasons why not to export
>> those symbols. Personally I don't see anything that one wouldn't be
>> able to do in their downstream without the symbols exported in
>> mainline (one can add the exports any time or if the kernel source
>> can't be modified can just load a wrapper module that exports its own
>> symbols...)
>
> Sorry, I just realized that last sentence might sound nonsense. By
> wrapper I meant reimplementing the missing functions using some
> already exported functions I mentioned in my post to another patch
> from this series, such as __get_vm_area() and map_vm_area().

We should look at the two aspects separately: one is how to drive
the IOMMU as part of a loadable driver, the other is how to handle
DMA to uncached memory on x86. You are in an unfortunate
position of needing both, but aside from that they seem unrelated.

      Arnd