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Date:   Wed, 25 Apr 2018 00:09:05 -0700
From:   Christoph Hellwig <hch@infradead.org>
To:     Daniel Vetter <daniel@ffwll.ch>
Cc:     Christoph Hellwig <hch@infradead.org>,
        Christian =?iso-8859-1?Q?K=F6nig?= <christian.koenig@amd.com>,
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        amd-gfx list <amd-gfx@lists.freedesktop.org>,
        Jerome Glisse <jglisse@redhat.com>,
        dri-devel <dri-devel@lists.freedesktop.org>,
        Dan Williams <dan.j.williams@intel.com>,
        Logan Gunthorpe <logang@deltatee.com>,
        "open list:DMA BUFFER SHARING FRAMEWORK" 
        <linux-media@vger.kernel.org>, Thierry Reding <treding@nvidia.com>
Subject: Re: [Linaro-mm-sig] [PATCH 4/8] dma-buf: add peer2peer flag
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On Wed, Apr 25, 2018 at 09:02:17AM +0200, Daniel Vetter wrote:
> Can we please not nack everything right away? Doesn't really motivate
> me to show you all the various things we're doing in gpu to make the
> dma layer work for us. That kind of noodling around in lower levels to
> get them to do what we want is absolutely par-for-course for gpu
> drivers. If you just nack everything I point you at for illustrative
> purposes, then I can't show you stuff anymore.

No, it's not.  No driver (and that includes the magic GPUs) has
any business messing with dma ops directly.

A GPU driver imght have a very valid reason to disable the IOMMU,
but the code to do so needs to be at least in the arch code, maybe
in the dma-mapping/iommu code, not in the driver.

As a first step to get the discussion started we'll simply need
to move the code Thierry wrote into a helper in arch/arm and that
alone would be a massive improvement.  I'm not even talking about
minor details like actually using arm_get_dma_map_ops instead
of duplicating it.

And doing this basic trivial work really helps to get this whole
mess under control.