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Date:   Thu, 29 Mar 2018 23:33:34 -0700
From:   Christoph Hellwig <hch@infradead.org>
To:     Jerome Glisse <jglisse@redhat.com>
Cc:     Logan Gunthorpe <logang@deltatee.com>,
        Christian =?iso-8859-1?Q?K=F6nig?= <christian.koenig@amd.com>,
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        linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org,
        linux-media@vger.kernel.org, Bjorn Helgaas <bhelgaas@google.com>
Subject: Re: [PATCH 2/8] PCI: Add pci_find_common_upstream_dev()
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On Thu, Mar 29, 2018 at 09:58:54PM -0400, Jerome Glisse wrote:
> dma_map_resource() is the right API (thought its current implementation
> is fill with x86 assumptions). So i would argue that arch can decide to
> implement it or simply return dma error address which trigger fallback
> path into the caller (at least for GPU drivers). SG variant can be added
> on top.

It isn't in general.  It doesn't integrate with scatterlists (see my
comment to page one), and it doesn't integrate with all the subsystems
that also need a kernel virtual address.