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Date:   Fri, 1 Nov 2019 12:28:25 +0000
From:   Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
To:     Jean-Philippe Brucker <jean-philippe@linaro.org>
Cc:     Saravana Kannan <saravanak@google.com>,
        Will Deacon <will@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        iommu@lists.linux-foundation.org,
        Bjorn Helgaas <bhelgaas@google.com>,
        Robin Murphy <robin.murphy@arm.com>
Subject: Re: [PATCH 0/7] iommu: Permit modular builds of ARM SMMU[v3] drivers
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On Fri, Nov 01, 2019 at 12:41:48PM +0100, Jean-Philippe Brucker wrote:

[...]

> > > I'm also wondering about ACPI support.
> > 
> > I'd love to add ACPI support too, but I have zero knowledge of ACPI.
> > I'd be happy to help anyone who wants to add ACPI support that allows
> > ACPI to add device links.
> 
> It's not as generic as device-tree, each vendor has their own table to
> describe the IOMMU topology. I don't see a nice way to transpose the
> add_links() callback there. Links need to be created either in a common
> path (iommu_probe_device()) or in the APCI IORT driver.

We can create a generic stub that calls into respective firmware
handling paths (eg iort_dma_setup() in acpi_dma_configure()).

There are three arches booting with ACPI so stubbing it out in
specific firmware handlers is not such a big deal, less generic
sure, but not catastrophically bad.

Obviously this works for IOMMU masters links - for resources
dependencies (eg power domains) it deserves some thought, keeping in
mind that IOMMUs are static table entries in ACPI and not device objects
so they are not even capable of expressing eg power resources and
suchlike.

Long story short: adding IOMMU masters links in ACPI should be
reasonably simple, everything else requires further thought.

Lorenzo