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From:   Saravana Kannan <saravanak@google.com>
Date:   Fri, 1 Nov 2019 14:26:05 -0700
Message-ID: <CAGETcx_U1huHHT=_xo6ArTWpmKMkr=rAy4ceoVUQv6XZGEDA_w@mail.gmail.com>
Subject: Re: [PATCH 0/7] iommu: Permit modular builds of ARM SMMU[v3] drivers
To:     Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc:     Jean-Philippe Brucker <jean-philippe@linaro.org>,
        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>
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On Fri, Nov 1, 2019 at 5:28 AM Lorenzo Pieralisi
<lorenzo.pieralisi@arm.com> wrote:
>
> 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.

Ok, good to know.

> Obviously this works for IOMMU masters links

It's unclear to me what you are referring to here and it's throwing me
off on the rest of the email.

Did you mean to say "IOMMU master's links"? As in the bus masters
whose accesses go through IOMMUs? And "links" as in device links?

OR

Do you mean device links from bus master devices to IOMMUs here?

> - 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.

If you can reword this sentence for me with more context or split it
into separate sentences, I'd appreciate that very much. I'd help me
understand this better and allow me to try to help out.

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

-Saravana