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From:   Saravana Kannan <saravanak@google.com>
Date:   Wed, 6 Nov 2019 21:55:50 -0800
Message-ID: <CAGETcx_0kwQTjM2BkW__ux8kRvazR-PuX4Hcp4fP_97QRLSkTQ@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 Mon, Nov 4, 2019 at 3:43 AM Lorenzo Pieralisi
<lorenzo.pieralisi@arm.com> wrote:
>
> On Fri, Nov 01, 2019 at 02:26:05PM -0700, Saravana Kannan wrote:
> > 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?
>
> I meant associating endpoints devices to the IOMMU they are connected
> to.
>
> In DT you do it through "iommus", "iommu-map" properties, in ACPI
> it is arch specific, doable nonetheless through ACPI (IORT on ARM)
> static tables data.
>
> > > - 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.
>
> In ACPI (at least on ARM but on x86 I suspect that's the same story with
> the DMAR table) an SMMU is presented in FW as an entry in a static
> table (eg IORT on ARM). I noticed that your patch series takes into
> account for instance eg clock dependencies in DT; this way the OS knows
> the clock(s) the SMMU depends on to be activated.
>
> In ACPI there is not a notion of "clock" (hopefully - unless someone
> sneaked that in using _DSD properties) but rather every device in the
> ACPI namespace (which is part of tables containing code that needs the
> ACPI interpreter to be used such as SSDT/DSDT - it is AML code) has ACPI
> objects describing power resources (ie ACPI specification 6.3, 7.2).
>
> The SMMU, since it is not itself an ACPI object in the ACPI namespace
> but rather an entry in a static ACPI table (IORT on ARM), can't have
> PowerResource object in it which means that at the moment there is no
> way you can detect a dependency on other power resources to be ON to
> build the device links you require to sort out the probe dependencies,
> which I *assume* that's the reason why you require to detect
> clock dependencies in DT.
>
> Maybe it is not even needed at all but in case it is I was giving
> a heads-up to say that clocks (or rather an all encompassing "power
> resource" dependency) dependencies in ACPI to build an SMMU as
> a module are not straightforward and most certainly will require
> firmware specifications updates.
>
> *Hopefully* in the short term all you need to detect is how endpoint
> devices are connected to an IOMMU and build device links to describe
> that probe dependency, if we need to throw power management into
> the picture there is more work to be done.
>
> I hope that's clearer, if it is not please let me know and I will
> try to be more precise.

Thanks for the detailed explanation and context Lorenzo. I understand
it better now.

-Saravana