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From:   Saravana Kannan <saravanak@google.com>
Date:   Wed, 6 Nov 2019 22:16:40 -0800
Message-ID: <CAGETcx8Y6-RGNWZ2qjC7-9UbfUZmQA2JYXDAJSsjpqw01qK_ug@mail.gmail.com>
Subject: Re: [PATCH 0/7] iommu: Permit modular builds of ARM SMMU[v3] drivers
To:     Will Deacon <will@kernel.org>
Cc:     Robin Murphy <robin.murphy@arm.com>,
        iommu@lists.linux-foundation.org,
        LKML <linux-kernel@vger.kernel.org>,
        Joerg Roedel <joro@8bytes.org>,
        Bjorn Helgaas <bhelgaas@google.com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
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On Wed, Oct 30, 2019 at 5:57 PM Saravana Kannan <saravanak@google.com> wrote:
>
> On Wed, Oct 30, 2019 at 8:54 AM Will Deacon <will@kernel.org> wrote:
> >
> > Hi Robin,
> >
> > On Wed, Oct 30, 2019 at 03:35:55PM +0000, Robin Murphy wrote:
> > > On 30/10/2019 14:51, Will Deacon wrote:
> > > > As part of the work to enable a "Generic Kernel Image" across multiple
> > > > Android devices, there is a need to seperate shared, core kernel code
> > > > from modular driver code that may not be needed by all SoCs. This means
> > > > building IOMMU drivers as modules.
> > > >
> > > > It turns out that most of the groundwork has already been done to enable
> > > > the ARM SMMU drivers to be 'tristate' options in drivers/iommu/Kconfig;
> > > > with a few symbols exported from the IOMMU/PCI core, everything builds
> > > > nicely out of the box. The one exception is support for the legacy SMMU
> > > > DT binding, which is not in widespread use and has never worked with
> > > > modules, so we can simply remove that when building as a module rather
> > > > than try to paper over it with even more hacks.
> > > >
> > > > Obviously you need to be careful about using IOMMU drivers as modules,
> > > > since late loading of the driver for an IOMMU serving active DMA masters
> > > > is going to end badly in many cases. On Android, we're using device links
> > > > to ensure that the IOMMU probes first.
> > >
> > > Out of curiosity, which device links are those? Clearly not the RPM links
> > > created by the IOMMU drivers themselves... Is this some special Android
> > > magic, or is there actually a chance of replacing all the
> > > of_iommu_configure() machinery with something more generic?
> >
> > I'll admit that I haven't used them personally yet, but I'm referring to
> > this series from Saravana [CC'd]:
> >
> > https://lore.kernel.org/linux-acpi/20190904211126.47518-1-saravanak@google.com/
> >
> > which is currently sitting in linux-next now that we're upstreaming the
> > "special Android magic" ;)
>
> Hi Robin,
>
> Actually, none of this is special Android magic. Will is talking about
> the of_devlink feature that's been merged into driver-core-next.
>
> A one line summary of of_devlink: the driver core + firmware (DT in
> this case) automatically add the device links during device addition
> based on the firmware properties of each device. The link that Will
> gave has more details.
>
> Wrt IOMMUs, the only missing piece in upstream is a trivial change
> that does something like this in drivers/of/property.c
>
> +static struct device_node *parse_iommus(struct device_node *np,
> +                                        const char *prop_name, int index)
> +{
> +        return parse_prop_cells(np, prop_name, index, "iommus",
> +                                "#iommu-cells");
> +}
>
> static const struct supplier_bindings of_supplier_bindings[] = {
>         { .parse_prop = parse_clocks, },
>         { .parse_prop = parse_interconnects, },
>         { .parse_prop = parse_regulators, },
> +        { .parse_prop = parse_iommus, },
>         {},
> };
>
> I plan to upstream this pretty soon, but I have other patches in
> flight that touch the same file and I'm waiting for those to get
> accepted. I also want to clean up the code a bit to reduce some
> repetition before I add support for more bindings.

As promised:
https://lore.kernel.org/lkml/20191105065000.50407-1-saravanak@google.com/

-Saravana