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Date:   Wed, 30 Oct 2019 15:54:45 +0000
From:   Will Deacon <will@kernel.org>
To:     Robin Murphy <robin.murphy@arm.com>
Cc:     iommu@lists.linux-foundation.org, linux-kernel@vger.kernel.org,
        Joerg Roedel <joro@8bytes.org>,
        Bjorn Helgaas <bhelgaas@google.com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>,
        saravanak@google.com
Subject: Re: [PATCH 0/7] iommu: Permit modular builds of ARM SMMU[v3] drivers
Message-ID: <20191030155444.GC19096@willie-the-truck>
References: <20191030145112.19738-1-will@kernel.org>
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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" ;)

Will