linux-kernel - Re: [RFC/PATCH 0/4] Initial support for modular IOMMU drivers

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Date:   Tue, 21 May 2019 17:07:18 +0100
From:   Jean-Philippe Brucker <jean-philippe.brucker@....com>
To:     "Isaac J. Manjarres" <isaacm@...eaurora.org>,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-pci@...r.kernel.org, iommu@...ts.linux-foundation.org,
        linux-arm-kernel@...ts.infradead.org
Cc:     kernel-team@...roid.com, robin.murphy@....com, joro@...tes.org,
        will.deacon@....com, lmark@...eaurora.org, robh+dt@...nel.org,
        bhelgaas@...gle.com, frowand.list@...il.com, pratikp@...eaurora.org
Subject: Re: [RFC/PATCH 0/4] Initial support for modular IOMMU drivers

Hi Isaac,

On 17/05/2019 19:47, Isaac J. Manjarres wrote:
> This series adds initial support for being able to use the ARM
> SMMU driver as a loadable kernel module. The series also adds
> to the IOMMU framework, so that it can defer probing for devices
> that depend on an IOMMU driver that may be a loadable module.
> 
> The primary reason behind these changes is that having the ARM
> SMMU driver as a module allows for the same kernel image to be
> used across different platforms. For example, if one platform
> contains an IOMMU that implements one version of the ARM SMMU
> specification, and another platform simply does not have an
> IOMMU, the only way that these platforms can share the same
> kernel image is if the ARM SMMU driver is compiled into the
> kernel image.
> 
> This solution is not scalable, as it will lead to bloating the
> kernel image with support for several future versions of the
> SMMU specification to maintain a common kernel image that works
> across all platforms. Having the ARM SMMU driver as a module allows
> for a common kernel image to be supported across all platforms,
> while yielding a smaller kernel image size, since the correct
> SMMU driver can be loaded at runtime, if necessary.

It can also be useful if IOMMU drivers want to rely on components that
distros usually build as modules. I have that problem with virtio-iommu,
where the whole virtio transport is usually modular.

> Patchset Summary:
> 
> 1. Since the ARM SMMU driver depends on symbols being exported from
> several subsystems, the first three patches are dedicated to exporting
> the necessary symbols.
> 
> 2. Similar to how the pinctrl framework handles deferring probes,
> the subsequent patch makes it so that the IOMMU framework will defer
> probes indefinitely if there is a chance that the IOMMU driver that a
> device is waiting for is a module. Otherwise, it upholds the current
> behavior of stopping probe deferrals once all of the builtin drivers
> have finished probing.
> 
> The ARM SMMU driver currently has support for the deprecated
> "mmu-masters" binding, which relies on the notion of initcall
> ordering for setting the bus ops to ensure that all SMMU devices
> have been bound to the driver. This poses a problem with
> making the driver a module, as there is no such notion with
> loadable modules. Will support for this be completely deprecated?
> If not, might it be useful to leverage the device tree ordering,
> and assign a property to the last SMMU device, and set the bus ops
> at that point? Or perhaps have some deferred timer based approach
> to know when to set the bus ops? 

Another problem is module unloading: if the user calls rmmod on an IOMMU
module, we have to ensure that endpoints aren't performing DMA anymore.
It could be solved by declaring consumers of an IOMMU with
device_link_add(), so that device drivers are unbound before the IOMMU
module is unloaded.

Thanks,
Jean

> 
> Thanks,
> Isaac
> 
> Isaac J. Manjarres (4):
>   of: Export of_phandle_iterator_args() to modules
>   PCI: Export PCI ACS and DMA searching functions to modules
>   iommu: Export core IOMMU functions to kernel modules
>   iommu: Add probe deferral support for IOMMU kernel modules
> 
>  drivers/iommu/iommu-sysfs.c | 3 +++
>  drivers/iommu/iommu.c       | 6 ++++++
>  drivers/iommu/of_iommu.c    | 8 ++++++--
>  drivers/of/base.c           | 1 +
>  drivers/pci/pci.c           | 1 +
>  drivers/pci/search.c        | 1 +
>  6 files changed, 18 insertions(+), 2 deletions(-)
>