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Although it's vanishingly unlikely that anyone would integrate an SMMU
within a coherent interconnect without also making the pagetable walk
interface coherent, the same effect happens if a coherent SMMU fails to
advertise CTTW correctly. This turns out to be the case on some popular
NXP SoCs, where VFIO started failing the IOMMU_CAP_CACHE_COHERENCY test,
even though IOMMU_CACHE *was* previously achieving the desired effect
anyway thanks to the underlying integration.
While those SoCs stand to gain some more general benefits from a
firmware update to override CTTW correctly in DT/ACPI, it's also easy
to work around this in Linux as well, to avoid imposing too much on
affected users - since the upstream client devices *are* correctly
marked as coherent, we can trivially infer their coherent paths through
the SMMU as well.
Reported-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Fixes: df198b37e7 ("iommu/arm-smmu: Report IOMMU_CAP_CACHE_COHERENCY better")
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Tested-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://lore.kernel.org/r/d6dc41952961e5c7b21acac08a8bf1eb0f69e124.1671123115.git.robin.murphy@arm.com
Signed-off-by: Will Deacon <will@kernel.org>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.