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CONFIG_ARM_SMMU_DISABLE_BYPASS_BY_DEFAULT: Default to disabling bypass on ARM SMMU v1 and v2

General informations

The Linux kernel configuration item CONFIG_ARM_SMMU_DISABLE_BYPASS_BY_DEFAULT:

Help text

Say Y here to (by default) disable bypass streams such that incoming transactions from devices that are not attached to an iommu domain will report an abort back to the device and will not be allowed to pass through the SMMU.

Any old kernels that existed before this KConfig was introduced would default to _allowing_ bypass (AKA the equivalent of NO for this config). However the default for this option is YES because the old behavior is insecure.

There are few reasons to allow unmatched stream bypass, and even fewer good ones. If saying YES here breaks your board you should work on fixing your board. This KConfig option is expected to be removed in the future and we'll simply hardcode the bypass disable in the code.

NOTE: the kernel command line parameter 'arm-smmu.disable_bypass' will continue to override this config.

Hardware

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