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Platforms that optionally provide an IOMMU protocol should do so by including a DXE driver (usually called IoMmuDxe) that produces either the IOMMU protocol -- if the underlying capabilities are available --, or gIoMmuAbsentProtocolGuid, to signal that the IOMMU capability detection completed with negative result (i.e., no IOMMU will be available in the system). In turn, DXE drivers (and library instances) that are supposed to use the IOMMU protocol if it is available should add the following to their DEPEX: gEdkiiIoMmuProtocolGuid OR gIoMmuAbsentProtocolGuid This ensures these client modules will only be dispatched after IOMMU detection completes (with positive or negative result). Cc: Jordan Justen <jordan.l.justen@intel.com> Cc: Laszlo Ersek <lersek@redhat.com> Cc: Leo Duran <leo.duran@amd.com> Contributed-under: TianoCore Contribution Agreement 1.0 Suggested-by: Jordan Justen <jordan.l.justen@intel.com> Suggested-by: Laszlo Ersek <lersek@redhat.com> Signed-off-by: Brijesh Singh <brijesh.singh@amd.com> Reviewed-by: Laszlo Ersek <lersek@redhat.com> Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>