IOMMU, AMD Family15h Model10-1Fh erratum 746 Workaround

commit 318fe782539c4150d1b8e4e6c9dc3a896512cb8a upstream.

The IOMMU may stop processing page translations due to a perceived lack
of credits for writing upstream peripheral page service request (PPR)
or event logs. If the L2B miscellaneous clock gating feature is enabled
the IOMMU does not properly register credits after the log request has
completed, leading to a potential system hang.

BIOSes are supposed to disable L2B micellaneous clock gating by setting
L2_L2B_CK_GATE_CONTROL[CKGateL2BMiscDisable](D0F2xF4_x90[2]) = 1b. This
patch corrects that for those which do not enable this workaround.

Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
Acked-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Joerg Roedel <joro@8bytes.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>