iommu/vt-d: Enable PCI ACS for platform opt in hint

PCI ACS is disabled if Intel IOMMU is off by default or intel_iommu=off
is used in command line. Unfortunately, Intel IOMMU will be forced on if
there're devices sitting on an external facing PCI port that is marked
as untrusted (for example, thunderbolt peripherals). That means, PCI ACS
is disabled while Intel IOMMU is forced on to isolate those devices. As
the result, the devices of an MFD will be grouped by a single group even
the ACS is supported on device.

[ 0.691263] pci 0000:00:07.1: Adding to iommu group 3
[ 0.691277] pci 0000:00:07.2: Adding to iommu group 3
[ 0.691292] pci 0000:00:07.3: Adding to iommu group 3

Fix it by requesting PCI ACS when Intel IOMMU is detected with platform
opt in hint.

Fixes: 89a6079df791a ("iommu/vt-d: Force IOMMU on for platform opt in hint")
Co-developed-by: Lalithambika Krishnakumar <[e-mail]>
Signed-off-by: Lalithambika Krishnakumar <[e-mail]>
Signed-off-by: Lu Baolu <[e-mail]>
Reviewed-by: Mika Westerberg <[e-mail]>
Cc: Mika Westerberg <[e-mail]>
Cc: Ashok Raj <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Joerg Roedel <[e-mail]>

drivers/iommu/intel/dmar.c

···
 	if (!ret)
 		ret = dmar_walk_dmar_table((struct acpi_table_dmar *)dmar_tbl,
 					   &validate_drhd_cb);
-	if (!ret && !no_iommu && !iommu_detected && !dmar_disabled) {
+	if (!ret && !no_iommu && !iommu_detected &&
+	    (!dmar_disabled || dmar_platform_optin())) {
 		iommu_detected = 1;
 		/* Make sure ACS will be enabled */
 		pci_request_acs();