x86, AMD IOMMU: ignore detection of GART IOMMU

One of the last IOMMU updates covered a bug in the AMD IOMMU code. The early
detection code does not succeed if the GART is already detected. This patch
fixes this.

Cc: Robert Richter <robert.richter@amd.com>
Cc: Bhavna Sarathy <Bhavna.Sarathy@amd.com>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
Cc: iommu@lists.linux-foundation.org
Cc: Joerg Roedel <joerg.roedel@amd.com>
Cc: Robert Richter <robert.richter@amd.com>
Cc: Bhavna Sarathy <Bhavna.Sarathy@amd.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/kernel/amd_iommu_init.c

@@ -828,7 +828,7 @@ static int __init early_amd_iommu_detect(struct acpi_table_header *table)
 
 void __init amd_iommu_detect(void)
 {
-	if (swiotlb || no_iommu || iommu_detected)
+	if (swiotlb || no_iommu || (iommu_detected && !gart_iommu_aperture))
 		return;
 
 	if (acpi_table_parse("IVRS", early_amd_iommu_detect) == 0) {