x86/amd-iommu: Add amd_iommu=off command line option

This patch adds a command line option to tell the AMD IOMMU
driver to not initialize any IOMMU it finds.
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>

Documentation/kernel-parameters.txt

@@ -324,6 +324,8 @@ and is between 256 and 4096 characters. It is defined in the file
 				    they are unmapped. Otherwise they are
 				    flushed before they will be reused, which
 				    is a lot of faster
+			off	  - do not initialize any AMD IOMMU found in
+				    the system
 
 	amijoy.map=	[HW,JOY] Amiga joystick support
 			Map of devices attached to JOY0DAT and JOY1DAT

arch/x86/kernel/amd_iommu_init.c

@@ -120,6 +120,7 @@ struct ivmd_header {
 bool amd_iommu_dump;
 
 static int __initdata amd_iommu_detected;
+static bool __initdata amd_iommu_disabled;
 
 u16 amd_iommu_last_bdf;			/* largest PCI device id we have
 					   to handle */
@@ -1372,6 +1373,9 @@ void __init amd_iommu_detect(void)
 	if (no_iommu || (iommu_detected && !gart_iommu_aperture))
 		return;
 
+	if (amd_iommu_disabled)
+		return;
+
 	if (acpi_table_parse("IVRS", early_amd_iommu_detect) == 0) {
 		iommu_detected = 1;
 		amd_iommu_detected = 1;
@@ -1401,6 +1405,8 @@ static int __init parse_amd_iommu_options(char *str)
 	for (; *str; ++str) {
 		if (strncmp(str, "fullflush", 9) == 0)
 			amd_iommu_unmap_flush = true;
+		if (strncmp(str, "off", 3) == 0)
+			amd_iommu_disabled = true;
 	}
 
 	return 1;