2 files changed, 22 insertions, 0 deletions

diff --git a/Documentation/x86_64/boot-options.txt b/Documentation/x86_64/boot-options.txt
index 4303e0c12476..e97a4c8fe6ea 100644
--- a/Documentation/x86_64/boot-options.txt
+++ b/Documentation/x86_64/boot-options.txt
@@ -199,6 +199,10 @@ IOMMU
    allowed  overwrite iommu off workarounds for specific chipsets.
    soft  Use software bounce buffering (default for Intel machines)
    noaperture Don't touch the aperture for AGP.
+   allowdac Allow DMA >4GB
+            When off all DMA over >4GB is forced through an IOMMU or bounce
+            buffering.
+   nodac    Forbid DMA >4GB
 
   swiotlb=pages[,force]
 
diff --git a/arch/x86_64/kernel/pci-dma.c b/arch/x86_64/kernel/pci-dma.c
index 4dcb671bd19f..225e19941b67 100644
--- a/arch/x86_64/kernel/pci-dma.c
+++ b/arch/x86_64/kernel/pci-dma.c
@@ -170,8 +170,20 @@ void dma_free_coherent(struct device *dev, size_t size,
 }
 EXPORT_SYMBOL(dma_free_coherent);
 
+static int forbid_dac __read_mostly;
+
 int dma_supported(struct device *dev, u64 mask)
 {
+#ifdef CONFIG_PCI
+        if (mask > 0xffffffff && forbid_dac > 0) {
+
+
+
+                printk(KERN_INFO "PCI: Disallowing DAC for device %s\n", dev->bus_id);
+                return 0;
+        }
+#endif
+
         if (dma_ops->dma_supported)
                 return dma_ops->dma_supported(dev, mask);
 
@@ -231,6 +243,8 @@ EXPORT_SYMBOL(dma_set_mask);
    allowed  overwrite iommu off workarounds for specific chipsets.
    soft  Use software bounce buffering (default for Intel machines)
    noaperture Don't touch the aperture for AGP.
+   allowdac Allow DMA >4GB
+   nodac    Forbid DMA >4GB
 */
 __init int iommu_setup(char *p)
 {
@@ -267,6 +281,10 @@ __init int iommu_setup(char *p)
                     iommu_merge = 0;
             if (!strncmp(p, "forcesac",8))
                     iommu_sac_force = 1;
+            if (!strncmp(p, "allowdac", 8))
+                    forbid_dac = 0;
+            if (!strncmp(p, "nodac", 5))
+                    forbid_dac = -1;
 
 #ifdef CONFIG_SWIOTLB
             if (!strncmp(p, "soft",4))