1 files changed, 11 insertions, 4 deletions

diff --git a/drivers/usb/core/message.c b/drivers/usb/core/message.c
index 25f63f1096b4..ad4b956380d2 100644
--- a/drivers/usb/core/message.c
+++ b/drivers/usb/core/message.c
@@ -411,15 +411,22 @@ int usb_sg_init (
                  * Some systems need to revert to PIO when DMA is temporarily
                  * unavailable.  For their sakes, both transfer_buffer and
                  * transfer_dma are set when possible.  However this can only
-                 * work on systems without HIGHMEM, since DMA buffers located
-                 * in high memory are not directly addressable by the CPU for
-                 * PIO ... so when HIGHMEM is in use, transfer_buffer is NULL
+                 * work on systems without:
+                 *
+                 *  - HIGHMEM, since DMA buffers located in high memory are
+                 *    not directly addressable by the CPU for PIO;
+                 *
+                 *  - IOMMU, since dma_map_sg() is allowed to use an IOMMU to
+                 *    make virtually discontiguous buffers be "dma-contiguous"
+                 *    so that PIO and DMA need diferent numbers of URBs.
+                 *
+                 * So when HIGHMEM or IOMMU are in use, transfer_buffer is NULL
                  * to prevent stale pointers and to help spot bugs.
                  */
                 if (dma) {
                         io->urbs [i]->transfer_dma = sg_dma_address (sg + i);
                         len = sg_dma_len (sg + i);
-#ifdef CONFIG_HIGHMEM
+#if defined(CONFIG_HIGHMEM) || defined(CONFIG_IOMMU)
                         io->urbs[i]->transfer_buffer = NULL;
 #else
                         io->urbs[i]->transfer_buffer =