1 files changed, 15 insertions, 1 deletions

diff --git a/drivers/usb/core/message.c b/drivers/usb/core/message.c
index da4ee07e0094..b7434787db5f 100644
--- a/drivers/usb/core/message.c
+++ b/drivers/usb/core/message.c
@@ -412,10 +412,24 @@ int usb_sg_init (
                 io->urbs [i]->status = -EINPROGRESS;
                 io->urbs [i]->actual_length = 0;
 
+                /*
+                 * 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
+                 * to prevent stale pointers and to help spot bugs.
+                 */
                 if (dma) {
-                        /* hc may use _only_ transfer_dma */
                         io->urbs [i]->transfer_dma = sg_dma_address (sg + i);
                         len = sg_dma_len (sg + i);
+#ifdef CONFIG_HIGHMEM
+                        io->urbs[i]->transfer_buffer = NULL;
+#else
+                        io->urbs[i]->transfer_buffer =
+                                page_address(sg[i].page) + sg[i].offset;
+#endif
                 } else {
                         /* hc may use _only_ transfer_buffer */
                         io->urbs [i]->transfer_buffer =