1 files changed, 10 insertions, 22 deletions

diff --git a/drivers/usb/core/message.c b/drivers/usb/core/message.c
index 9720e699f47..da718e84d58 100644
--- a/drivers/usb/core/message.c
+++ b/drivers/usb/core/message.c
@@ -459,35 +459,23 @@ int usb_sg_init(struct usb_sg_request *io, struct usb_device *dev,
                         io->urbs[i]->context = io;
 
                         /*
-                         * 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:
+                         * 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.
                          *
-                         *  - 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.
+                         * Note that if IOMMU coalescing occurred, we cannot
+                         * trust sg_page anymore, so check if S/G list shrunk.
                          */
+                        if (io->nents == io->entries && !PageHighMem(sg_page(sg)))
+                                io->urbs[i]->transfer_buffer = sg_virt(sg);
+                        else
+                                io->urbs[i]->transfer_buffer = NULL;
+
                         if (dma) {
                                 io->urbs[i]->transfer_dma = sg_dma_address(sg);
                                 len = sg_dma_len(sg);
-#if defined(CONFIG_HIGHMEM) || defined(CONFIG_GART_IOMMU)
-                                io->urbs[i]->transfer_buffer = NULL;
-#else
-                                io->urbs[i]->transfer_buffer = sg_virt(sg);
-#endif
                         } else {
                                 /* hc may use _only_ transfer_buffer */
-                                io->urbs[i]->transfer_buffer = sg_virt(sg);
                                 len = sg->length;
                         }