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-rw-r--r--drivers/usb/core/message.c32
1 files changed, 10 insertions, 22 deletions
diff --git a/drivers/usb/core/message.c b/drivers/usb/core/message.c
index 9720e699f472..da718e84d58d 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;
}