[ARM] Don't call consistent_sync() for DMA coherent memory

Memory allocated by the coherent memory allocators will be marked
uncacheable, which means it's pointless calling consistent_sync()
to perform cache maintainence on this memory; it's just a waste of
CPU cycles.

Moreover, with the (subsequent) merge of outer cache support, it
actually breaks things to call consistent_sync() on anything but
direct-mapped memory.
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

arch/arm/common/dmabounce.c

@@ -281,9 +281,13 @@ map_single(struct device *dev, void *ptr, size_t size,
 		ptr = buf->safe;
 
 		dma_addr = buf->safe_dma_addr;
-	}
-
+	} else {
+		/*
+		 * We don't need to sync the DMA buffer since
+		 * it was allocated via the coherent allocators.
+		 */
 		consistent_sync(ptr, size, dir);
+	}
 
 	return dma_addr;
 }
@@ -397,7 +401,10 @@ sync_single(struct device *dev, dma_addr_t dma_addr, size_t size,
 		default:
 			BUG();
 		}
-		consistent_sync(buf->safe, size, dir);
+		/*
+		 * No need to sync the safe buffer - it was allocated
+		 * via the coherent allocators.
+		 */
 	} else {
 		consistent_sync(dma_to_virt(dev, dma_addr), size, dir);
 	}