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-rw-r--r--kernel/dma/swiotlb.c12
1 files changed, 7 insertions, 5 deletions
diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
index 2048194a03..33d942615b 100644
--- a/kernel/dma/swiotlb.c
+++ b/kernel/dma/swiotlb.c
@@ -1309,11 +1309,13 @@ phys_addr_t swiotlb_tbl_map_single(struct device *dev, phys_addr_t orig_addr,
pool->slots[index + i].orig_addr = slot_addr(orig_addr, i);
tlb_addr = slot_addr(pool->start, index) + offset;
/*
- * When dir == DMA_FROM_DEVICE we could omit the copy from the orig
- * to the tlb buffer, if we knew for sure the device will
- * overwrite the entire current content. But we don't. Thus
- * unconditional bounce may prevent leaking swiotlb content (i.e.
- * kernel memory) to user-space.
+ * When the device is writing memory, i.e. dir == DMA_FROM_DEVICE, copy
+ * the original buffer to the TLB buffer before initiating DMA in order
+ * to preserve the original's data if the device does a partial write,
+ * i.e. if the device doesn't overwrite the entire buffer. Preserving
+ * the original data, even if it's garbage, is necessary to match
+ * hardware behavior. Use of swiotlb is supposed to be transparent,
+ * i.e. swiotlb must not corrupt memory by clobbering unwritten bytes.
*/
swiotlb_bounce(dev, tlb_addr, mapping_size, DMA_TO_DEVICE);
return tlb_addr;