pci: document pci_dma_flags_set_dmaflush()

index e07f253..32f88e5 100644
@@ -745,7 +745,30 @@ to "Closing".
    2.5., page+offset is always used, and the "address" field has been
    deleted.
 
-2) More to come...
+2) Platforms that permit DMA reordering
+
+   At least one platform (SGI SN2) allows DMA to be reordered between
+   device and host memory. This reordering can happen in the NUMA
+   interconnect, and it usually results in correct operation and improved
+   performance. In some situations it may be necessary to explicitly
+   synchronize DMA from the device. For example, if the device does a DMA
+   write to indicate that data is ready, the DMA of the "completion
+   indication" can race with DMA of data. In this situation,
+   pci_dma_flags_set_dmaflush() can be used to associate a "dmaflush"
+   attribute with a memory region. The dmaflush attribute will cause in-
+   flight DMA from the device to be flushed to host memory when the
+   associated memory region is written. (On platforms that don't permit
+   DMA reordering, this is a no-op.)
+
+   For example, you might map the memory region used for completion
+   indications as follows:
+
+	int count, flags = pci_dma_flags_set_dmaflush(DMA_BIDIRECTIONAL);
+	.....
+	count = dma_map_sg(dev, sglist, nents, flags);
+
+
+3) More to come...
 
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