patch-2.4.9 linux/Documentation/DMA-mapping.txt

• Lines: 20
• Date: Wed Aug 15 01:54:44 2001
• Orig file: v2.4.8/linux/Documentation/DMA-mapping.txt
• Orig date: Sat May 19 17:43:05 2001

diff -u --recursive --new-file v2.4.8/linux/Documentation/DMA-mapping.txt linux/Documentation/DMA-mapping.txt
@@ -180,6 +180,19 @@
   to memory is immediately visible to the device, and vice
   versa.  Consistent mappings guarantee this.
 
+  IMPORTANT: Consistent DMA memory does not preclude the usage of
+             proper memory barriers.  The cpu may reorder stores to
+	     consistent memory just as it may normal memory.  Example:
+	     if it is important for the device to see the first word
+	     of a descriptor updated before the second, you must do
+	     something like:
+
+		desc->word0 = address;
+		wmb();
+		desc->word1 = DESC_VALID;
+
+             in order to get correct behavior on all platforms.
+
 - Streaming DMA mappings which are usually mapped for one DMA transfer,
   unmapped right after it (unless you use pci_dma_sync below) and for which
   hardware can optimize for sequential accesses.