common: DMA-mapping: Add strongly ordered memory attribute
Strongly ordered memory is occasionally needed for some DMA
allocations for specialized use cases. Add the corresponding
DMA attribute.
Change-Id: Idd9e756c242ef57d6fa6700e51cc38d0863b760d
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>
diff --git a/Documentation/DMA-attributes.txt b/Documentation/DMA-attributes.txt
index 725580d..d62d0c6 100644
--- a/Documentation/DMA-attributes.txt
+++ b/Documentation/DMA-attributes.txt
@@ -67,3 +67,12 @@
Since it is optional for platforms to implement
DMA_ATTR_NO_KERNEL_MAPPING, those that do not will simply ignore the
attribute and exhibit default behavior.
+
+DMA_ATTR_STRONGLY_ORDERED
+-------------------------
+
+DMA_ATTR_STRONGLY_ORDERED allocates memory with a very restrictive type
+of mapping (no unaligned accesses, no re-ordering, no write merging, no
+buffering, no pre-fetching). This has severe performance penalties and
+should not be used for general purpose DMA allocations. It should only
+be used if one of the restrictions on strongly ordered memory is required.