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>

Documentation/DMA-attributes.txt

@@ -67,3 +67,12 @@ set on each call.
 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.

include/linux/dma-attrs.h

@@ -16,6 +16,7 @@ enum dma_attr {
 	DMA_ATTR_WRITE_COMBINE,
 	DMA_ATTR_NON_CONSISTENT,
 	DMA_ATTR_NO_KERNEL_MAPPING,
+	DMA_ATTR_STRONGLY_ORDERED,
 	DMA_ATTR_MAX,
 };