2 files changed, 25 insertions, 0 deletions

diff --git a/Documentation/DMA-attributes.txt b/Documentation/DMA-attributes.txt
index 725580de75a..f50309081ac 100644
--- a/Documentation/DMA-attributes.txt
+++ b/Documentation/DMA-attributes.txt
@@ -67,3 +67,27 @@ 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_SKIP_CPU_SYNC
+----------------------
+
+By default dma_map_{single,page,sg} functions family transfer a given
+buffer from CPU domain to device domain. Some advanced use cases might
+require sharing a buffer between more than one device. This requires
+having a mapping created separately for each device and is usually
+performed by calling dma_map_{single,page,sg} function more than once
+for the given buffer with device pointer to each device taking part in
+the buffer sharing. The first call transfers a buffer from 'CPU' domain
+to 'device' domain, what synchronizes CPU caches for the given region
+(usually it means that the cache has been flushed or invalidated
+depending on the dma direction). However, next calls to
+dma_map_{single,page,sg}() for other devices will perform exactly the
+same sychronization operation on the CPU cache. CPU cache sychronization
+might be a time consuming operation, especially if the buffers are
+large, so it is highly recommended to avoid it if possible.
+DMA_ATTR_SKIP_CPU_SYNC allows platform code to skip synchronization of
+the CPU cache for the given buffer assuming that it has been already
+transferred to 'device' domain. This attribute can be also used for
+dma_unmap_{single,page,sg} functions family to force buffer to stay in
+device domain after releasing a mapping for it. Use this attribute with
+care!
diff --git a/include/linux/dma-attrs.h b/include/linux/dma-attrs.h
index a37c10cc51c..f83f793223f 100644
--- a/include/linux/dma-attrs.h
+++ b/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_SKIP_CPU_SYNC,
         DMA_ATTR_MAX,
 };