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Diffstat (limited to 'Documentation/DMA-attributes.txt')
| -rw-r--r-- | Documentation/DMA-attributes.txt | 24 |
1 files changed, 24 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! |