[3/4] dma: document dma_flags_set_dmabarrier()

From: akepner
Date: Thu Sep 27 2007 - 21:14:20 EST

Document dma_flags_set_dmabarrier().

Signed-off-by: Arthur Kepner <akepner@xxxxxxx>

DMA-API.txt | 26 ++++++++++++++++++++++++++
1 files changed, 26 insertions(+)

diff --git a/Documentation/DMA-API.txt b/Documentation/DMA-API.txt
index cc7a8c3..5fc0bba 100644
--- a/Documentation/DMA-API.txt
+++ b/Documentation/DMA-API.txt
@@ -544,3 +544,29 @@ size is the size (and should be a page-sized multiple).
The return value will be either a pointer to the processor virtual
address of the memory, or an error (via PTR_ERR()) if any part of the
region is occupied.
+dma_flags_set_dmabarrier(int dir)
+Amend dir (one of the enum dma_data_direction values), with a
+platform-specific "dmabarrier" attribute. The dmabarrier attribute
+forces a flush of all in-flight DMA when the associated memory
+region is written to (see example below.)
+This provides a mechanism to enforce ordering of DMA on platforms that
+permit DMA to be reordered between device and host memory (within a
+NUMA interconnect). On other platforms this is a nop.
+The dmabarrier would be set when the memory region is mapped for DMA,
+ int count, flags = dma_flags_set_dmabarrier(DMA_BIDIRECTIONAL);
+ ....
+ count = dma_map_sg(dev, sglist, nents, flags);
+As an example of a situation where this would be useful, suppose that
+the device does a DMA write to indicate that data is ready and
+available in memory. The DMA of the "completion indication" could
+race with data DMA. Using a dmabarrier on the memory used for
+completion indications would prevent the race.
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