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-rw-r--r--Documentation/memory-barriers.txt17
1 files changed, 9 insertions, 8 deletions
diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index 6dafc8085acc..34c1970908a5 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -1920,9 +1920,6 @@ There are some more advanced barrier functions:
/* assign ownership */
desc->status = DEVICE_OWN;
- /* force memory to sync before notifying device via MMIO */
- wmb();
-
/* notify device of new descriptors */
writel(DESC_NOTIFY, doorbell);
}
@@ -1930,11 +1927,15 @@ There are some more advanced barrier functions:
The dma_rmb() allows us guarantee the device has released ownership
before we read the data from the descriptor, and the dma_wmb() allows
us to guarantee the data is written to the descriptor before the device
- can see it now has ownership. The wmb() is needed to guarantee that the
- cache coherent memory writes have completed before attempting a write to
- the cache incoherent MMIO region.
-
- See Documentation/DMA-API.txt for more information on consistent memory.
+ can see it now has ownership. Note that, when using writel(), a prior
+ wmb() is not needed to guarantee that the cache coherent memory writes
+ have completed before writing to the MMIO region. The cheaper
+ writel_relaxed() does not provide this guarantee and must not be used
+ here.
+
+ See the subsection "Kernel I/O barrier effects" for more information on
+ relaxed I/O accessors and the Documentation/DMA-API.txt file for more
+ information on consistent memory.
MMIO WRITE BARRIER