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-rw-r--r--Documentation/memory-barriers.txt22
1 files changed, 11 insertions, 11 deletions
diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index cc621decd943..06e14efd8662 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -1910,7 +1910,8 @@ There are some more advanced barrier functions:
These are for use with consistent memory to guarantee the ordering
of writes or reads of shared memory accessible to both the CPU and a
- DMA capable device.
+ DMA capable device. See Documentation/core-api/dma-api.rst file for more
+ information about consistent memory.
For example, consider a device driver that shares memory with a device
and uses a descriptor status value to indicate if the descriptor belongs
@@ -1931,22 +1932,21 @@ There are some more advanced barrier functions:
/* assign ownership */
desc->status = DEVICE_OWN;
- /* notify device of new descriptors */
+ /* Make descriptor status visible to the device followed by
+ * notify device of new descriptor
+ */
writel(DESC_NOTIFY, doorbell);
}
- The dma_rmb() allows us guarantee the device has released ownership
+ The dma_rmb() allows us to guarantee that 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 dma_mb() implies both a dma_rmb() and
- a dma_wmb(). 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/core-api/dma-api.rst file for
- more information on consistent memory.
+ a dma_wmb().
+
+ Note that the dma_*() barriers do not provide any ordering guarantees for
+ accesses to MMIO regions. See the later "KERNEL I/O BARRIER EFFECTS"
+ subsection for more information about I/O accessors and MMIO ordering.
(*) pmem_wmb();