summaryrefslogtreecommitdiff
path: root/Documentation/memory-barriers.txt
diff options
context:
space:
mode:
Diffstat (limited to 'Documentation/memory-barriers.txt')
-rw-r--r--Documentation/memory-barriers.txt42
1 files changed, 42 insertions, 0 deletions
diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index 7ee2ae6d5451..70a09f8a0383 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -1633,6 +1633,48 @@ There are some more advanced barrier functions:
operations" subsection for information on where to use these.
+ (*) dma_wmb();
+ (*) dma_rmb();
+
+ 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.
+
+ For example, consider a device driver that shares memory with a device
+ and uses a descriptor status value to indicate if the descriptor belongs
+ to the device or the CPU, and a doorbell to notify it when new
+ descriptors are available:
+
+ if (desc->status != DEVICE_OWN) {
+ /* do not read data until we own descriptor */
+ dma_rmb();
+
+ /* read/modify data */
+ read_data = desc->data;
+ desc->data = write_data;
+
+ /* flush modifications before status update */
+ dma_wmb();
+
+ /* 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);
+ }
+
+ The dma_rmb() allows us guarantee the device has released ownership
+ before we read the data from the descriptor, and he 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.
+
MMIO WRITE BARRIER
------------------