dmaengine: add fence support

Some engines optimize operation by reading ahead in the descriptor chain
such that descriptor2 may start execution before descriptor1 completes.
If descriptor2 depends on the result from descriptor1 then a fence is
required (on descriptor2) to disable this optimization.  The async_tx
api could implicitly identify dependencies via the 'depend_tx'
parameter, but that would constrain cases where the dependency chain
only specifies a completion order rather than a data dependency.  So,
provide an ASYNC_TX_FENCE to explicitly identify data dependencies.

Signed-off-by: Dan Williams <dan.j.williams@intel.com>

1 files changed, 3 insertions, 0 deletions

diff --git a/include/linux/async_tx.h b/include/linux/async_tx.h
index 866e61c4e2e0..a1c486a88e88 100644
--- a/include/linux/async_tx.h
+++ b/include/linux/async_tx.h
@@ -58,11 +58,14 @@ struct dma_chan_ref {
  * array.
  * @ASYNC_TX_ACK: immediately ack the descriptor, precludes setting up a
  * dependency chain
+ * @ASYNC_TX_FENCE: specify that the next operation in the dependency
+ * chain uses this operation's result as an input
  */
 enum async_tx_flags {
         ASYNC_TX_XOR_ZERO_DST    = (1 << 0),
         ASYNC_TX_XOR_DROP_DST    = (1 << 1),
         ASYNC_TX_ACK             = (1 << 2),
+        ASYNC_TX_FENCE           = (1 << 3),
 };
 
 /**