[PATCH RESEND 2/2] Documentation: dmaengine: document DMA_CTRL_ACK

[Robert Jarzmik <robert.jarzmik@free.fr>]

Add documentation about acking the transfers, and their
reusability.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Cc: Maxime Ripard <maxime.ripard@free-electrons.com>
---
 Documentation/dmaengine/provider.txt | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/Documentation/dmaengine/provider.txt b/Documentation/dmaengine/provider.txt
index 05d2280..ca67b0f 100644
--- a/Documentation/dmaengine/provider.txt
+++ b/Documentation/dmaengine/provider.txt
@@ -345,11 +345,12 @@ where to put them)
       that abstracts it away.
 
   * DMA_CTRL_ACK
-    - Undocumented feature
-    - No one really has an idea of what it's about, besides being
-      related to reusing the DMA transaction descriptors or having
-      additional transactions added to it in the async-tx API
-    - Useless in the case of the slave API
+    - If set, the transfer can be reused after being completed.
+    - There is a guarantee the transfer won't be freed until it is acked
+      by async_tx_ack().
+    - As a consequence, if a device driver wants to skip the dma_map_sg() and
+      dma_unmap_sg() in between 2 transfers, because the DMA'd data wasn't used,
+      it can resubmit the transfer right after its completion.
 
 General Design Notes
 --------------------
-- 
2.1.4