linux-kernel - [PATCH v3 6/6] Documentation: dmaengine: document DMA_CTRL

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Date:	Thu, 14 May 2015 19:53:26 +0200
From:	Robert Jarzmik <robert.jarzmik@...e.fr>
To:	Vinod Koul <vinod.koul@...el.com>,
	Jonathan Corbet <corbet@....net>,
	Daniel Mack <daniel@...que.org>,
	Haojian Zhuang <haojian.zhuang@...il.com>,
	Robert Jarzmik <robert.jarzmik@...e.fr>
Cc:	dmaengine@...r.kernel.org, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: [PATCH v3 6/6] Documentation: dmaengine: document DMA_CTRL_ACK

Add documentation about acking the transfers, and their
reusability.

Signed-off-by: Robert Jarzmik <robert.jarzmik@...e.fr>
---
 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..33a81ce 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 TX transfer can be reused after being completed.
+    - there is a guarantee the TX 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() because the DMA'd data wasn't used, it can resubmit the
+      transfer right after its completion.
 
 General Design Notes
 --------------------
-- 
2.1.4

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