Documentation: dmaengine: Add DMA_CTRL_REUSE documentation

Signed-off-by: Vinod Koul <[e-mail]>
Acked-by:Robert Jarzmik <[e-mail]>

Documentation/dmaengine/provider.txt

···
     - This can be acked by invoking async_tx_ack()
     - If set, does not mean descriptor can be reused
 
+  * DMA_CTRL_REUSE
+    - If set, the descriptor can be reused after being completed. It should
+      not be freed by provider if this flag is set.
+    - The descriptor should be prepared for reuse by invoking
+      dmaengine_desc_set_reuse() which will set DMA_CTRL_REUSE.
+    - dmaengine_desc_set_reuse() will succeed only when channel support
+      reusable descriptor as exhibited by capablities
+    - 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.
+    - Descriptor can be freed in few ways
+	- Clearing DMA_CTRL_REUSE by invoking dmaengine_desc_clear_reuse()
+	  and submitting for last txn
+	- Explicitly invoking dmaengine_desc_free(), this can succeed only
+	  when DMA_CTRL_REUSE is already set
+	- Terminating the channel
+
 
 General Design Notes
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