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-rw-r--r--Documentation/dmaengine/provider.txt23
1 files changed, 20 insertions, 3 deletions
diff --git a/Documentation/dmaengine/provider.txt b/Documentation/dmaengine/provider.txt
index ca67b0f04c6e..67d4ce4df109 100644
--- a/Documentation/dmaengine/provider.txt
+++ b/Documentation/dmaengine/provider.txt
@@ -345,12 +345,29 @@ where to put them)
that abstracts it away.
* DMA_CTRL_ACK
- - 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().
+ - If clear, the descriptor cannot be reused by provider until the
+ client acknowledges receipt, i.e. has has a chance to establish any
+ dependency chains
+ - 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
--------------------