1 files changed, 48 insertions, 0 deletions
diff --git a/Documentation/driver-api/dmaengine/provider.rst b/Documentation/driver-api/dmaengine/provider.rst
index dfc4486b5743..790a15089f1f 100644
--- a/Documentation/driver-api/dmaengine/provider.rst
+++ b/Documentation/driver-api/dmaengine/provider.rst
@@ -247,6 +247,54 @@ after each transfer. In case of a ring buffer, they may loop
 (DMA_CYCLIC). Addresses pointing to a device's register (e.g. a FIFO)
 are typically fixed.
 
+Per descriptor metadata support
+-------------------------------
+Some data movement architecture (DMA controller and peripherals) uses metadata
+associated with a transaction. The DMA controller role is to transfer the
+payload and the metadata alongside.
+The metadata itself is not used by the DMA engine itself, but it contains
+parameters, keys, vectors, etc for peripheral or from the peripheral.
+
+The DMAengine framework provides a generic ways to facilitate the metadata for
+descriptors. Depending on the architecture the DMA driver can implement either
+or both of the methods and it is up to the client driver to choose which one
+to use.
+
+- DESC_METADATA_CLIENT
+
+  The metadata buffer is allocated/provided by the client driver and it is
+  attached (via the dmaengine_desc_attach_metadata() helper to the descriptor.
+
+  From the DMA driver the following is expected for this mode:
+  - DMA_MEM_TO_DEV / DEV_MEM_TO_MEM
+    The data from the provided metadata buffer should be prepared for the DMA
+    controller to be sent alongside of the payload data. Either by copying to a
+    hardware descriptor, or highly coupled packet.
+  - DMA_DEV_TO_MEM
+    On transfer completion the DMA driver must copy the metadata to the client
+    provided metadata buffer before notifying the client about the completion.
+    After the transfer completion, DMA drivers must not touch the metadata
+    buffer provided by the client.
+
+- DESC_METADATA_ENGINE
+
+  The metadata buffer is allocated/managed by the DMA driver. The client driver
+  can ask for the pointer, maximum size and the currently used size of the
+  metadata and can directly update or read it. dmaengine_desc_get_metadata_ptr()
+  and dmaengine_desc_set_metadata_len() is provided as helper functions.
+
+  From the DMA driver the following is expected for this mode:
+  - get_metadata_ptr
+    Should return a pointer for the metadata buffer, the maximum size of the
+    metadata buffer and the currently used / valid (if any) bytes in the buffer.
+  - set_metadata_len
+    It is called by the clients after it have placed the metadata to the buffer
+    to let the DMA driver know the number of valid bytes provided.
+
+  Note: since the client will ask for the metadata pointer in the completion
+  callback (in DMA_DEV_TO_MEM case) the DMA driver must ensure that the
+  descriptor is not freed up prior the callback is called.
+
 Device operations
 -----------------