dmaengine: doc: Add sections for per descriptor metadata support

Update the provider and client documentation with details about the
metadata support.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Reviewed-by: Tero Kristo <t-kristo@ti.com>
Tested-by: Keerthy <j-keerthy@ti.com>
Reviewed-by: Grygorii Strashko <grygorii.strashko@ti.com>
Link: https://lore.kernel.org/r/20191223110458.30766-4-peter.ujfalusi@ti.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>

1 files changed, 87 insertions, 0 deletions

diff --git a/Documentation/driver-api/dmaengine/client.rst b/Documentation/driver-api/dmaengine/client.rst
index 45953f171500..a9a7a3c84c63 100644
--- a/Documentation/driver-api/dmaengine/client.rst
+++ b/Documentation/driver-api/dmaengine/client.rst
@@ -151,6 +151,93 @@ The details of these operations are:
      Note that callbacks will always be invoked from the DMA
      engines tasklet, never from interrupt context.
 
+  Optional: per descriptor metadata
+  ---------------------------------
+  DMAengine provides two ways for metadata support.
+
+  DESC_METADATA_CLIENT
+
+    The metadata buffer is allocated/provided by the client driver and it is
+    attached to the descriptor.
+
+  .. code-block:: c
+
+     int dmaengine_desc_attach_metadata(struct dma_async_tx_descriptor *desc,
+				   void *data, size_t len);
+
+  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.
+
+    Becasue the DMA driver manages the memory area containing the metadata,
+    clients must make sure that they do not try to access or get the pointer
+    after their transfer completion callback has run for the descriptor.
+    If no completion callback has been defined for the transfer, then the
+    metadata must not be accessed after issue_pending.
+    In other words: if the aim is to read back metadata after the transfer is
+    completed, then the client must use completion callback.
+
+  .. code-block:: c
+
+     void *dmaengine_desc_get_metadata_ptr(struct dma_async_tx_descriptor *desc,
+		size_t *payload_len, size_t *max_len);
+
+     int dmaengine_desc_set_metadata_len(struct dma_async_tx_descriptor *desc,
+		size_t payload_len);
+
+  Client drivers can query if a given mode is supported with:
+
+  .. code-block:: c
+
+     bool dmaengine_is_metadata_mode_supported(struct dma_chan *chan,
+		enum dma_desc_metadata_mode mode);
+
+  Depending on the used mode client drivers must follow different flow.
+
+  DESC_METADATA_CLIENT
+
+    - DMA_MEM_TO_DEV / DEV_MEM_TO_MEM:
+      1. prepare the descriptor (dmaengine_prep_*)
+         construct the metadata in the client's buffer
+      2. use dmaengine_desc_attach_metadata() to attach the buffer to the
+         descriptor
+      3. submit the transfer
+    - DMA_DEV_TO_MEM:
+      1. prepare the descriptor (dmaengine_prep_*)
+      2. use dmaengine_desc_attach_metadata() to attach the buffer to the
+         descriptor
+      3. submit the transfer
+      4. when the transfer is completed, the metadata should be available in the
+         attached buffer
+
+  DESC_METADATA_ENGINE
+
+    - DMA_MEM_TO_DEV / DEV_MEM_TO_MEM:
+      1. prepare the descriptor (dmaengine_prep_*)
+      2. use dmaengine_desc_get_metadata_ptr() to get the pointer to the
+         engine's metadata area
+      3. update the metadata at the pointer
+      4. use dmaengine_desc_set_metadata_len()  to tell the DMA engine the
+         amount of data the client has placed into the metadata buffer
+      5. submit the transfer
+    - DMA_DEV_TO_MEM:
+      1. prepare the descriptor (dmaengine_prep_*)
+      2. submit the transfer
+      3. on transfer completion, use dmaengine_desc_get_metadata_ptr() to get
+         the pointer to the engine's metadata area
+      4. read out the metadata from the pointer
+
+  .. note::
+
+     When DESC_METADATA_ENGINE mode is used the metadata area for the descriptor
+     is no longer valid after the transfer has been completed (valid up to the
+     point when the completion callback returns if used).
+
+     Mixed use of DESC_METADATA_CLIENT / DESC_METADATA_ENGINE is not allowed,
+     client drivers must use either of the modes per descriptor.
+
 4. Submit the transaction
 
    Once the descriptor has been prepared and the callback information