Documentation: dmaengine: Document new dma_vec API

Document the dmaengine_prep_peripheral_dma_vec() API function, the
device_prep_peripheral_dma_vec() backend function, and the dma_vec
struct.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Link: https://lore.kernel.org/r/20240620122726.41232-8-paul@crapouillou.net
Signed-off-by: Vinod Koul <vkoul@kernel.org>

Author: pcercuei

Documentation/driver-api/dmaengine/client.rst

@@ -80,6 +80,10 @@ The details of these operations are:
 
   - slave_sg: DMA a list of scatter gather buffers from/to a peripheral
 
+  - peripheral_dma_vec: DMA an array of scatter gather buffers from/to a
+    peripheral. Similar to slave_sg, but uses an array of dma_vec
+    structures instead of a scatterlist.
+
   - dma_cyclic: Perform a cyclic DMA operation from/to a peripheral till the
     operation is explicitly stopped.
 
@@ -102,6 +106,11 @@ The details of these operations are:
 		unsigned int sg_len, enum dma_data_direction direction,
 		unsigned long flags);
 
+     struct dma_async_tx_descriptor *dmaengine_prep_peripheral_dma_vec(
+		struct dma_chan *chan, const struct dma_vec *vecs,
+		size_t nents, enum dma_data_direction direction,
+		unsigned long flags);
+
      struct dma_async_tx_descriptor *dmaengine_prep_dma_cyclic(
 		struct dma_chan *chan, dma_addr_t buf_addr, size_t buf_len,
 		size_t period_len, enum dma_data_direction direction);

Documentation/driver-api/dmaengine/provider.rst

@@ -433,6 +433,12 @@ supported.
     - residue: Provides the residue bytes of the transfer for those that
       support residue.
 
+- ``device_prep_peripheral_dma_vec``
+
+  - Similar to ``device_prep_slave_sg``, but it takes a pointer to a
+    array of ``dma_vec`` structures, which (in the long run) will replace
+    scatterlists.
+
 - ``device_issue_pending``
 
   - Takes the first transaction descriptor in the pending queue,
@@ -544,6 +550,10 @@ dma_cookie_t
 - Not really relevant any more since the introduction of ``virt-dma``
   that abstracts it away.
 
+dma_vec
+
+- A small structure that contains a DMA address and length.
+
 DMA_CTRL_ACK
 
 - If clear, the descriptor cannot be reused by provider until the