dmaengine: Add documentation for new memcpy scatter-gather function

Documentation describes semantics, limitations and a typical use case
scenario.

Signed-off-by: Adrian Larumbe <adrianml@alumnos.upm.es>
Link: https://lore.kernel.org/r/20211101180825.241048-2-adrianml@alumnos.upm.es
Signed-off-by: Vinod Koul <vkoul@kernel.org>

Documentation/driver-api/dmaengine/provider.rst

@@ -162,6 +162,29 @@ Currently, the types available are:
 
   - The device is able to do memory to memory copies
 
+- - DMA_MEMCPY_SG
+
+  - The device supports memory to memory scatter-gather transfers.
+
+  - Even though a plain memcpy can look like a particular case of a
+    scatter-gather transfer, with a single chunk to copy, it's a distinct
+    transaction type in the mem2mem transfer case. This is because some very
+    simple devices might be able to do contiguous single-chunk memory copies,
+    but have no support for more complex SG transfers.
+
+  - No matter what the overall size of the combined chunks for source and
+    destination is, only as many bytes as the smallest of the two will be
+    transmitted. That means the number and size of the scatter-gather buffers in
+    both lists need not be the same, and that the operation functionally is
+    equivalent to a ``strncpy`` where the ``count`` argument equals the smallest
+    total size of the two scatter-gather list buffers.
+
+  - It's usually used for copying pixel data between host memory and
+    memory-mapped GPU device memory, such as found on modern PCI video graphics
+    cards. The most immediate example is the OpenGL API function
+    ``glReadPielx()``, which might require a verbatim copy of a huge framebuffer
+    from local device memory onto host memory.
+
 - DMA_XOR
 
   - The device is able to perform XOR operations on memory areas